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Hi all,

I built a TC-15 kit last year, and other than some dumb wiring mistakes, it has worked without issues since.

I noticed one mistake fairly quickly, because the amp had no headroom, and sounded awful. It turns out I had neglected to wire pin 3 on V5, once I corrected that it sounded much better but not quite right. After some time and some fiddling with bright cap values, I realized I had swapped the cut control pot and channel 2 treble pot, once I corrected this, I restored the bright caps back to their original values, and the amp sounded fantastic.

I have been using the amp for a while since with no issues, but about a month ago, I was sitting at my shop and noticed that when I turned either channel up past about 2 oclock, the volume would drop slightly whenever I hit a strong chord. It wasn't all that noticeable at first, and sounded like hard knee compression or a limiter, but it has gradually gotten worse to the point that is now distorts and compresses at anything above bedroom levels. If I keep the volume very low, it sounds normal, and the distortion actually doesn't sound all that terrible, it's actually very tubelike, but the amp is now very, very quiet.

I have replaced all the tubes except for the rectifier, and I verified the transformers are both fine. I read that it could be parasitic oscillation, but I don't have a scope to check this out, and I don't know why it would have developed it in the first place. I also read it could be blocking distortion, but once again, why would it gradually develop this?

can anyone offer some some ideas of where to start? At this point, I'm thinking about just replacing all of the wiring and components on the board, but I prefer to not replace everything if it's a simple problem. I can post pictures of the internals if that would help.

Rectifier tube is dying, replace it.

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http://pdfelectronics.com

Get a Chinese 5AR4. JJ has issues (in case you are shopping)

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Sounds good, thanks for the replies.

That would be the obvious next step I guess. I just can't believe the rectifier is crapping out already....

premature for sure. you could measure the b+ under the circumstances. might be fun. should not drop too much under heavy chord.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Replaced the Rectifier, still having the issue. A few people have suggested filter caps, but like i said, I'm hesitant to just start replacing stuff without a scope. If I'm going to replace components I might as well just do the whole board to rule everything out.

What is your B+? Is any cap hot ?

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Sorry about the long delay in response, I actually haven't had the amp for a few months. One of my business partners decided to "do me a favor" and took my amp to a local tech. He had the amp for a month and then called up and said he couldn't figure out what was wrong with it and I would owe him $75 for bench time and replacement parts . I think he replaced all the filter caps and thought it was an output transformer problem, but I'm not 100% sure. I do know the B+ is fine.

Long story short, I was not too pleased that he took the amp without giving any indication he would be charging us even if he couldn't fix it, but I wasn't sure what to do. He's been paid his $75, and I will hopefully have the amp back tonight. Not sure what my next steps are. I'm thinking I should replace the OT, since I've tried virtually everything else I can think of...

Any suggestions?

So I finally tracked it down. Turned out to be a faulty crunch/munch switch. It was taking the ac output voltage from over 200v to less than 20v, not sure how it could do that, but now that's it removed the amp is working perfectly. I'm also not sure how an amp tech with a scope missed that with 4 hours of bench time, and I caught it with nothing but a fluke 87, and a guitar...

...but at least now I have my amp back!