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