Nick Gorham wrote:

Assuming that the decoupling is done with zero ESR caps, and there is no
inductance in either the cap or resistor, both of which will be false.

Given the assumed wonderful decoupling, maybe its worth thinking how the
switching spikes got to the valve?


They got in through the grid circuit of the first tube as I explained in
the original post.

My point was in the context of your statement

"Don't forget this is a mic pre not a power amp so current draw is less
than 10mA and as its class A it is also constant."

That on its own is false.

Agreed on its own it is and I have no wish to labour the point.


Cheers

Ian