100:1 probe for measuring ripple in a tube amp power supply

[Andreax1985]

Hi, I wish to measure b+ ripple in a tube amp. Since the b+ rails are at 350V, I was considering buying a 100:1 passive probe for my scope (rated for 400V input). Nevertheless, here is my doubt: I'll need to use the scope in AC coupling mode to clearly see the ripple. This means that I cannot safely use a cheap 100Mohm 100:1 probe. I'm on a budget and I can't afford a differential probe. Could you suggest me a suitable 100:1 probe which does not give issues when used in AC coupling mode?

[Richard Crowley]

It is not clear whether you are saying that your (unidentified) scope already has AC mode, or whether you think you need a new probe to get AC coupling?

Ripple is typically a very small signal.  Using a 100x probe will probably make the ripple nearly invisible.

You don't need a 100x probe unless you are trying to measure a very large signal (350V).  But you are not asking about measuring the 350V. That should be measured with a DMM.  You are asking about measuring the ripple which is a very small AC component resting on top of the 350V.  You need a 1x or 10x probe with AC coupling.

[Andreax1985]

It is not clear whether you are saying that your (unidentified) scope already has AC mode, or whether you think you need a new probe to get AC coupling?

Ripple is typically a very small signal.  Using a 100x probe will probably make the ripple nearly invisible.

You don't need a 100x probe unless you are trying to measure a very large signal (350V).  But you are not asking about measuring the 350V. That should be measured with a DMM.  You are asking about measuring the ripple which is a very small AC component resting on top of the 350V.  You need a 1x or 10x probe with AC coupling.

My scope has AC coupling mode. As I said, I want to visualize ripple in the (high voltage) b+ stage of a tube amp (350V+). Of course the ripple component will (hopefully) be very small compared to the signal, but I think you are well aware that in AC coupling mode the DC component is not attenuated at all. So, if I use a cheap 100:1 passive probe for probing a 350+ V rectified signal in AC coupling mode I have good chances of damaging the scope. Hence, my question.

[Martin.M]

you can use a Tektronix P6006 Probe from flea market , 10:1 , <600V 
It is build for glowing scopes but useful with any 1M input BNC
Martin

[Andreax1985]

you can use a Tektronix P6006 Probe from flea market , 10:1 , <600V 
It is build for glowing scopes but useful with any 1M input BNC
Martin

I already have 10:1 probes (CAT I, 600V), but perhaps I'm not being clear enough on what I think the problem is...

[Richard Crowley]

I think you are well aware that in AC coupling mode the DC component is not attenuated at all. So, if I use a cheap 100:1 passive probe for probing a 350+ V rectified signal in AC coupling mode I have good chances of damaging the scope. Hence, my question.

Quite to the contrary, AC coupling completely "attenuates" the DC component 100%.  That is its whole purpose.  It completely blocks any DC component and allows only the AC signal through.  That is exactly what you want when measuring the small AC ripple signal on top of the large DC signal.

[Andreax1985]

Quite to the contrary, AC coupling completely "attenuates" the DC component 100%.  That is its whole purpose.  It completely blocks any DC component and allows only the AC signal through.  That is exactly what you want when measuring the small AC ripple signal on top of the large DC signal.

Yes, but the oscilloscope's AC coupling capacitor will charge to the voltage at the probe's tip, so that, if the signal has an high DC offset, the input voltage rating of the oscilloscope can be exceeded causing high voltage breakdown in the oscilloscope, possibly damaging it.

[Richard Crowley]

You have not revealed what scope you are asking about.  Do you have some scope that has no input protection?  If the capacitor charges up to the 350V, that means it is doing its job with 350V DC at the tube amp end, and 0V DC at the oscilloscope end.  That is exactly how AC coupling works. 

Are you saying that the AC coupling capacitor in your mystery scope is not rated for 350V?  If so, then simply add a high voltage capacitor between the amp and your 1x scope probe.

If you use DC coupling with a 100x probe, you will never see the ripple.

[Andreax1985]

You have not revealed what scope you are asking about.  Do you have some scope that has no input protection?  If the capacitor charges up to the 350V, that means it is doing its job with 350V DC at the tube amp end, and 0V DC at the oscilloscope end.  That is exactly how AC coupling works. 

Are you saying that the AC coupling capacitor in your mystery scope is not rated for 350V?  If so, then simply add a high voltage capacitor between the amp and your 1x scope probe.

If you use DC coupling with a 100x probe, you will never see the ripple.

My scope is a Siglent 1104x-e, whose input is rated for 400V pk-pk. However, I'm only estimating the voltage on the amp b+ rails at 350V and I feel that my estimate is dangerously close to the limit of my scope (and I'm not even accounting for possible spikes). Hence my desire to operate with a slightly larger saftey margin.

[MaxFrister]

Here is my understanding of the issue.

A 10:1 probe is a complicated 1/10 voltage divider.

You want to apply a 350V AC RMS voltage and look at the ripple.

That means you are applying 350*2*sqrt(2) AC peek-to-peek at the probe tip, or 990V -- Let's call it 1KV.

Then, 1/10 of 1KV is 100V -- well bellow the scopes spec of 400V. 

p.s., make sure that there is no way the ground clip on the probe can be disconnected while probing high voltages.

[Andreax1985]

Here is my understanding of the issue.

A 10:1 probe is a complicated 1/10 voltage divider.

You want to apply a 350V AC RMS voltage and look at the ripple.

That means you are apply 350*2*sqrt(2) AC peek-to-peek at the probe tip, or 990V -- Let's call it 1KV.

Then, 1/10 of 1KV is 100V -- well bellow the scopes spec of 400V. 

p.s., make sure that there is no way the ground clip on the probe can be disconnected while probing high voltages.

Let's say that IF the DC component of my signal is 350V and I'm using AC coupling mode, the capacitor in the scope (rated for 400v) will charge at 350V. But if I'm wrong or if a voltage spike takes place (how likely is it?) then I'm blowing the scope.

[MaxFrister]

Sorry, not enough coffee this morning.  Your B+ is DC, so it is just a straight 350V.

[MaxFrister]

Your scope input will only see 35V.

 Just check that that probe is rated for the DC voltage.  Someone already mentioned that probes from older scopes were all rated for 500V+

[capt bullshot]

As pointed out, the AC mode of the scope places the full DC voltage across the scope's AC coupling capacitor, regardless of the probe ratio or rating.
If you are unsure about the max. rating of this capacitor (which is expected to include the specified max. voltage of the scope alone plus some headroom), leave the scope in DC coupling and place a suitable capacitor between the B+ rail and the probe tip.

So if you scope is rated for 400V max. input voltage, this would be safe to use on 350V DC in AC coupling mode with any probe. Usually one wants to use a 1:1 probe here for best display resolution of the ripple voltage.

[Andreax1985]

As pointed out, the AC mode of the scope places the full DC voltage across the scope's AC coupling capacitor, regardless of the probe ratio or rating.
If you are unsure about the max. rating of this capacitor (which is expected to include the specified max. voltage of the scope alone plus some headroom), leave the scope in DC coupling and place a suitable capacitor between the B+ rail and the probe tip.

So if you scope is rated for 400V max. input voltage, this would be safe to use on 350V DC in AC coupling mode with any probe. Usually one wants to use a 1:1 probe here for best display resolution of the ripple voltage.

Thanks!

[floobydust]

I've had spectacular ka-bangs come out of scopes  when I disconnected the probe and switched back from AC to DC coupling. This hard shorts the scope's input blocking cap as it discharges through the switch.

The 1MEG input resistor is in the scope, and AFTER the blocking cap when you are on AC-coupling. So the blocking cap charges right up to the full HVDC potential on B+ even though on x10. You can measure ripple at 350VDC but you have to be careful. X1 is very risky, I would not use it without a jig with a blocking cap and voltage divider and bleeder resistor.

