Grid chokes at the input stage

xcortes · 3418

0 Members and 1 Guest are viewing this topic.

Offline xcortes

  • Sr. Member
  • ****
    • Posts: 997
on: March 21, 2013, 09:14:45 PM
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?
« Last Edit: March 22, 2013, 03:13:29 AM by xcortes »

Xavier Cortes

Offline Jim R.

  • Hero Member
  • *****
    • Posts: 2194
  • Blind Bottlehead
Reply #1 on: March 22, 2013, 02:09:52 AM

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

Jim Rebman -- recovering audiophile

Equitech balanced power; uRendu, USB processor -> Musette DAC -> 5670 tube buffer -> Finale Audio F138 FFX -> Cain and Cain Abbys near-field).

s.e.x. 2.1 under construction.  Want list: Stereomour II

All ICs homemade (speaker and power next)

Offline Paul Birkeland

  • Global Moderator
  • Hero Member
  • *****
    • Posts: 19452
Reply #2 on: March 22, 2013, 06:41:11 AM
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.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man