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

--- Quote from: Peter S on December 04, 2022, 03:14:22 pm ---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?

--- End quote ---

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:

--- Quote from: Peter S on December 04, 2022, 03:14:22 pm ---   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?
--- End quote ---
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.
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