Connecting the scope to the HVDC present is hard on the scope's input protection (clamp) as you are fast charging the input cap to the HVDC present. Disconnecting (and touching GND or anything else in the circuit) the blocking capacitor discharges through the scope's protection clamp which is also hard on the scope and can damage it.

[Andreax1985]

I've had spectacular ka-bangs come out of scopes  when I disconnected the probe and switched back from AC to DC coupling. This hard shorts the scope's input blocking cap as it discharges through the switch.

The 1MEG input resistor is in the scope, and AFTER the blocking cap when you are on AC-coupling. So the blocking cap charges right up to the full HVDC potential on B+ even though on x10. You can measure ripple at 350VDC but you have to be careful. X1 is very risky, I would not use it without a jig with a blocking cap and voltage divider and bleeder resistor.

Connecting the scope to the HVDC present is hard on the scope's input protection (clamp) as you are fast charging the input cap to the HVDC present. Disconnecting (and touching GND or anything else in the circuit) the blocking capacitor discharges through the scope's protection clamp which is also hard on the scope and can damage it.

Perhaps I could simply switch off the amp while leaving everything connected, so that the blocking capacitor slowly discharges through the amp resistors...

[floobydust]

That would work, but one last trap.

Keep in mind changing VOLTS/DIV switch does switch in and out divider resistors, so I would start and end (measurements) at high settings like 10V/DIV.
Don't do this (turn amp power on/off or connect probe) at sensitive settings like 50mV/DIV where the scope's front end is vulnerable.

Looking at B+ you will see mains ripple, rectifier switching noise and the DC voltage will bob up and down due to mains voltage moving around, as well as load from music playing.

[Andreax1985]

That would work, but one last trap.

Keep in mind changing VOLTS/DIV switch does switch in and out divider resistors, so I would start and end (measurements) at high settings like 10V/DIV.
Don't do this (turn amp power on/off or connect probe) at sensitive settings like 50mV/DIV where the scope's front end is vulnerable.

Looking at B+ you will see mains ripple, rectifier switching noise and the DC voltage will bob up and down due to mains voltage moving around, as well as load from music playing.

Thanks! My 10x probes are rated for 300Vrms CAT II, are they enough for 320V DC?  (I measured the voltage with a multimeter)

[vk6zgo]

Here is my understanding of the issue.

A 10:1 probe is a complicated 1/10 voltage divider.

You want to apply a 350V AC RMS voltage and look at the ripple.

That means you are applying 350*2*sqrt(2) AC peek-to-peek at the probe tip, or 990V -- Let's call it 1KV.

Then, 1/10 of 1KV is 100V -- well bellow the scopes spec of 400V. 

p.s., make sure that there is no way the ground clip on the probe can be disconnected while probing high voltages.

No, the OP wants to look at low level ac "hum" riding on top of around 350v DC.

I will, however, address your comment:-
Peak to peak isn't a real thing with ac signals, as the positive & negative half cycles do not exist at the same time.
The highest voltage at any time is the peak voltage on any half cycle---with your values, about 495v.

What I believe the OP was worried about was that the coupling capacitor in circuit in the "ac coupled" mode could charge up to the full  350v DC voltage over approx 5t=CR.

Here, it is assumed that "R"is made up of the series resistance of the probe, in turn in series with the input impedance of the 'scope vertical input,(assuming the latter to be a pure resistance.).

This further assumes that the coupling capacitor's DC voltage rating is less than 350v.
Once the cap breaks down, possibly, all sorts of nasty stuff can happen.
As the 'scope is being used to look at low level ac, the volts/div will be set to a quite sensitive range,which will be overloaded by the 350v DC, with probable damage.

Against this, is the fact that, over many decades, people have used analog Oscilloscopes set to "ac coupling" to look at low level ac superimposed upon DC voltages of the same order as those the OP has.

If you have a fairly sophisticated analog 'scope, it will have adjustable "DC offset", which allows you to stay "DC coupled", but most of the analog 'scopes used for this sort of thing don't have that function.

If damage in the way anticipated was common, it would have become part of the "folklore" of the industry, but it hasn't.

Possibly this is more of a problem with DSOs.
I have read a lot of warnings about this as a problem, but I have not heard of a lot of people killing their DSOs in this fashion.

[Andreax1985]

Exactly. Anyway, let me be more precise. I measured my DC current at 320V DC with a multimeter. My scope input (Siglent 1004x-e) is rated for 400Vpk.

So the question is: is a 400Vpk rated input enough to withstand a full 320V DC signal in AC coupling mode?

[DaJMasta]

When you're using a 1x probe, it's closer than I'd cut it but it's within spec.  When you're using a 10x probe, sure, cause that's only 32V on the actual frontend of the scope.  Just make sure your probe is rated well enough.

[Andreax1985]

When you're using a 1x probe, it's closer than I'd cut it but it's within spec.  When you're using a 10x probe, sure, cause that's only 32V on the actual frontend of the scope.  Just make sure your probe is rated well enough.

Please notice that I'am referring to AC coupling mode, so the blocking cap is charged at full 320V even when using attenuating probes. So, since I'm not sure of the blocking cap's rating, I can only infer it from the scope input rating (400V pk). BUT: is 400V pk enough for 320V DC?

[DaJMasta]

Please notice that I'am referring to AC coupling mode, so the blocking cap is charged at full 320V even when using attenuating probes. So, since I'm not sure of the blocking cap's rating, I can only infer it from the scope input rating (400V pk). BUT: is 400V pk enough for 320V DC?

I don't know why you reask this or are so insistent, myself and several others have answered your question accurately and you just keep second guessing it.  Unless you're expecting your ripple component to be more than 160V peak to peak on top of your 320VDC, then you are still within the spec of the frontend with a 1x probe (or just a BNC cable going straight into the front).  If you just use a 10x probe, it has an internal 9M Ohm internally, so with the inbuilt voltage divider, the scope's frontend, at the BNC right where the AC cap can connect, only ever sees 1/10th of the voltage at the tip of the probe.  That means that with a normal 10x probe, you're going to be more worried about the dielectric breakdown of the parts at the very tip and the clearances around the resistor string in the probe itself because of your input voltage (the probe's voltage rating) than the scope's frontend, because 4kV on the tip of a 10x probe is required before the frontend of your connected scope will see that 400V maximum voltage.


The answer really has nothing to do with the coupling mode - as the rating of the frontend should be for any coupling mode.  It would say in the datasheet/user manual if the AC coupled input mode had a different maximum voltage.

[rf-loop]

When you're using a 1x probe, it's closer than I'd cut it but it's within spec.  When you're using a 10x probe, sure, cause that's only 32V on the actual frontend of the scope.  Just make sure your probe is rated well enough.

Please notice that I'am referring to AC coupling mode, so the blocking cap is charged at full 320V even when using attenuating probes. So, since I'm not sure of the blocking cap's rating, I can only infer it from the scope input rating (400V pk). BUT: is 400V pk enough for 320V DC?

Input rating is NOT 400V pk.
It is:

1 MΩ: ≤400 Vpk(DC + Peak AC <=10 kHz)

If this is difficult to understand and you are working with over 300V voltages I think it is not oscilloscope at all what you need first.
First you need some basic fundamentals study book. Yes I know reading is boring but... 
but also not alone reading, all need also understand.  But it is good you ask. This is wise.

But then you do not listen, you repeat and repeat same question without giving any more real facts about signals what you need measure. If you only tell 320V DC and you need measure ripple how you think it can answer anymore than example +320V +  +80V is +400V  (400Vpk)  also -320V + -80V is  -400V (400Vpk) You can even alternate between these as long as you stay below 10kHz (sine). (so <10kHz sine <800Vpk-pk (note small but important difference between pk and pk-pk) is accepted and with specification limits)
Without more details I think many peoples can repeat these answers infinitely.



