Hum pots

vetmed · 1080

0 Members and 1 Guest are viewing this topic.

Offline vetmed

  • Full Member
  • ***
    • Posts: 245
on: November 02, 2016, 07:37:52 AM
I have seen values from 20 ohms to 100 and was wondering if there there is a method to determine value used, or do people just use what is in the parts box? TIA

Robert Lees

Offline Paul Birkeland

  • Global Moderator
  • Hero Member
  • *****
    • Posts: 19507
Reply #1 on: November 03, 2016, 06:18:25 AM
You have to consider that the plate current in your output stage will travel through the hum pot, and hum pots are generally not bypassed by the cathode bypass capacitor.  If you use too high of a value, you'll have a bit of unbypassed cathode resistor, which can cause some abnormalities during operation.

On the other end of the spectrum, if you had a 10V filament and used a 20 Ohm hum pot, it would burn up half an amp of extra heater current when 100 Ohms would be a tenth of an amp.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man