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?
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?
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