You can connect 400V DC to scope input. But remember also what is DC.  When you connect it, it is not DC at all. How long time after step you can call it DC.  Think carefully this 10kHz. When you switch from 0V to 400V it depends this edge what is maximum frequency what there exist. So you can "slowly" (yes 10kHz is slow) rise it from 0V to 400V..  to +400 or  to -400V (ref GND)  and same when you change it from this voltage back to 0. You need stay below 10kHz if you are workin there near max limits.

(btw, pure DC is possible only in human imagination. In practice there is not DC at all but yes, perhaps 1pHz can think in practice as "DC" perhaps even nHz or uHz).  

But, can you explain more what you really need measure. You have tild 320V (dc) and then you want measure some ripple in this "dc".  What kind of ripple. What is this ripple level range and max freq.

Btw, as far as I know Siglent use 500V rated capacitors in front end these positions what need it.
(it can also see in this old SDS1202X-E "compensation issue" case)

But, can you explain more what you really need measure. You have tild 320V (dc) and then you want measure some ripple in this "dc".  What kind of ripple. What is this ripple level range and max freq.

If you afraid this internal AC coupling capacitor you can of course use external capacitor and then oscilloscope DC coupled. This give also one more advantage. You can select  what is low frequency corner what you need instead of Siglent very low corner freq around 1.2-1.3Hz. (more low than many other scopes and if your DC have low f fluctuations then ripple measurement may be extremely difficult if you need look low levels.) Is is possible or useful to use scope DC coupled and external coupling capacitor?

What is this ripple (over this 320V) what you want measure. Ripple pk-pk level and something about freq range and about what V/div,  mV/div range you need for ripple measurement?

[Hydron]

The OP is 100% correct to be thinking of the 400V rating - coupling mode DOES matter and AC coupling will negate any probe scaling factor for DC input for most probe/frontend designs. He/she understands a subtle gotcha in the way the AC coupling mode works and is asking a reasonable question rather than trying to start an argument.

I'd also agree with the suggestion above to use DC coupling and an external capacitor at the tip however.

[Andreax1985]

When you're using a 1x probe, it's closer than I'd cut it but it's within spec.  When you're using a 10x probe, sure, cause that's only 32V on the actual frontend of the scope.  Just make sure your probe is rated well enough.

Please notice that I'am referring to AC coupling mode, so the blocking cap is charged at full 320V even when using attenuating probes. So, since I'm not sure of the blocking cap's rating, I can only infer it from the scope input rating (400V pk). BUT: is 400V pk enough for 320V DC?

Input rating is NOT 400V pk.
It is:

1 MΩ: ≤400 Vpk(DC + Peak AC <=10 kHz)

If this is difficult to understand and you are working with over 300V voltages I think it is not oscilloscope at all what you need first.
First you need some basic fundamentals study book. Yes I know reading is boring but... 
but also not alone reading, all need also understand.  But it is good you ask. This is wise.

But then you do not listen, you repeat and repeat same question without giving any more real facts about signals what you need measure. If you only tell 320V DC and you need measure ripple how you think it can answer anymore than example +320V +  +80V is +400V  (400Vpk)  also -320V + -80V is  -400V (400Vpk) You can even alternate between these as long as you stay below 10kHz (sine). (so <10kHz sine <800Vpk-pk (note small but important difference between pk and pk-pk) is accepted and with specification limits)
Without more details I think many peoples can repeat these answers infinitely.



You can connect 400V DC to scope input. But remember also what is DC.  When you connect it, it is not DC at all. How long time after step you can call it DC.  Think carefully this 10kHz. When you switch from 0V to 400V it depends this edge what is maximum frequency what there exist. So you can "slowly" (yes 10kHz is slow) rise it from 0V to 400V..  to +400 or  to -400V (ref GND)  and same when you change it from this voltage back to 0. You need stay below 10kHz if you are workin there near max limits.

(btw, pure DC is possible only in human imagination. In practice there is not DC at all but yes, perhaps 1pHz can think in practice as "DC" perhaps even nHz or uHz).  

But, can you explain more what you really need measure. You have tild 320V (dc) and then you want measure some ripple in this "dc".  What kind of ripple. What is this ripple level range and max freq.

Btw, as far as I know Siglent use 500V rated capacitors in front end these positions what need it.
(it can also see in this old SDS1202X-E "compensation issue" case)

But, can you explain more what you really need measure. You have tild 320V (dc) and then you want measure some ripple in this "dc".  What kind of ripple. What is this ripple level range and max freq.

If you afraid this internal AC coupling capacitor you can of course use external capacitor and then oscilloscope DC coupled. This give also one more advantage. You can select  what is low frequency corner what you need instead of Siglent very low corner freq around 1.2-1.3Hz. (more low than many other scopes and if your DC have low f fluctuations then ripple measurement may be extremely difficult if you need look low levels.) Is is possible or useful to use scope DC coupled and external coupling capacitor?

What is this ripple (over this 320V) what you want measure. Ripple pk-pk level and something about freq range and about what V/div,  mV/div range you need for ripple measurement?

Thanks for the 500V bit of info on the coupling capacitor rating. As for the rest I can't be precise because I can't know before I probe the circuit. But since what I want to probe is a classic power supply stage in a tube (audio) amp I think ripple will be very low compared to the Dc signal (so no worries about it's magnitude). Also, I expect it to be at 100hz frequency as typical in this kind of applications. So I have no worries related to its frequency either.

[capt bullshot]

Thanks for the 500V bit of info on the coupling capacitor rating. As for the rest I can't be precise because I can't know before I probe the circuit. But since what I want to probe is a classic power supply stage in a tube (audio) amp I think ripple will be very low compared to the Dc signal (so no worries about it's magnitude). Also, I expect it to be at 100hz frequency as typical in this kind of applications. So I have no worries related to its frequency either.

A 10:1 300Vrms CAT II probe is enough for your measurement, including quite a good deal of margin. This is because 300Vrms equals to 420Vpk, and CAT II includes headroom for overvoltage. And your DC source is of limited energy (discharge of the filter cap, gives you a loud bang when shorted, but no violent explosion).
I agree to the others, a 10:1 probe is safer than a 1:1 probe here (for measuring at 320V DC) and will give you enough display resolution as one expects the ripple voltage in the "some volts" range here. The internal resistance of the 10:1 probe will do a good job of protecting the scope's input.

The general strategy for measuring ripple on a supply rail would be:
Set the scope to a high enough voltage / div scale, to cover the DC voltage, set coupling to DC.
Connect the probe, turn on the DUT (in any order you like)
Watch the signal for plausibility, check it's not exceeding the range.
As long as you can display the signal in DC coupling, one can assume it's safe to switch to AC coupling.
Change to AC coupling and increase sensitivity (less volts / div) until you can watch the ripple.
Disconnect and discharge the AC coupling capacitor by setting the scope to DC coupling again.

This procedure takes care of safely charging / discharging the scope's AC coupling capacitor. Usually these steps are skipped, because most general purpose scope's inputs are robust enough to not require them. Especially when you're using a probe (even the 1:1 ones should have some internal losses to dampen the charge / discharge currents) and operating at typical supply voltages up to 20V. For your 320V, I'd recommend to be more careful in general, since a charged capacitor at 320V bites you and your electronics. A charged AC coupling capacitor (from measuring B+) can damage small signal semiconductors if you move the probe from B+ to a small signal node without discharging the AC coupling capacitor, even when using a 10:1 probe.

