Microphonic wires in SF Twin Reverb

[sixs929]

Well I seem to be going backwards, here. I've been blackfacing an SF Twin reverb for some time now, and all seemed to be going OK as far as quieting the amp down - the biggest gain was discovering the heater circuit hum pot was half open, and changing it over to dual 100 ohm resistors did help a lot as far as noise.
Next I took out the Master Volume pot and associated circuitry - suffice to say it's all going back in. All I did was trade one set of buzzes for another.

BUT - my big question is - since pulling the MV out now the reverb sits there "mumbling". I don't know what else to call it. It carries on quietly all the time like it's sensing my heartbeat or something. And even with the reverb knob on 2, god forbid you tap the chassis (it's on blocks, with the tank nearby) - the reverb reacts like you shook the tank. Nice sustain, though 
And finally - the two wires going from pins 6 and 7 on V4 are highly microphonic. Tapping or rubbing on them sounds like your rubbing the speaker cones, and as my finger gets closer to those wires, the amp's buzz gets progressively louder, and shreiks pretty good if you touch either one.

whatta ya think?

Thanks much
Six

[kriswel]

Don't put your finger in a live amp! Besides high risk of death you might be acting as an antenna. Removing the MV should not give you any problems if done properly. It does have a switch on it that is connected to the reverb circuit for your "boost", look hard at this.  If you didn't remove it properly then you might put it back in wrong as well so I would concentrate on making sure you have made the right connections post removal at this point.

If there is other problems in the amp removing the MV might make them louder but it will not be the cause. Compare the schematics and layouts and make sure the new connections are right.

Have you tackled basic maintenance issues with the amp first? Sounds like you had some noise issues and you think that BF'ing will fix them. A SF amp in good working condition should be as quiet as a BF in good working condition. I would fix the amp first then make changes. caps, tubes, clean pots, tighten tube sockets, correct heater wire polarity...

Do not put your fingers in a live amp. Chop stick and one hand behind back. You might push those wires around a bit that are now going from the 220k R's to the .01 cap in the phase inverter. It might be crossing something it doesn't like (think antenna). You could also try using shielded wire. Only ground one end of the shield. But I would bet there is a wrong connection in the BF'ing attempt. I've been there before.