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

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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:

--- Quote from: Andreax1985 on January 29, 2019, 07:21:37 am ---
--- Quote from: rf-loop on January 29, 2019, 06:52:40 am ---
--- Quote from: Andreax1985 on January 29, 2019, 04:25:49 am ---
--- Quote from: DaJMasta on January 29, 2019, 04:18:10 am ---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.

--- End quote ---

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?

--- End quote ---

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?

--- End quote ---

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.

--- End quote ---

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:

--- Quote from: vk6zgo on January 30, 2019, 02:39:11 am ---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.
--- End quote ---

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.

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