[Wolfgang]

Hi, I wish to measure b+ ripple in a tube amp. Since the b+ rails are at 350V, I was considering buying a 100:1 passive probe for my scope (rated for 400V input). Nevertheless, here is my doubt: I'll need to use the scope in AC coupling mode to clearly see the ripple. This means that I cannot safely use a cheap 100Mohm 100:1 probe. I'm on a budget and I can't afford a differential probe. Could you suggest me a suitable 100:1 probe which does not give issues when used in AC coupling mode?

This is how I did it (to measure ripple in an 500V HV power supply):

- a took a *big* foil cap (10uF@1000V) and connected one end to the PSU and grounded the other end. At the grounded side
it looks like a resistor (e.g., 10kOhm) and a pair of antiparallel diodes (1N4007) that are bridged by a switch, now closed.
- I connected the load to the power supply
- I power up the PSU. The cap now charges up thru the closed ground switch
- I open the ground switch. AC now appears across the 10kOhm resistor and can be measured. Lower bandwidth limit is ca. 10Hz.
- Performing measurements
- Afterwards, close the ground switch again
- Disconnect scope
- Power down the PSU
- After coupling cap is empty, disconnect it
- Done

[trobbins]

The B+ ripple in a valve amp may well get to 5-10Vrms for a capacitor input filter, especially with a valve diode.  Be very careful to check if the power supply uses a choke input filter, as the diode output voltage that connects to the choke will be a large DC+AC level.  Most valve amp people just get a nice high voltage cap of about 10nF and place it between the B+ and a 10:1 probe, and use DC coupling mode to ensure the probe and scope parts are all ground referenced.  Clipping the cap/probe to the test point, and then energising the amp, and then de-energising, allows the interface cap to charge up and down more sedately and safely.  A 10:1 probe presents 10Meg loading, which is usually high enough not to noticeably alter voltage rail levels in the preamp sections of an amp (whereas 1Meg loading can have a noticeable influence).

[2N3055]

Choose right tool for the job.
You cannot rely on AC decoupling in the scope to do this without blowing up.

You should buy a book "Building Valve Amplifiers, Second Edition" by Morgan Jones.
Books are our friends.

In attachment excerpt from the book about your particular task.

Regards,
Sinisa

[rf-loop]

When you're using a 1x probe, it's closer than I'd cut it but it's within spec.  When you're using a 10x probe, sure, cause that's only 32V on the actual frontend of the scope.  Just make sure your probe is rated well enough.

Please notice that I'am referring to AC coupling mode, so the blocking cap is charged at full 320V even when using attenuating probes. So, since I'm not sure of the blocking cap's rating, I can only infer it from the scope input rating (400V pk). BUT: is 400V pk enough for 320V DC?

Input rating is NOT 400V pk.
It is:

1 MΩ: ≤400 Vpk(DC + Peak AC <=10 kHz)

If this is difficult to understand and you are working with over 300V voltages I think it is not oscilloscope at all what you need first.
First you need some basic fundamentals study book. Yes I know reading is boring but... 
but also not alone reading, all need also understand.  But it is good you ask. This is wise.

But then you do not listen, you repeat and repeat same question without giving any more real facts about signals what you need measure. If you only tell 320V DC and you need measure ripple how you think it can answer anymore than example +320V +  +80V is +400V  (400Vpk)  also -320V + -80V is  -400V (400Vpk) You can even alternate between these as long as you stay below 10kHz (sine). (so <10kHz sine <800Vpk-pk (note small but important difference between pk and pk-pk) is accepted and with specification limits)
Without more details I think many peoples can repeat these answers infinitely.



You can connect 400V DC to scope input. But remember also what is DC.  When you connect it, it is not DC at all. How long time after step you can call it DC.  Think carefully this 10kHz. When you switch from 0V to 400V it depends this edge what is maximum frequency what there exist. So you can "slowly" (yes 10kHz is slow) rise it from 0V to 400V..  to +400 or  to -400V (ref GND)  and same when you change it from this voltage back to 0. You need stay below 10kHz if you are workin there near max limits.

(btw, pure DC is possible only in human imagination. In practice there is not DC at all but yes, perhaps 1pHz can think in practice as "DC" perhaps even nHz or uHz).  

But, can you explain more what you really need measure. You have tild 320V (dc) and then you want measure some ripple in this "dc".  What kind of ripple. What is this ripple level range and max freq.

Btw, as far as I know Siglent use 500V rated capacitors in front end these positions what need it.
(it can also see in this old SDS1202X-E "compensation issue" case)

But, can you explain more what you really need measure. You have tild 320V (dc) and then you want measure some ripple in this "dc".  What kind of ripple. What is this ripple level range and max freq.

If you afraid this internal AC coupling capacitor you can of course use external capacitor and then oscilloscope DC coupled. This give also one more advantage. You can select  what is low frequency corner what you need instead of Siglent very low corner freq around 1.2-1.3Hz. (more low than many other scopes and if your DC have low f fluctuations then ripple measurement may be extremely difficult if you need look low levels.) Is is possible or useful to use scope DC coupled and external coupling capacitor?

What is this ripple (over this 320V) what you want measure. Ripple pk-pk level and something about freq range and about what V/div,  mV/div range you need for ripple measurement?

Thanks for the 500V bit of info on the coupling capacitor rating. As for the rest I can't be precise because I can't know before I probe the circuit. But since what I want to probe is a classic power supply stage in a tube (audio) amp I think ripple will be very low compared to the Dc signal (so no worries about it's magnitude). Also, I expect it to be at 100hz frequency as typical in this kind of applications. So I have no worries related to its frequency either.

You can use PP510 or PP215 siglent probe.
Set oscilloscope to DC cxoupling.
Set probe to 10x
Set scope for 10x probe
Connect 2.2nF capacitor rated least 500V (or more V)  to DUT test point and other end of capacitor to probe tip. BW low end corner frequency is bit under 10Hz now. (if calculate it supoer simply using RC formula) (many scopes AC coupling is also in this ballpark. But Siglent 1000X-E AC coupling corner freq is around 1.2Hz and this is quite low. Due to this if your 320V DC have some low freq instability is make very difficult to measure ripple because if you use low vertical setting like 5 - 20mV/div your trace difficult to observe because it is jumping out from display.

And if you look AC line frequency related ripple, as some told previously. Using trigger source "AC line" is useful.

If you want use 1x probe then these Siglent probes are not suitable. But under 300V same with 220nF capacitor result is same and then can use down to 0.5mV/div with 20MHz BW on..  If really need go so high sensitivity. 20MHz BW can turn on always when use 1x probe because probe itself have more low freq response buit 20MHz BW reduce scope front end wide BW noise.

Of course also you can - if really want - use scope own AC coupling with voltages up to 400Vpk.  Scope data sheet limits are still there (400Vpk  400V dc. ). If there is different limit for AC coupling it need also read in data sheet. There is no, so if you connect 320V DC to input and it is AC coupled and it blows ask new main board or repair from Siglent because data sheet (promise) tell it can do.

I want also sidenote one thing about probes. I have seen many peoples who do not look other than probe "nameplate" rating. There is other thing but because there all users do not read these "small things".
There is same kind of thing with what ever brand probes, including Keysight, Tektronix etc.. some differencies in numbers but same principle. Max voltage is highly dependent of frequency (mostly)
Example probe PP215 (200MHz probe what come with 1000X-E 200MHz models.
There nicely read in probe. 10x 600VDC, Pk, AC.
BUT
It is only for up to roughly around 50kHz
100kHz 200V
1MHz it is only 50V and 100MHz only around 25V

[David Hess]

As pointed out several times, common x10 and x100 probes do not attenuate the DC component when the oscilloscope input is set to AC coupled mode and this is a serious hazard to the oscilloscope.

Better high voltage probes including x10 models include a shunt resistance but unfortunately they are expensive and rare.  They can be identified from their input resistance specification which will be lower so a x10 probe might only be 1 megohm and a x100 probe might be 10 megohms.

