Microphonics issues retrofitting fet inputs.

[screamersusa]

 
Greetings all.

I whipped up a fetzer valve and also the mps102 version as well. I also have been toying with a Behringer GMX amp which appears to have a fetzer valve type preamp attached to the input jack.
I'm noticing microphonic issues in the following experiments.

1) The behringer's original circuit is microphonic and the amp feeds back on itself on high gain settings at the preamp level.
    Tapping on anything related to the input jack and board yields consistent microphonics. Since it's smd I assume the caps are ceramic.

2) Retrofitting a DIY fetzer valve has the same effect but different frequency of feedback and less microphponics when tapping.

3) I tried both DIY jfet preamps on the input of a Marshall Valvestate. The clean channel sounded much warmer and ferdery however the od channels seemed to have too much low end breakup. The od channels feed back however at high gain. This time however instead of the fet preamp microphonics, the actual casing neat the od channel showed the highest sensitivity to microphonics. With the fet removed it seems to go away.

Some input on how to interface these things properly would be appreciated.
The GMX actually is a fantastic amo and has saved numerous gigs when peoples expensive tube amps failed. The fet input stage is a problem though and while no one has complained, I know its got to be fixed. The GMX is now the only acceptable solid state  "required" backup amp per many of the shows I do.
Cheers