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In Reply to: RE: Sounds like an oscillation. posted by Ralph on June 04, 2020 at 10:27:49
As you know, there are 3200V lurking at the outputs of this amplifier. Consequently, when I have it on my benchtop, I am running it unloaded. It exhibits no abnormalities in that mode. DC voltages are all stable and as expected. To go anywhere near the outputs with any of my oscilloscope probes would be not wise, as I am sure you know. Do you know any tricks whereby I could detect oscillations without a 'scope? I do use a 6000V probe when I measure DCV with my Fluke. I was thinking I might use that probe and look at ACV magnitude only, on the benchtop. Was thinking of making a dummy speaker load using 3000pF HV capacitors; the speaker measures as two 3000pF capacitors in series, with the screen as center tap.
Follow Ups:
Then its a good idea to have one of these.
Thanks, Ralph. Is that probe specifically and only for "Keysight" oscilloscopes, as the data sheet suggests? I currently own a Hitachi, a Sencore, and a Tektronix. I just bought the Tektronix, and I would hate to damage it. And is that thing really insulated well enough for 3200V?Your idea of oscillation sounds better and better to me, the more I think about it, because in the process of eliminating causes of the original mode of malfunction, which was nothing like what I have going on now, I de-soldered several parts in order to test their integrity out of circuit. Then I re-soldered them. Perhaps one of the new joints I made is faulty. That's the first thing I will look for when I get the amp back on my bench. When I had the old problem of oscillation, 3-4 years ago, which was due to a bad solder joint, I could pick up the oscillation on my scope without even touching the probe to the circuit traces. All I had to do was to hold the probe about 6-8 inches away from the source of the problem, and the scope would go crazy with VHF. Maybe the same condition will apply here, if I do have an oscillation. I'll use the Hitachi, since that is the least valuable.
EDIT. I just looked on eBay for "high voltage probe for tektronix", and I found several up to 15kV, for between $200-$300. I should have thought of this. Thanks, again.
Edits: 06/05/20
I admit that I didn't look that closely- I just googled 'high voltage oscilloscope probe' and that was one of the first that showed up.
Another way you could look for this sort of thing would be to wind some coils on a pencil and then remove the pencil. Tie each end to your scope probe and then hold it near the suspected area of oscillation (RF energy).
IMO if the amp is really oscillating then the feedback network needs to be addressed. A Pi filter network could be installed that cuts off feedback above a certain frequency. Might be a bit of a hatchet job but you really don't want an amplifier to oscillate- Bad Things happen.
The amps do not oscillate when in proper operating condition, but they are always a threat to oscillate, because you have a low voltage solid-state front end feeding a very high voltage tube output stage. So, everything has to be copacetic. I am not about to put in any sort of output filter, in any case. That's way above my skill-set. But Harold Beveridge has a low pass filter built in to the input and driver stages. So very hf stuff that might get into the amplifier from the preamp and cause oscillation, does not get there. Like I said, first it remains to be seen that the amp IS oscillating. Then, if it is oscillating, the most likely cause is a bad solder joint in the high voltage section or perhaps a bad NOS power tube or etc. (As you know better than I, finding the cause can be difficult.)
That's what I did last time, made a coil of wire and attached it to my scope probe and just held it near to the HV section of the amplifier without touching it. Oscillation was easy to demonstrate on the scope. I did find some HV probes for Tektronix, at around $1000, for 5kV.
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