11-02-2017, 12:15 PM
I've had that before, top cap being the grid, I think the valve is oscillating and you dampen it with the finger.
Check all the caps around EF39 and the resistor feeding the grid. If all is good try a small pf value cap to ground on the grid. But these valves are in my experience prone to spurious oscillation, some more than others. If it is oscillating that would account for the volume going down with higher HT, the valve is being driven harder at the expense of the intended signal.
I'm assuming that the regeneration on the 1st stage is not the cause?
Check all the caps around EF39 and the resistor feeding the grid. If all is good try a small pf value cap to ground on the grid. But these valves are in my experience prone to spurious oscillation, some more than others. If it is oscillating that would account for the volume going down with higher HT, the valve is being driven harder at the expense of the intended signal.
I'm assuming that the regeneration on the 1st stage is not the cause?
Boater Sam.