A better and safer way to make this measurement without a suitable probe is to use an external AC coupling capacitor and shunt resistance to remove the high voltage DC component with a standard x1 or x10 probe.  The higher input resistance of a x10 probe will yield a lower cutoff frequency for a given coupling capacitance which may be important in line frequency applications for accurate results.

[vk6zgo]

Most of the time, if you had an old analog 'scope,you could pretty much "wing it" as they don't seem to be as fragile as DSOs.
Additionally, the really old Tektronix probes (the big,clunky ones) were designed for use with tube equipment, hence their voltage ratings are better, removing that source of worry.

As I mentioned before, some Oscilloscopes have a DC offset function, which allows you to view small ac signals superimposed upon large DC voltages, whilst in the "DC coupled" mode.

You can also do this with your 'scope in "A-B" mode, using two channels, with one being fed a DC voltage which becomes the "DC offset".
The problem here, is finding an adjustable high voltage supply which is beyond reproach as far as hum, etc are concerned---you don't want to be seeing imperfections in your "offset voltage"!

A way to use your "ac coupled" mode without,any threat to your coupling capacitor, is to make up a voltage divider, with a fairly high total resistance, but with the bottom, & smallest, resistance having a voltage drop across it well within the 'scope ratings.
Hang the voltage divider across the HT line, & look at the hum level on that lower voltage to your heart's content.

Obviously, the total resistance of the divider must be high enough not to draw too high a current, otherwise, your results will be in error.

[David Hess]

As I mentioned before, some Oscilloscopes have a DC offset function, which allows you to view small ac signals superimposed upon large DC voltages, whilst in the "DC coupled" mode.

A Tektronix 7A13 differential comparator has an offset range of +/-500 volts (and maximum input of +/-500 volts) at a 0.1V/div sensitivity.  Or a x100 probe could be used with DC coupling and 1mV/div sensitivity to yield 0.1V/div sensitivity with a +/-1000 volt offset range although it would be pretty noisy unless the 5MHz bandwidth limit is used.

The same 7A13 has AC input coupling capacitors good to 500 volts though which is higher than common oscilloscope inputs so it could also be used directly in AC coupled mode for 1mV/div sensitivity (noisy again) with a suitable x1 probe if one could be found.

[Andreax1985]

All in all, I think that the safest solution is to use DC coupling mode with an external AC coupling capacitor (even if it sickens me not to being able to safely use the AC coupling mode in my scope, if they feel that I cannot use full input in AC mode they should have specified it in the datasheet). Now, for computing the minimum value of this capacitor I'm using the formula f = 1/(2*pi*R*C) where f is the desired -3db frequency of the high pass filter (let's say 2Hz), and R is the total resistance (let's say 10Mohm when using 10x probes). This yields C = 10nF. If I wanted to cut frequencies below 15Hz I'd choose C = 1nF. Is this correct?

[trobbins]

To minimise amplitude differences between mains and 2nd harmonic, perhaps aim for one decade below mains frequency.

[rf-loop]

All in all, I think that the safest solution is to use DC coupling mode with an external AC coupling capacitor (even if it sickens me not to being able to safely use the AC coupling mode in my scope, if they feel that I cannot use full input in AC mode they should have specified it in the datasheet). Now, for computing the minimum value of this capacitor I'm using the formula f = 1/(2*pi*R*C) where f is the desired -3db frequency of the high pass filter (let's say 2Hz), and R is the total resistance (let's say 10Mohm when using 10x probes). This yields C = 10nF. If I wish to cut frequencies below 15Hz I'd choose C = 1nF. Is this correct?

Siglent internal AC coupling is designed so that it meets front panel information. 400Vpk. You can connect 400V DC to oscilloscope input using internal coupling DC or AC.

How ever it is, it is still good practice to use external coupling capacitor in Your case what you have told also for reduce scope internal parts stress.

Previously I have also told about external coupling capacitor and freq response.
Yes your calculus is ok. (Roughly because it is complex network and not only ideal R and C. In practice still well enough accurate.)



Also this can use
You can give known/wanted values and it calculate unknown.
http://www.learningaboutelectronics.com/Articles/High-pass-filter-calculator.php

As told previously Siglent this model internal AC coupling have quite low corner frequency. Around 1.2Hz what is low. Perhaps too low for many applications. (if example testing ripple this "dc" under test may have too much drift/fluctuation so with sensitive V/div settings it is difficult to observe due to large vertical movements. So, with external AC coupling it is easy to select suitable corner frequency.

Btw, when use input coupling DC with also external DC coupling,  it is good to note that you can select vertical mode for fixed offset voltage or for fixed vertical position and this last one is handy in many cases when looking some small signal riding over DC. Now when adjust V/div range it keep trace position. This feature can use even for lot of more high resolution DC level (indirect) measurements using offset because offset DAC is lot of more than 8bit. 10x probe, 5mV/div and offset max 20V and offset resolution 100uV.
(and same for horizontal, there can select fixed delay time mode or fixed position mode (example for keep trigger position other than center of screen. Example fixed position in nearly left side of screen and it stay fixed independent of t/div adjusting)

[trobbins]

Also note that valve amp turn on can cause a much higher voltage than measured at idle,  as diodes conduct before typical amp valves.   A scope interface should aim to cope with at least the peak of the power transformer hv secondary ac voltage, plus margin.

[2N3055]

I would not use anything that is not safe up to 1000V on valve amplifiers..
If you include peak voltages, lifted grounds, grid voltage variations, wrongly positioned voltage selectors, and you need some reserve..

[David Hess]

All in all, I think that the safest solution is to use DC coupling mode with an external AC coupling capacitor (even if it sickens me not to being able to safely use the AC coupling mode in my scope, if they feel that I cannot use full input in AC mode they should have specified it in the datasheet).

400 volts is the most common design specification because it allows work on 340 volt DC produced from rectified 240 volt AC power like would be found in an off-line switching power supply.

Now, for computing the minimum value of this capacitor I'm using the formula f = 1/(2*pi*R*C) where f is the desired -3db frequency of the high pass filter (let's say 2Hz), and R is the total resistance (let's say 10Mohm when using 10x probes). This yields C = 10nF. If I wanted to cut frequencies below 15Hz I'd choose C = 1nF. Is this correct?

10nF for 1.6Hz and 1nF for 16Hz for a x10 probe, yes, I get the same result.  Using a x10 probe with its 10 megohm input resistance makes the external AC coupling capacitor much more manageable.

[vk6zgo]

Also note that valve amp turn on can cause a much higher voltage than measured at idle,  as diodes conduct before typical amp valves.   A scope interface should aim to cope with at least the peak of the power transformer hv secondary ac voltage, plus margin.

I think a better word would be a "somewhat" higher voltage.

Traditionally, valve amplifiers used thermionic rectifiers, which heated up at a similar rate to the other valves.
They also used LC filtering which tended to limit any excessive voltage at the output of the supply.
Silicon diodes & RC filtering may not be as good at this.

The easy way to avoid such higher voltages is to turn the amp "on" first, but that means you have to connect the probes whilst the HT line is powered up, which would not be advisable for the beginner.

It is still an undeniable fact that people used Oscilloscopes to probe valve device HT lines for decades with no adverse effect upon either the instrument, or the operator.

[vk6zgo]

 author=2N3055 link=topic=166216.msg2162602#msg2162602 date=1548849382]
I would not use anything that is not safe up to 1000V on valve amplifiers..
If you include peak voltages, lifted grounds, grid voltage variations, wrongly positioned voltage selectors, and you need some reserve..
[/quote]

I have never seen a "1000v rated" Oscilloscope probe used over the many years in which I, & many others,  worked on valve equipment of all kinds.
The old Tektronix probes were 500v rated, which was quite adequate.

Any tech worth their salt would check for "wrongly positioned voltage selectors", which would be quite likely to have caused damage to the amplifier anyway.

