Single ended amp microphonic problem.

[Tapsnap]

I built this simple single ended practice amp a few years ago. It worked quite well, but it started getting this high pitched feedback a while ago which was present even without a guitar plugged in. I investigated and found that the 12AT7 preamp tube was microphonic when I tapped it. I bought a replacement Mullard 12AT7 but it still has the same problem. In fact the whole chassis is microphonic. I can tap the chassis with a stick and the tapping is amplified.
Any ideas what might be causing this?

[labb]

I would look for a bad solder joint

[Bieworm]

i had the same problem recently. Turned out to be a bad solder between a filter cap and the ground buss. It was near invisible.. but the whole construction was microphonic. I found the problem by yanking the turretboard..

not saying that IS your problem, but well worth checking that anyway

[glass54]

Dear Tapsnap,
When I looked at your turret board in maximum size, I noticed that you're using pure copper wire for inter-turret connections as opposed to tin plated wire. I have done this before and I found the copper wire oxidizing, potentially giving you a poor joint, especially IF your soldered joints are poorly "wetted" Might be worth a further inspection and reflowing any suspicious joints using a good flux and very clean soldering iron tip 
Again, I am not saying this is your problem BUT be suspicious 
Kind regards
Mirek

[j_bruce]

I think Mirek is onto it.

I notice that the single cap near the bottom has a very dicey looking solder joint on its left end (left as shown in the pic).

I would firm that up and then I would do just a Mirek suggests on the bus wire: flux and resolder every turret contact.

Good luck,
Jerry

[Colas LeGrippa]

I had the same problem this week . I replaced the 3 12ax7 with new ones but with same issue.
My problem was at V1 where the connection at the B+ capacitor was really close to the chassis,
to the point where poking gently with a chopstick was enough to make sparks. The tone was tiny with a squeal, just like a microphonic tube would have done.
Thanx to my neighbour Ho Ming for the chopsticks.

[shooter]

Thanx to my neighbour Ho Ming for the chopsticks.

 
 
I get mine from my neighbor, he does a lot of whittling n banjo playing