dealing with problematic gain stages that are susceptible to noise and microphonics

[SoulFetish]

Certain amps I've worked on seem to have at least one stage which is really sensitive to noise issues and microphonics. On the Silvertone 1474 I was recently working on, the summing/recovery gain stage directly before the cathodyne phase inverter was really touchy and prone to noise problems. If I shined my headlamp on the plates from a couple of feet away, I could hear it radiate audibly through the amp, which was weird. And on my own amp, my input stage is dead quiet. But that drives a FMV tone stack, and the second stage is quiet "hissy" and really sensitive to microphonics. I've ruled out it being a tube, plate resistor, and coupling capacitors.
Actually, I just realized that in both cases there is a tone stack directly feeding the problematic stage.
You all run into this at all and can shed some more light on different causes and solutions?

[nevetslab]

Certain amps I've worked on seem to have at least one stage which is really sensitive to noise issues and microphonics. On the Silvertone 1474 I was recently working on, the summing/recovery gain stage directly before the cathodyne phase inverter was really touchy and prone to noise problems. If I shined my headlamp on the plates from a couple of feet away, I could hear it radiate audibly through the amp, which was weird. And on my own amp, my input stage is dead quiet. But that drives a FMV tone stack, and the second stage is quiet "hissy" and really sensitive to microphonics. I've ruled out it being a tube, plate resistor, and coupling capacitors.
Actually, I just realized that in both cases there is a tone stack directly feeding the problematic stage.
You all run into this at all and can shed some more light on different causes and solutions?

I had to read thru my notes on the restoration of a friend's Silvertone 1474 on this (back in Oct 2012). I had found their 'stock' reverb circuit with that 'back door spring' reverb tank just had to go. I revised the circuit to add a Fender Reverb Tank drive xfmr, and added an Accutronics 82B3C1B short 3-spring tank, which getting it to NOT behave like a microphone down in the speaker compartment was a bit tedious, but succeeded. I've attached the revised schematic of the end result.

1474 AMPLIFIER-REVISED REVERB SCH-1.pdf



I had forgotten about the block of foam I placed over the 6CG7 reverb tube. I never did cobble together an isolation pouch for the tank but did have to mechanically 'tune it' with regards to position and height for minimum pickup from the speakers.

One thing I forgot to do was to scrap the open ckt input phone jacks for the switching type, allowing the muting of the input stage and the 6dB pad on one of the inputs (per ch).