Products > Test Equipment
100:1 probe for measuring ripple in a tube amp power supply
David Hess:
--- Quote from: Andreax1985 on January 30, 2019, 09:23:06 am ---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).
--- End quote ---
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.
--- Quote ---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?
--- End quote ---
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:
--- Quote from: trobbins on January 30, 2019, 10:42:58 am ---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.
--- End quote ---
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:
--- Quote from: vk6zgo on January 30, 2019, 12:46:03 pm ---
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?
--- End quote ---
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:
--- Quote from: 2N3055 on January 30, 2019, 03:17:09 pm ---
--- Quote from: vk6zgo on January 30, 2019, 12:46:03 pm ---
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?
--- End quote ---
There are many 1000V + rated probes, passive and active. Passive ones will be 100:1.
--- End quote ---
... 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.
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