"Peak voltages".
Do you mean resonant peaks?
Or just the normal peak voltage of 1.414 x RMS?

"Grid voltage variations?" They may cause changes in the current draw of tubes, but I can't see a situation where that would increase the voltage above that of the HT supply.

"Lifted grounds? "In what context?

[2N3055]

I have never seen a "1000v rated" Oscilloscope probe used over the many years in which I, & many others,  worked on valve equipment of all kinds.
The old Tektronix probes were 500v rated, which was quite adequate.

Any tech worth their salt would check for "wrongly positioned voltage selectors", which would be quite likely to have caused damage to the amplifier anyway.

"Peak voltages".
Do you mean resonant peaks?
Or just the normal peak voltage of 1.414 x RMS?

"Grid voltage variations?" They may cause changes in the current draw of tubes, but I can't see a situation where that would increase the voltage above that of the HT supply.

"Lifted grounds? "In what context?

There are many 1000V +  rated probes, passive and active. Passive ones will be 100:1.
Any tech worth of salt also sometimes makes mistakes. Old amps used to have several voltages 110/120/130--210/220/230/240V. Selector on 210V plugged into today's 230V distribution in Croatia  (Un +10 % / -15% = max.: 253 V, min: 199,5 V) would not mean instant explosion usually, but would mean that voltage on the secondary would rise 20%, so 400V would become 480V.
I'w seen valve power amplifiers with 500V+ inside... 20% on top of that is a lot.
Both resonances, fast unloading of large inductors in power filters, and yes P-P as opposed to RMS voltages.
Lifted grounds as in parts of circuits left floating, and having weird voltages depending where you measure from...

Thank you for sharing your experience.
You obviously know much more about topic than me so you can avoid all pitfalls. I like to be sure that my lack of such wholesome knowledge doesn't end up killing expensive piece of equipment.
That's all. And also, "old school" CRT scopes were much more robust than those made in last, let's say, 20 years. Partially because they were made to service such equipment, and partially because of high quality and over-engineering.  You could use it without worry.

My (very expensive) MSO3000T says 135V RMS on front panel, next to input BNC.  In manual it says: "CAUTION When measuring voltages over 30 V, use a 10:1 probe."


Not meant to be used on tube amps, at least not without high voltage active probe...

Regards,

[tggzzz]

I have never seen a "1000v rated" Oscilloscope probe used over the many years in which I, & many others,  worked on valve equipment of all kinds.
The old Tektronix probes were 500v rated, which was quite adequate.

Any tech worth their salt would check for "wrongly positioned voltage selectors", which would be quite likely to have caused damage to the amplifier anyway.

"Peak voltages".
Do you mean resonant peaks?
Or just the normal peak voltage of 1.414 x RMS?

"Grid voltage variations?" They may cause changes in the current draw of tubes, but I can't see a situation where that would increase the voltage above that of the HT supply.

"Lifted grounds? "In what context?

There are many 1000V +  rated probes, passive and active. Passive ones will be 100:1.

... except those that are 1000:1 passive probes, e.g.
http://w140.com/tekwiki/wiki/P6013
http://w140.com/tekwiki/wiki/P6015

Note that those do have the full potential divider inside the probe (i.e. don't rely on the scope's 1Mohm input resistance), so they can safely be used with the scope internally AC coupled.

[2N3055]

... except those that are 1000:1 passive probes, e.g.
http://w140.com/tekwiki/wiki/P6013
http://w140.com/tekwiki/wiki/P6015

Note that those do have the full potential divider inside the probe (i.e. don't rely on the scope's 1Mohm input resistance), so they can safely be used with the scope internally AC coupled.

Sorry, I meant to relay that they won't be 10:1 or 1:1 probes..

But we are digressing.

OP asked how to measure ripple on top for 300-500V power supply. Way to do it (with common scopes that are widespread today) is to have a probing solution that will decouple DC in a safe manner and won't attenuate AC. Also you want to highpass filter it so anything less than 10-20 Hz doesn't mess up measurements.

In a book I quoted there is a good chapter on topic.

[David Hess]

And except those old proper Tektronix x100 passive probes:

http://w140.com/tekwiki/wiki/P6007
http://w140.com/tekwiki/wiki/P6009

Which are safe to use with any coupling up to 1.5 kilovolts because they also include an internal divider.

[cvanc]

What am I missing here?  Wouldn't measuring it with a meter be both easier and more repeatable?

[rf-loop]

What am I missing here?  Wouldn't measuring it with a meter be both easier and more repeatable?

You have missed OP's first message in this thread.

[eliocor]

Take a look at the whole application note AN118:
https://www.analog.com/media/en/technical-documentation/application-notes/AN118fb.pdf
lots of suggestions on how to measure ripple voltages and noise on HV power supplies.

[Andreax1985]

I have successfully measured ripple using an external capacitor and DC coupling. If you are interested in seeing the results and following my "investigation", I started a new thread in the Beginners section:

https://www.eevblog.com/forum/beginners/tracking-back-100hz-hum-in-a-tube-guitar-amp/

[Peter S]

Hi Folks,  Just joined this forum specifically for this thread.   That is exactly my dillema;  working on tube amps with 460 V (and  higher)  B+,  looking for ripple.   My scope is also 400V max DC+ AC.   Some very interesting points learned here.  If I place a probe on a 460V  B+,  there will be a momentary 460V spike that would be passed by a DC blocking capacitor.
   At the risk of beating a dead horse,  why not just build a simple resistor voltage divider,  two  1 Meg resistors with the scope at the centre tap.   Would this not reduce everything to safe limits,  AC and DC?
   Thanks,  Peter in Canada.  Sorry if this is redundant

[bdunham7]

Hi Folks,  Just joined this forum specifically for this thread.   That is exactly my dillema;  working on tube amps with 460 V (and  higher)  B+,  looking for ripple.   My scope is also 400V max DC+ AC.   Some very interesting points learned here.  If I place a probe on a 460V  B+,  there will be a momentary 460V spike that would be passed by a DC blocking capacitor.
   At the risk of beating a dead horse,  why not just build a simple resistor voltage divider,  two  1 Meg resistors with the scope at the centre tap.   Would this not reduce everything to safe limits,  AC and DC?

I've never seen this thread before, but it is worth commenting on.

First, the original OP had a Siglent SDS1104X-E and thus the whole worry about the DC blocking capacitor was irrelevant--that scope, along with many modern DSOs, has a 1M DC-coupled input impedance regardless of whether DC or AC coupling is selected.  So he could have simply used the standard 10X probes and AC coupling and taken his reading.

Second, if you do have a scope with true DC blocking capacitor that gets charged up to the DC bias present on the input, the safest solution is instead of a 100X/100M probe that is basically a 99M resistor, use a 100X/10M probe that has a built-in voltage divider that will result in the DC bias at the scope of ~0.11X whatever is at the probe tipe.  The Probemaster 4910-2 works this way, as do a number of older Tek probes.

Of course this still leaves you with 100X attenuation, which may be too much.  In that case, if you can live with a very limited BW, you can make your own divider probe.  I'd suggest something like a 4.5M and 1M resistor in series with the scope across the 1M, that gives you 5M total input resistance and 10X voltage division with a 5.5X reduction of the DC component across the scope's capacitor.

[magic]

   At the risk of beating a dead horse,  why not just build a simple resistor voltage divider,  two  1 Meg resistors with the scope at the centre tap.   Would this not reduce everything to safe limits,  AC and DC?

You could, but the scope likely contains another 1MΩ to ground internally so division ratio would be 3:1 rather than 2:1.
Frequency response would be limited due to input capacitance of the scope, probably good enough for 100Hz or similar.
Real passive probes contain additional (partly adjustable) capacitors for tuning AC response to match DC response, that's the whole "probe compensation" thing.

