Bias adjust for my P1 eXtreme

Variable cathode bias option for P1 variant amplifiers

Variable cathode bias option for P1 variant amplifiers

As my P1eX is pretty much done (I have to revisit the grounding scheme as there’s a fair amount of hum when cranked) I’ve been meaning to get around to trying different output tubes to see what differences I get in tone. Initially I chose the 6V6 tube which should give me a “Fendery” tone…whatever that means. Admittedly it does sound “Fendery” when it’s doing “light crunch” with a Strat in position 2 or 4, but when it’s cranked it gets “Marshally” very quickly. Not that I’m complaining.

So here’s some tubes I can try , with their recommended cathode resistor values:

  • 200R / 5W – KT88, 6550
  • 330R / 5W – EL34/6L6
  • 400R / 5W – KT66
  • 600R / 5W – 6V6

Right now the amp is cathode biased, so I must also add the fixed bias modification. This will allow me to measure voltage across a 1 Ohm resistor going from the cathode pin to ground (therefore giving me current). See the diagram to the right for the circuit. It’s quite simple really.

UPDATE

I was having difficulty finding a 1kOhm/2 watt potentiometer, but I have located a part here on the Newark site.

I can get “bias tip jacks” from the Hoffman site with no problems.

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One thought on “Bias adjust for my P1 eXtreme

  1. Thanks for finally writing about >Bias adjust for my P1 eXtreme
    Hot Bottles <Loved it!

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