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1st amp power amp problems...

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8/27/2000 2:30 AM

1st amp power amp problems...

Hi everyone, just finished my first scratch build and there seems to be a few problems with the power amp that I just can't seem to figure out. First off, it's a 60-watt 2x6L6 (Tesla) in class AB1 PP with a pentode / triode switch, the speaker outputs are isolated from the chassis.

-Problem 1: in triode mode, the amp isn't noisy at all, when in pentode mode, it squeals (but you can still hear the guitar signal come trough). What could this be? Is it related to my next problem?

-Problem 2: The output jacks are isolated from the chassis (and any ground). When I ground the common (black) wire to it's respective star point, the amp squeals uncontrollably at what seems to be full volume. Doesn't this commone wire need to be grounded for proper operation?

-Problem 3: The amp has prescence and resonance controls connected to the 4ohm tap on the OPT (a hammond 1650N), however they seem to have very very little (if any) effect, is this related to the OPT common wire not being grounded?

If anyone could even scratch the surface on these issues, I'd really appreciate it as I'm really dissapointed that after 6 months of building + designing the thing I still can't use it! Doh!

Thanks in advance everyone! ...Spencer

8/27/2000 3:20 AM

Hi Spencer

I think when your using negative feedback around the power tubes you have to ground the output trans through the chassis. I believe this should not be the star ground where the first input filter is.

Your problem number three: the presence and resonance controls decouple the feedback loop, each in the high or low frequencies. you might try different values of feedback resistors or temporarily wire a one meg pot. find a good value and substitute a resistor.

you do have a feedback resistor hooked to the phase inverter driver tube?

Tracy

8/27/2000 3:33 AM
Les	Problem 1: is probably related to problem 2. Problem 2: you need to swap the plate leads on the output tubes around. Problem 3: ground the speaker jacks. the presence control WONT work unless the secondary of the output tranny is grounded. good luck!

8/27/2000 4:34 AM
Steve A.	Out-of-phase OT? Les: Problem 2: you need to swap the plate leads on the output tubes around. That is my guess, too. And as you said the OT secondary must be connected to ground if the amp uses NFB. The only choice is whether to connect the ground through the speaker jack, or using an isolated jack and running a wire to the star ground point. --Thanks! Steve Ahola

8/28/2000 5:50 AM

Re: 1st amp power amp problems...

Thanks guys, the switching of the plate leads fixed the squeal in pentode mode and the grounding of the secondary's common wire fixed the prescence + resonence controls. However, now, when I switch the amp into triode mode, it shrieks, any suggestions? BTW, in case it matters, there's 550v on the plates, with the tubes sucking up 70ma of current (2x6L6).
Thanks again! ...Spencer

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