Changing bias circuit in JTM-45 to accommodate EL34s

[dehughes]

You probably want to change the bias pot to 100K to give you more sweep range, and then the resistor before the diode sets the extent of that sweep so you might want to change this too to get a good usable range for the tubes you want to use.

Good deal...I think I'll just start with getting some high idle current EL34s and see if that works.

Also, I had this backwards....with the tubes biased up as hot as I can get them, the pot is reading ZERO, not max. So, I'm left with decreasing either the 220k or the 56k resistor, NOT increasing the pot. Sorry...I process slowly with bias stuff.

[Mykkus]

33K resistor.

31 should really be fine though. Depends on the tubes. I run the old seimens around 28MA with 410 on the plates in my JTM clone, they sound really good that way and should last longer.

[dehughes]

Cool. Thanks.

I picked up some high idle current EL34B-STR tubes, and they bias up just fine with the circuit unchanged. So, I think so long as I look for higher idle current EL34s, I should be fine.

As for bias level....I'm liking 35ma at 420v. But I see your 28ma at 410 is much lower. I was told by Greg Germino that in my amp 32-34ma is a good spot for EL34s, but I suppose it depends on the ear of the beholder, to a certain extent.

Don't I need to bias up to 60% to avoid crossover distortion, anyway? 28ma at 410v is only 46%....