When audio probing a screen resistor fixes the amp....

[Mr_bibbles]

Hey, this one goes out to the real experts out there....

What does it mean when you can return a very low output amplifier to full volume just by probing the screen resistor? Like, as soon as I touch the lead with the probe BANG full volume.

To clarify, I've been scratching my head trying to return a low-output AC30 to full volume ( customer described it as intermittently cutting out before staying dead). All preamp tubes are good, new power tubes are fine, screen resistors all measure well, bias voltage is present, bias resistor measures fine, fx patched no change, OT measures fine with basic tests, filter caps ok, dropping resitors ok, able to probe signal all the way up to the screen grids before it ZAPPED back on.

This has happened a couple times to me before but I've never been able to identify why!

[Helmholtz]

Does this happen with all screen resistors?

I understand you're using a scope. Any signs of HF oscillation?

[pdf64]

Had you checked for oscillation beforehand?
I recall some modern AC30 models having their grid stoppers remote from the valve socket.
Which isn’t good practice.

[Mr_bibbles]

Had you checked for oscillation beforehand?
I recall some modern AC30 models having their grid stoppers remote from the valve socket.
Which isn’t good practice.

I am not using an oscilloscope, I am using a probe so it is tough for me to identify oscillation. However the resistors ARE on the socket, so this should enhance their protection of HFO right? Also, would probing a signal that is oscillating above hearing range shunt it off and cause the circuit to work properly?? That would explain this phenomenon.

Does this happen with all screen resistors?

I understand you're using a scope. Any signs of HF oscillation?

I am actually using a cheap audio probe, just a film cap tied to a 1/4 jack fed into another amplifier. I'm not sure if it happens with the other ones since after it came back, it STAYED back....

But you both seem to expect high frequency oscillation, I will do some research into the possibilities!

[Helmholtz]

Of course it could also be a bad contact or connection. Try chopsticking or freeze spray (don't spray hot tubes).

When in failure mode, can you measure screen voltages with your meter (directly at tube sockets)?

[Mr_bibbles]

Of course it could also be a bad contact or connection. Try chopsticking or freeze spray (don't spray hot tubes).

When in failure mode, can you measure screen voltages with your meter (directly at tube sockets)?

Strangely enough I cannot get it to fail again, after having no signal for two days of testing. I had chopsticked it very thoroughly, but I don't have freeze spray, but I believe it did have the correct voltages on the screens while I was testing, (not to mention the plate and bias voltages)

[Enzo]

And you have resoldered the resistor at both ends?

[Jazz P Bass]

And you have resoldered the resistor at both ends?

Ditto!

An open screen resistor circuit will prevent that tube from working.

[drewl]

Check the socket pins to see if they are loose or intermittent.

[Helmholtz]

THe AC30 has 4 output tubes.
So if only one screen resistor or socket contact was loose it should still put out some power.

[Mr_bibbles]

Thank you for all the replies.

I have not re soldered it at both ends and am suspecting a different issue, because I had chopsticked the amp THOROUGHLY before this with no change, not even a pop or crackle. I have also measured the grid resistors individually and they're all on point, that was one of the first things I had checked. It went from maybe 2% output to %100, so like Helmholtz said it probably was not one of the 4 tubes failing to conduct since we would have more output (if it was a single tube on the stage failing.

IS there any chance the film resistor on the end of the audio probe shunted something off, or forced something to conduct?? I know that's a long shot but I chopsticked all the resistors in that section (and the ones not in that section) after it came back to life and now I cannot get it into failure mode again. This amplifier has several IC stages in the effects loop....

I will see if I can get connectivity from the backside of the sockets! seems unlikely since nothing could bring the signal back EXCEPT touching the grids with the audio probe but open to all possibilities.

[Helmholtz]

IS there any chance the film resistor on the end of the audio probe shunted something off, or forced something to conduct??

The added capacitance when connecting a probe to screens or plates may stop (or start) oscillation.