Parallel Triodes in Preamp

[dbishopbliss]

I have a DaBeck Fat45 amplifier which is essentially a JTM45 with some tweaks. I started trying to document the circuit a while ago, but it proved to be challenging without being able to see the underside of the circuit board so I don't have the complete schematic.


The amp has an extra preamp tube. It does not use the tube as an additional gain stage but instead it runs both triodes in parallel. If I recall the description on the Way Back Machines says this is supposed to lower noise and give a fatter tone. That said... I'm thinking that perhaps the plate and cathode values may not be correct for parallel operation. Essentially, the plate and cathode resistors are the same values used for when the triodes are not in parallel. That is, a 100K plate resistor, 820R cathode resistor and 250uF bypass cap.





The original JTM45 had a shared cathode resistor/bypass cap for both triodes so perhaps those values are fine. Should I swap the 100K resistor for a 50K? What difference should I hear if I make the change?

[Merlin]

Should I swap the 100K resistor for a 50K? What difference should I hear if I make the change?

Mainly you'll get a bit less gain. The existing values look fine to me, it's basically equivalent to running a single triode with a 200k load, which is fine if you like the extra gain. Mess with it all you like, there are no 'correct' values in a guitar amp, only what sounds best to you.

[dbishopbliss]

Mainly you'll get a bit less gain. The existing values look fine to me, it's basically equivalent to running a single triode with a 200k load, which is fine if you like the extra gain. Mess with it all you like, there are no 'correct' values in a guitar amp, only what sounds best to you.

Generally I prefer higher gain at lower volumes. I suppose I can just clip a 100K resistor in parallel for testing.