[Kleinstein]

The Scope would only have 1 M to ground. So a 1 M resistor would give a 1:2 divider, not a 1:3 divider.
The Question is a little if the AC coupling before / after the resistors. Old style there was just a capacitors beform. Newer could split the 1 M and have something like 10 M to ground before the capacitor and 1.1 M after the capacitor. So one would still discharge the capacitor and still have not less than 1 M impedance to ground.

Jumping between 0 V and 460 V with AC coupling could still cause significant transient spikes. Many scopes will survive, but I have damages a scope input this way (go from some +150 to -150 V on a scope with specs for 200 V max.)
If possible use a 10:1 probe to be on the safe side, though the AC coupling capacitor could still charge up to the full DC votlage !

For ripple voltage measurements one sometimes looks for pretty low AC voltage on top of high DC. So a 1:100 divider is not ideal.

[Fungus]

At the risk of beating a dead horse,  why not just build a simple resistor voltage divider,  two  1 Meg resistors with the scope at the centre tap.   Would this not reduce everything to safe limits,  AC and DC?

The impedance of the average oscilloscope input is also about 1Meg so when you connect up your probe you'll be loading the circuit down.

A 100:1 probe will have a very high impedance (99MOhms) so that won't happen.

You can do it with resistors but you have to take all the impedances into account.

Make sure your probe is rated for high voltage. There are 100:1 probes out there which are sold for their impedance properties, not their high voltage abilities.

[TimFox]

Again, I hate to re-animate a dead horse, but I have successfully used a good polypropylene capacitor and normal metal-film resistor to form a high-pass network to pass frequencies above, say, 10 Hz to measure ripple on a Vbb supply at a few hundred volts.

For AC at 50 to 120 Hz, this is essentially 1:1, and the resistor can be, say 100 kohm to get only a 10% fudge factor (frequency response only) with a scope impedance of 1 megohm.
For the high-pass capacitor, 220 nF or so will suffice.
The AC loading of 100 kohms should be negligible for a Vbb rectifier circuit.

As always, proper high-voltage safety precautions must be taken.

[tautech]

Again, I hate to re-animate a dead horse, but I have successfully used a good polypropylene capacitor and normal metal-film resistor to form a high-pass network to pass frequencies above, say, 10 Hz to measure ripple on a Vbb supply at a few hundred volts.

For AC at 50 to 120 Hz, this is essentially 1:1, and the resistor can be, say 100 kohm to get only a 10% fudge factor (frequency response only) with a scope impedance of 1 megohm.
For the high-pass capacitor, 220 nF or so will suffice.
The AC loading of 100 kohms should be negligible for a Vbb rectifier circuit.

As always, proper high-voltage safety precautions must be taken.

100% however for the novice I consider this a very risky practice as there is zero indication on the scope display or within measurements to remind the user they are working with elevated voltages.
Therefore best advice is to invest in 100x probes that will pass DC for the scope to clearly display the elevated voltages worked with.
As a secondary reminder but primary to the user is selection of a brightly colored probe as it's not just about getting the job done but protecting yourself and the instrument.
Recommend these if you can find them locally:
http://www.pintek.com.tw/productDetail/land-ctop-2/index/pscsn/17073/psn/19272

[TimFox]

Again, I hate to re-animate a dead horse, but I have successfully used a good polypropylene capacitor and normal metal-film resistor to form a high-pass network to pass frequencies above, say, 10 Hz to measure ripple on a Vbb supply at a few hundred volts.

For AC at 50 to 120 Hz, this is essentially 1:1, and the resistor can be, say 100 kohm to get only a 10% fudge factor (frequency response only) with a scope impedance of 1 megohm.
For the high-pass capacitor, 220 nF or so will suffice.
The AC loading of 100 kohms should be negligible for a Vbb rectifier circuit.

As always, proper high-voltage safety precautions must be taken.

100% however for the novice I consider this a very risky practice as there is zero indication on the scope display or within measurements to remind the user they are working with elevated voltages.
Therefore best advice is to invest in 100x probes that will pass DC for the scope to clearly display the elevated voltages worked with.
As a secondary reminder but primary to the user is selection of a brightly colored probe as it's not just about getting the job done but protecting yourself and the instrument.
Recommend these if you can find them locally:
http://www.pintek.com.tw/productDetail/land-ctop-2/index/pscsn/17073/psn/19272

In my projects building vacuum-tube preamps, I needed to check for ripple on the Vbb supply, but a 100:1 probe into a 5 mV/div scope input would not be sufficiently sensitive.
I did not build these hp filters as "probes", but as a circuit that could be connected to the power supply and CRO before turning on the supply.
Safety first!

[trobbins]

A well made or restored valve amp should have a bleed resistor across the B+ supply. Add a fixed divider resistor to that bleed, and confirm it provides a 10:1 or 100:1 ratio, and then just probe the safe end of the bleed. Make the probing process as safe as you can. I use that form of divider to extend to a maintence socket that can easily connect to an external meter for checking rails and bias levels.

[Peter S]

 Being close to Christmas,  I thought I deserved a new toy.     Personally,  I am leaning towards a simple voltage divider (maybe 2:1)  taking into consideration the internal impedance of the scope,  hopefully bringing things into safe limits.  To be even safer,  I thought I would buy myself a second scope to be my 'daily driver'  and save my Leader LBO-516 for 'special occasions'.  It is a 100 MHz scope with 8 trace capability and features mostly beyond me.  It has been working great for me for over 25 years.
   I found its poor cousin,  an LBO-522 for $160 CDN and snapped it up.  It has a max input voltage of 600V.   Most scopes I've seen  (like my 516,  are only 400V)    The only issue is the trace intensity has to be at or close to max for a good visible trace.   The manual shows internal adjustments for  intensity and HV.    Somehow I thought they would be closely related and the two pots  VR1  and VR2  are not far apart on the schematic.   The scope  otherwise seems to be perfectly calibrated.   I was hoping that a slight tweak of one of these controls would brighten up the trace....but probably more to it?   Just wondering if anyone had experience with this?   The CRT does not seem to have any dim areas that I would imagine would develop if it had been left on for years with a trace at the zero line.
     Thanks again everyone for the great information

[wasedadoc]

I would tack a 100nF cap (of adequate voltage rating) in series with a 4.7M resistor between the amp supply and zero Volts.  (Capacitor to supply and resistor to zero Volts.) After powering up amp wait 60 seconds before clipping an ordinary x1 scope probe across the resistor. Measure and note amplitude of ripple.  Disconnect probe before powering off amp.  Correct measured value to compensate slight reduction in measured ripple due to capacitor reactance at whichever of 50, 60, 100 or 120 Hz is appropriate.

[floobydust]

OP, I've measured ripple on tube amps with Leader scopes and I don't recommend it without a few precautions.
Once you select AC-coupling, the scope's input cap charges up to the DC potential and is a problem because there is no discharge path. So the either the cap holds a stray charge and kills the next thing probed (circuit under test or the scope's front-end if X1) or discharges (ka-bang!) into the AC/GND/DC switch, I think it was a LBO-510 (0.1uF) that does that.

What I've done is after the measurement, touch the scope probe to my hand, =a lazy man's 10MEG resistor to GND and the scope trace will go off screen for a few seconds and then come back to center. The scope's front-end is overloaded but at low current, so the input clamp is not overloaded. This is the cowboy way to discharge that cap and not recommended at all.
But it shows the problem - either that cap is dumped into the scope's front end or the next circuit element probed.

It's too bad this thread did not come up with a schematic for a simple add-on capacitor jig for such measurements.
I would not go 100:1 because you lose the ability to measure low level AC just because of the very large DC component. Even worse is 100:1 scope probes are cheap like borsch on Ali.

