Microphonics question

[vibrovox7reverb]

I'm having trouble with an amp I'm working on. The problem is that if the amp is tapped anywhere on the cabinet or chassis you can hear it come through the speaker. I've changed out tubes like crazy even to replacing every tube in the amp with known good tubes and the problem is exactly the same. It's so microphonic that as I turn the pots the sound of my fingers touching the pots can be heard through the speaker, not a crackling but like a ringing noise. I'm wondering if the chassis itself can be microphonic, and if that's even possible because this phenomenon happens no matter where the chassis is touched. With how loud the sound is it seems like it would have to be early in the signal chain for it to be a capacitor, but I've changed those out. I've also changed the power supply caps as well (which were already new) thinking that since this was a silverface fender they might be the culprit in that under-chassis cap compartment.

Basically I have no idea why this amp rings like this whenever it's tapped, and I haven't the slightest clue what to do about it.

[trobbins]

If the microphonics is from a bad joint/connection, or cracked resistor, or ceramic capacitor, then 'chopsticking' wires and pins and parts may identify the problem spot. If it's just the input valve then you have a number of control techniques that could be deployed. There are many references on the topic.
eg:
http://www.ozvalveamps.org/microphonics.htm

[soundguruman]

A bad plate resistor in preamp stages will do that, and so will a bad reverb unit.
Unplug V1 preamp tube and see if the ringing stops, unplug the reverb unit...

[vibrovox7reverb]

After pulling tubes I've found that the phase inverter is the culprit. I also wonder, can the socket itself be the cause of the problem? The other tubes when tapped softly with my finger make almost no sound while the phase inverter makes quite a loud noise. I've already done some part replacement and reflowed all joints to no avail, so that has me wondering. Other tubes in this slot all make noise, so I know it's not the tube...I'm using solid core wire and I've heard that if they vibrate they can be slightly microphonic or something.

[trobbins]

It could be bad socket contacts, or a bad component attached to the socket terminals, perhaps?

[vibrovox7reverb]

Sorry, I forgot to reply to this awhile back. The ringing stops whenever the tube shield is on the phase inverter socket. I had left it off previously to keep that tube cooler. I get the feeling that this is not normal, however the problem has subsided for now, thanks for the help you guys.