SF Twin Reverb with VERY microphonic V2 stage w/Norm & Vib Vol's CCW

[nevetslab]

During my preventative maintenance work going thru the rental inventory of Fender Twin Reverb amps, I came upon one Silver Face Twin that was plagued with a lot of microphonic tubes. After first replacing the power tube set with another batch that both matched AND was absent of any microphonics, I went thru the rest of the tubes....none of them had tube shields in place....V2 was the biggest offender, and had to replace the tube socket. I re-tensioned the remainder of the 9-pin tube sockets (closing down the gap using jewler's bladed screwdriver) so there was nice, tight fit with the tubes installed. And restored tube shields to keep them tight).

What has effectively stymied me is being unable to get V2 to NOT be microphonic. Tried many tubes there (after replacing the socket with a new tight-fitting one), some new tubes. Both Normal AND Vibrato Ch Volume controls are turned off, and only the Master Volume is turned up.

Apart from the Master Volume circuit, I don't see any difference in the preamp circuit in the circuit leading up to the input to the LTPI stage.

twin_reverb_sf_100_schem.pdf 65_Twin_Reverb Sch.pdf

While both V1B and V2B each follow their respective Volume controls, they are active, sharing common cathode circuit, and V2B is followed with additional gain from V4B, where it mixes with the circuit from V1B, only this one SF Twin is flat out obnoxious with V2 tube being VERY microphonic. I'm not hearing or seeing that with any of the other 65 Re-issue Twins I'm going thru. When I moved the chassis back into the cabinet, to see just how bad it is (will it sing from vibration or not), it didn't seem to be objectionable....I couldn't excite it banging on the top of the cabinet any more. So I did at least make improvements....but rubbing your fingers over the tube shield tells you immediately it ain't solved yet.

Any suggestions?

[Mick Bailey]

This reminds me of a similar situation where it turned out that an associated coupling cap had gone extremely microphonic. Even the gentlest tap with a fingertip sounded like it was the tube, though I'd tried a few known-good examples. It was so bad I could speak into the amp chassis and it would come through the speaker.

[Chuck H]

This reminds me of a similar situation where it turned out that an associated coupling cap had gone extremely microphonic. Even the gentlest tap with a fingertip sounded like it was the tube, though I'd tried a few known-good examples. It was so bad I could speak into the amp chassis and it would come through the speaker.

That was my thought too. Possible microphonic component. The trouble is locating it!. IME probing the components often produces an equal "thump" no matter what you tap. I isolated a microphonic cap in a Marshall once by just sort of rubbing and shoving each component with a chopstick rather than tapping on them.

[Leo_Gnardo]

That was my thought too. Possible microphonic component. The trouble is locating it!. IME probing the components often produces an equal "thump" no matter what you tap. I isolated a microphonic cap in a Marshall once by just sort of rubbing and shoving each component with a chopstick rather than tapping on them.

Tap scrape & prod with the mighty chopstick has helped me a lot, and much thanks to MEF also for introducing me to the technique.

One of the unexpected microphonic offenders can be a piece of wire - specifically the one connecting the dry/reverb mix point to grid of V4. The solution that works for me is just a bit radical. I got to thinking "if there's no wire, then it can't be microphonic can it?" I mount 220K, 470K, 3M3 + 10pF right on the tube socket connection & run leads from the other ends of those resistors to their sources.

Granted this probably isn't the source of nevets' problem but, you never know.

Disc caps, especially the big ones, often act as (awful) condenser mics, just active enough to play hob with noise/microphonics. Swap out for a good film cap & you'll often find the amp sounds more like you expect it should.

[Jazz P Bass]

I had a microphonic amp that had a capacitor lead 'soldered' to the front panel that 'wasn't'.

[g1]

Sometimes the microphonics are so severe that even the lightest touch of a chopstick is too much. I save my old soft-bristled toothbrushes for such situations, among other uses.
Also, cracked/broken leads being held in place by goop can be an absolute nightmare to find. Probably more so for crackling rather than microphonics though.

[nevetslab]

After putting another Fender Twin Reverb to bed, I found it interesting that V2 on this amp had absolutely NO microphonic tendencies, while some others since I began this thread had some...not as radical as that SF Twin, but still very touch sensitive...rub your finger over the tube shield with the vol pots down, and it would sing. Not so on this last one. So, as speculation goes....tube socket wires, routed to microphonic components on the PCB? wires themselves? The mystery goes on, but may yet find this one out.

[Tone Meister]

... So, as speculation goes....tube socket wires, routed to microphonic components on the PCB? wires themselves? The mystery goes on, but may yet find this one out.

I've have capacitors in a Fender tone stack to become excessively microphonic and appear to be the tube or socket itself. In posts #2 and #3, respectively, Pete and Chuck mentioned coupling caps as well. Of course, all caps will be microphonic to some degree.