JTM45 build lacking sustain

[Paul G.]

In upper registers, notes seem to sputter and die prematurely. Otherwise the amp sounds strong and correct.

I did check all voltages before firing up (will recheck all, of course) and bias is good.

Just wondering if anyone has encountered this and if there's an area I should concentrate on.

P.

[beasleybodyshop]

First thought would be a bad tube. Second would be a faulty coupling cap or poor solder joint, and thirdly would be to test the output transformer turns ratio.

[Paul G.]

thnks. Will do.

[beasleybodyshop]

For solder joint issues, using chopsticks on the circuit will let you know if it's the culprit. Good luck!

[Paul G.]

Not entirely sure, but I may just be a dope.

Before Sunday's gig, I was playing the amp quietly at home, and noticed some crossover distortion. Impossible!! I set the bias when build was completed!

Meter on, bias set at 33/35mA. Apparently as amp burns in, the bias will drift. Reset to 39/41 (near calculated 40mA), made sure it was holding, then I packed up and went to gig.

Amp was much much better. Loud (very loud, actually), full, no raspiness and better sustain.

Will still go back and double check everything, just 'cause.

But…one lesson learned.

P.

[NickC]

If the bias adjustment pot slips, you may want to consider something like this:

https://www.state-elec.com/store/index. ... 010K_STATE

[Paul G.]

If the bias adjustment pot slips, you may want to consider something like this:

https://www.state-elec.com/store/index. ... 010K_STATE

Thanks, but I'm pretty sure it was the tubes. Doing a bit of research I found the JJ KT66 drift at first, it's a known issue. An hour born so burning in and they stay at bias.

P.

[Stevem]

A matched tested PI tube will help in that dept also.