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

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floobydust:
Don't worry about asking questions here, we share what we know and can always learn more about the art  :)
My main concern is damaging vintage test equipment doing these measurements because there can be several traps which we are discussing.

You can add the 2:1 divider ahead of things, but it will actually read 1/3 because the scope ends up a 1MEG load in parallel with the 1MEG lower divider resistor giving 500k.
If you try measure signal on a gain stage's plate, it will get loaded down somewhat.

I realized the 100:1 probe is actually a bad idea. AC-coupled, a scope's input cap will charge to the full DC potential regardless of the probe attenuation, if it's 1x, 10x, 100x because the cap is ahead of the scope's 1MEG front-end resistor. This is a trap.


If you have any drama with the Leader, check it's PSU rail voltages are OK and then I go after the old Marcon caps.

vk6zgo:

--- Quote from: Peter S on December 20, 2022, 02:52:12 pm ---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

--- End quote ---

Of course, it can't be ---it will always be less than half, as the 'scope input Z will be in parallel with the bottom resistor.

vk6zgo:

--- Quote from: floobydust on December 20, 2022, 04:21:30 pm ---Don't worry about asking questions here, we share what we know and can always learn more about the art  :)
My main concern is damaging vintage test equipment doing these measurements because there can be several traps which we are discussing.

You can add the 2:1 divider ahead of things, but it will actually read 1/3 because the scope ends up a 1MEG load in parallel with the 1MEG lower divider resistor giving 500k.
If you try measure signal on a gain stage's plate, it will get loaded down somewhat.

I realized the 100:1 probe is actually a bad idea. AC-coupled, a scope's input cap will charge to the full DC potential regardless of the probe attenuation, if it's 1x, 10x, 100x because the cap is ahead of the scope's 1MEG front-end resistor. This is a trap.

If you have any drama with the Leader, check it's PSU rail voltages are OK and then I go after the old Marcon caps.

--- End quote ---

If the cap can charge via the probe attenuation, it can equally discharge via that path.
As soon as you have done your test, clip the probe tip to the 'scope ground, & it will discharge harmlessly.

jonpaul:
look for the P6006..6009 1960s X10 designed in vacuum tube era. Rated at 600V.

https://w140.com/tekwiki/wiki/P6006

as in film "2001: A Space Oddssey"
Can easily probe DC or AC at the plates of amplifiers.

Low cost as so big and old!


Jon

vk6zgo:

--- Quote from: jonpaul on December 21, 2022, 09:21:04 am ---look for the P6006..6009 1960s X10 designed in vacuum tube era. Rated at 600V.

https://w140.com/tekwiki/wiki/P6006

as in film "2001: A Space Oddssey"
Can easily probe DC or AC at the plates of amplifiers.

Low cost as so big and old!


Jon

--- End quote ---

I used them a lot in the old days, teamed with a 545B & 1A5 plugin.
We didn't "need no steenkin' AC coupling"----the 1A5 "offset" could handle anything we threw at it!

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