Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Tapsnap on October 20, 2021, 09:38:46 pm

Title: Single ended amp microphonic problem.
Post by: Tapsnap on October 20, 2021, 09:38:46 pm

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?

Title: Re: Single ended amp microphonic problem.
Post by: labb on October 20, 2021, 10:17:15 pm

I would look for a bad solder joint

Title: Re: Single ended amp microphonic problem.
Post by: Bieworm on October 21, 2021, 01:21:05 am

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

Title: Re: Single ended amp microphonic problem.
Post by: glass54 on October 21, 2021, 01:36:52 am

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  :icon_biggrin:
Again, I am not saying this is your problem BUT be suspicious  :laugh:
Kind regards
Mirek

Title: Re: Single ended amp microphonic problem.
Post by: j_bruce on October 21, 2021, 03:34:35 pm

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

Title: Re: Single ended amp microphonic problem.
Post by: Colas LeGrippa on October 22, 2021, 12:12:25 am

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.

Title: Re: Single ended amp microphonic problem.
Post by: shooter on October 22, 2021, 03:35:15 am

Quote

Thanx to my neighbour Ho Ming for the chopsticks.

 
 :l2:
I get mine from my neighbor, he does a lot of whittling n banjo playing