Does your Leader scope have the old Marcon oil-filled electrolytic caps in the HV section? These are known to croak with age and cause dim displays.
https://www.opweb.de/english/company/Leader/LBO-522

[coromonadalix]

Bk precision PR100A

Input resistance of 100Mohm (probe resistance 99 Mohm ±1%)
Input capacitance of 6.5pF
Maximum input voltage of 1200VDC including peak AC derating with frequency IEC cat I
Cable length of 1.2m


Bk precision  PR2000B

The PR2000B is a high voltage Oscilloscope Probe with 200MHz bandwidth.
2KV
100MΩ/5pF Input impedance

[jonpaul]

use AC..DC..GRD switch in GRD
Pos
attach 10x probe to tube
Turn on amp, allow B+ to precharge the scopes AC coupling cap
move switch to AC
enjoy

[wasedadoc]

It's too bad this thread did not come up with a schematic for a simple add-on capacitor jig for such measurements.
I would not go 100:1 because you lose the ability to measure low level AC just because of the very large DC component.

Read the post immediately above the one where you wrote that.  Did that really need a diagram in addition to the words?

[floobydust]

It's too bad this thread did not come up with a schematic for a simple add-on capacitor jig for such measurements.
I would not go 100:1 because you lose the ability to measure low level AC just because of the very large DC component.

Read the post immediately above the one where you wrote that.  Did that really need a diagram in addition to the words?

That works OK but remembering a four or five step procedure, with great calamity if not followed, may be too much.
It's common to measure ripple in many locations, or look at audio on the plate, in one session. I'm not turning power on/off every time.

use AC..DC..GRD switch in GRD
Pos
attach 10x probe to tube
Turn on amp, allow B+ to precharge the scopes AC coupling cap
move switch to AC
enjoy

You need the part where you discharge that AC coupling cap after disconnecting the probe.

[jonpaul]

We measure  PARD on HV DC or_B+ bus, with external cap,10x probe, cap discharge add 1m bleeder with a switch.   For RMS, use AC meter like Fluke, or P.

best référence on PS measurement is the 1980s Tektronix and  HP application note like AN90, power supply measurements.

Jon

[David Hess]

Not so old Tektronix oscilloscopes include a 1 megohm resistor in series with the coupling capacitor when the coupling switch is set to GND, which serves to precharge the coupling capacitor when switching between AC and DC coupling.

[Peter S]

Thanks again for all the info.  The online manual for the LBO-522 seems to be incomplete but the LBO-516 service manual  (hopefully related) had an adjustment procedure for the internal intensity control.  I followed this and all is great!   The pot required only 10 or 15 degrees of rotation to get brightness back to perfect.  There still seemed to be lots of range left on either side of the pot's current position so I will assume all is good.    Further;  if this scope has sat for years with the trace brightness limited by this control,  may it have extended the life of the CRT?  (just being optimistic here!).

   At the risk of wearing everyone's patience;   if I simply had voltage divider using two  1 Meg resistors with the scope attached to the centre-tap,  how is it possible for the DC potential at centre-tap (scope connection) to be anything more than half of the potential at the top (input) of the series string?
  I feel I may be 'booted' off this thread,  as  you have all tried so hard to explain this!    Merry Christmas and thanks again!
Peter

[floobydust]

Don't worry about asking questions here, we share what we know and can always learn more about the art 
My main concern is damaging vintage test equipment doing these measurements because there can be several traps which we are discussing.

You can add the 2:1 divider ahead of things, but it will actually read 1/3 because the scope ends up a 1MEG load in parallel with the 1MEG lower divider resistor giving 500k.
If you try measure signal on a gain stage's plate, it will get loaded down somewhat.

I realized the 100:1 probe is actually a bad idea. AC-coupled, a scope's input cap will charge to the full DC potential regardless of the probe attenuation, if it's 1x, 10x, 100x because the cap is ahead of the scope's 1MEG front-end resistor. This is a trap.


If you have any drama with the Leader, check it's PSU rail voltages are OK and then I go after the old Marcon caps.

[vk6zgo]

Thanks again for all the info.  The online manual for the LBO-522 seems to be incomplete but the LBO-516 service manual  (hopefully related) had an adjustment procedure for the internal intensity control.  I followed this and all is great!   The pot required only 10 or 15 degrees of rotation to get brightness back to perfect.  There still seemed to be lots of range left on either side of the pot's current position so I will assume all is good.    Further;  if this scope has sat for years with the trace brightness limited by this control,  may it have extended the life of the CRT?  (just being optimistic here!).

   At the risk of wearing everyone's patience;   if I simply had voltage divider using two  1 Meg resistors with the scope attached to the centre-tap,  how is it possible for the DC potential at centre-tap (scope connection) to be anything more than half of the potential at the top (input) of the series string?
  I feel I may be 'booted' off this thread,  as  you have all tried so hard to explain this!    Merry Christmas and thanks again!
Peter

Of course, it can't be ---it will always be less than half, as the 'scope input Z will be in parallel with the bottom resistor.

[vk6zgo]

Don't worry about asking questions here, we share what we know and can always learn more about the art 
My main concern is damaging vintage test equipment doing these measurements because there can be several traps which we are discussing.

You can add the 2:1 divider ahead of things, but it will actually read 1/3 because the scope ends up a 1MEG load in parallel with the 1MEG lower divider resistor giving 500k.
If you try measure signal on a gain stage's plate, it will get loaded down somewhat.

I realized the 100:1 probe is actually a bad idea. AC-coupled, a scope's input cap will charge to the full DC potential regardless of the probe attenuation, if it's 1x, 10x, 100x because the cap is ahead of the scope's 1MEG front-end resistor. This is a trap.

If you have any drama with the Leader, check it's PSU rail voltages are OK and then I go after the old Marcon caps.

If the cap can charge via the probe attenuation, it can equally discharge via that path.
As soon as you have done your test, clip the probe tip to the 'scope ground, & it will discharge harmlessly.

[jonpaul]

look for the P6006..6009 1960s X10 designed in vacuum tube era. Rated at 600V.

https://w140.com/tekwiki/wiki/P6006

as in film "2001: A Space Oddssey"
Can easily probe DC or AC at the plates of amplifiers.

Low cost as so big and old!


Jon

[vk6zgo]

look for the P6006..6009 1960s X10 designed in vacuum tube era. Rated at 600V.

https://w140.com/tekwiki/wiki/P6006

as in film "2001: A Space Oddssey"
Can easily probe DC or AC at the plates of amplifiers.

Low cost as so big and old!


Jon

I used them a lot in the old days, teamed with a 545B & 1A5 plugin.
We didn't "need no steenkin' AC coupling"----the 1A5 "offset" could handle anything we threw at it!

[David Hess]

High voltage x100 probes with 10 to 50 megohm input resistance indicating a shunt resistabance to attenuate the DC voltage still exist, at least in theory, but they cost almost $1000 now.  I think Testec used to have a lower cost one but apparently no more.

[vk6zgo]

The only time I've needed a x100 probe was as a "cheap & nasty" substitute for an active probe, back when such devices were prohibitively costly.
They were, of course, conventional ones, but the signal I was looking at required a device with around 100 M input impedance, & was of fairly high level, so it was (just) adequate.

[tautech]

The only time I've needed a x100 probe was as a "cheap & nasty" substitute for an active probe, back when such devices were prohibitively costly.
They were, of course, conventional ones, but the signal I was looking at required a device with around 100 M input impedance, & was of fairly high level, so it was (just) adequate.

Yes, an often overlooked advantage of the 100x probe is the low input capacitance, often well under 5pf.

[floobydust]

This looked reasonable, without the danger of AC-coupling being a problem.

[David Hess]

That is basically the P6009 schematic, shown below.  It takes a little more than the bare essentially to get 100+ MHz performance.