Products > Test Equipment
100:1 probe for measuring ripple in a tube amp power supply
wasedadoc:
--- Quote from: floobydust on December 19, 2022, 08:24:11 pm ---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.
--- End quote ---
Read the post immediately above the one where you wrote that. Did that really need a diagram in addition to the words?
floobydust:
--- Quote from: wasedadoc on December 19, 2022, 10:19:14 pm ---
--- Quote from: floobydust on December 19, 2022, 08:24:11 pm ---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.
--- End quote ---
Read the post immediately above the one where you wrote that. Did that really need a diagram in addition to the words?
--- End quote ---
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.
--- Quote from: jonpaul on December 19, 2022, 09:56:46 pm ---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
--- End quote ---
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
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