[R-390] Microphonic Parts?

[Bernie Doran]

With a 10 Meg resister in the grid, they must be using the electron cloud 
to generate the bias for the stage.  so the flow through the resister is 
well under a micro amp. I can see almost anything causing noise with that 
setup. it would be interesting to check that cap with a Megger. That is what 
I use to check caps, at 500 V it reads to about 2000 Meg Ohms   Bernie W8RPW