Koch Twintone2 microphonic?

[guile]

A friend asked me to look at a problem in his Koch Twintone 2 guitar-amplifier.
https://en.audiofanzine.com/tube-combo- ... r_reviews/

It's very sensitive to even the slightest touch, notably when tapping on the powertubes and some of the pre-amp tubes.

I built some amps a long time ago, so I know my way around a little, but I dont know where to start.
Please advise
Thanks

[pompeiisneaks]

First thoughts are that the tubes may have gone microphonic. If there are specific tubes that make the noise the most, they could be the culprit. Tapping anywhere that could eventually hit the microphonic tube would make the more sensitive one ring.

~Phil

[guile]

Thanks.
I forgot to mention that I swapped out all of the tubes.
And the owner says that hitting it hard often cures the problem for a while.
I did not tested that..

[pompeiisneaks]

Hitting it means some connections are 'suspect' maybe clean and/or retension the sockets? Double check that some of the resistors or capacitors may not have dry solder joints or the like. Chopsticking around may prove useful.

~Phil

[xtian]

Yes, poking around with a chopstick is a good idea. Also I have had luck playing a sine wave via my smart phone into the amp, sweeping the frequency while adjusting volume until the amp misbehaves. Then you have two hands free to chopstick and find the issue.

[guile]

I cleaned all the tube sockets with contact fluid. This seemed to work but after a while the problem comes back.

The owner told me that with the mentioned problems, the amp also makes a loud pop when turned off. It didn't do that before.

Poking around did not help either..

[Phil_S]

It's hard to know from just a little information, but I think it may be a bad solder joint. Heat up the soldering iron and retouch anything you might suspect in the signal path.

[guile]

I wil try that. First I will put in a whole new set of tubes.

[guile]

The new tubes came in and it works and sounds fine now!
Thanks all for your help.