Grid chokes at the input stage

[xcortes]

Who's tried grid chokes at the input stage? I think PB did at Hank's amps?

I imagine the risk for hum pickup is huge at that position?

Mike has recommended them in the past.

Any experiences?

[Jim R.]

Xavier,

You must be reading my mind -- I was going to ask this same question.  I'm guessing that the m3/4 cores would be best in this position because of the noise pickup issues you mention, but let's see what the wonks have to say.

-- Jim

[Paul Birkeland]

Who's tried grid chokes at the input stage? I think PB did at Hank's amps?

I did indeed use them in Hank's amp, but I also knew that I had a 150 Ohm source driving them.

A grid choke is really the most useful when it is placed between grid and ground on a tube that may want some grid current while operating.

For a driver tube in a typical Bottlehead amp, you may find 1.5V-2.5V of bias with an input sensitivity of just under 1V, making the presence of abnormal grid current unlikely.

If we move up a stage to the grid of a DHT power tube, as we approach clipping, we may hit the 0V G/K line, and in attempting to operate beyond that line, the grid will be positive with respect to cathode, and grid current will be drawn.  The grid choke is a low impedance source of current for the grid if this happens, which allows the current to be drawn without shifting the bias.