Amp 'Gak' with hard notes...Heathkit

[petemoore]

Heathkit/Daestrom preamp/amplifier distorts [gakky sound] when source is inputted.
 I switched the tubes around [enter 12ax7 replacement] to see if a bad tube might be it.
 I tuned [dialed up/down] the input control [like a volume control], the amp input [like a volume control] bias knobs [put back where they were] and Volume control [front of the amp], but it just doesn't want to produce clean output.
 The amp was 'fixed' and returned 'not fixed', with replaced output tubes [I had to raise the top of the amp to fit the larger than stock/same type tubes in it]....long time ago, long story short...
 I twiggled all the tubes, and poked at all parts looking for cold solder scritches but found none.
 The 'Fix this old amp' thing is getting to me now, I don't know where to start next on it.
 I think there's just enough wrong with it to make it a 'professional' job, as you read I already tried that...
 When I first got the thing, with the bass turned less than 8/10, the amp had a superb tone to it, turning the bass or volume past a certain 'limit' caused the gakky distortion to occur...
 I guess I'm just randomly typeing about an OT subject here...I have all day, for 5 long years to work on it, but I don't have any idea where to start looking in it.
 Heck I'd even rip it apart totally and put new circuit boards in it if I thought it'd work right again.
 The fact that it 1/2 works makes me 1/2 believe I should try again to fix it. So I have it apart, connected and...it still ';misdistorts

[object88]

Not to dismiss any of the tube gurus roaming around these parts, but the folk over at ax84.com are pretty damned clever.  Tube amps is all they do, just about.  Ask in the non-AX84 discussion forum.

Sorry if this has been recommended before; I don't recall the older threads.

[aron]

It's just like a pedal. Sounds like one of the tube stages is misbiased or going into blocking distortion.

If this is a high voltage amp, I'd take it to a repair center unless you really know what you are doing.

You are right, sometimes amps are NO FUN!

[WorkBench]

Sounds like your main filter caps to me, try replacing all electrolytic caps in the thing.  Also, the forum over at Hoffman amps.com is great for this kind of stuff.  Very knowledgable and friendly folk, as i mentioned to you in your other post on this matter.  
Chris

[petemoore]

Thanks for the help on this, I've replaced a resistor, and found one of the output tubes has that slight orange glow [where it shouldn't] because it's not biased, I sure like the tone of the thing, I'll probably continue working on it, but I'm thinking the output tubes [7159] may be failing. :x
 Each side of the stereo amp gets two output tubes..one of which the red paint that reads 'Sovtek' has faded to white...I'm thinking this is a temperature indicator.
 The amp definitely got run hot, because it looks like it, PCB has a darker color under the output tube sockets, and a trace got totally blown off there too. I tested continuity across the suspect traces, but I'm thinking that probably changes as the amp gets warmed up and starts acting wierder as seconds pass...'till I turn it off again to try something else.
 I've decided to concentrate my efforts to the circuit around that glowing tube, and it's probably about due for a general recapping.
 Nice thing about the stereo amp is there is always the other side to compare this's and that's in the amp to one another.