amp debugging help!

So I'm still fussing with trying to get this old Fender Dual Showman up and running. It came into my hands many years ago as a "can you repair this?" amp, and has mostly gathered dust since. When I got it, it was blowing fuses, which someone had "fixed" by shorting out the fuse with some tinfoil. :shock: The fuse-blowing was caused by a bad power tube, but it had a number of other problems that made it downright unsafe, so it got set aside.

A few years later, some cocky dumba** :mrgreen: :oops: with a head full of DIY hi-fi tried to "improve" it by reducing the power section to two tubes, and triode-wiring and cathode-biasing them. This seemed unreliable too, so it got put away again.

Some years after that, an older and supposedly wiser cocky dumba** starts playing pedal steel, and wants to nurse it back to health for use as a steel amp. But it's still being flaky. It'll fire up, and voltages seem okay - for a while. Volume seems low and the tone seems dull, but it's working, right? But after a couple of minutes, one of the power tube plates gets cherry-red, and it pops a fuse again.

So here's the question... might this be a blown output transformer? Some sort of winding burn-through maybe? I haven't tried moving power tubes around or swapping them yet, so it may be that I have a bad tube as well, but I thought I'd bring it up here, see if the collective brain of TDPRI might have some suggestions.

The only way you're going to know for sure is to put the power section back to stock and to make sure that it's had a proper cap job. That way, you're starting from square one and you can work the trouble shooting knowing that you have fresh caps and a stock layout which matches up to known drawings and voltages. Working on it as is...it's a crap shoot.

+1 on what Phat says...

The only way you're going to know for sure is to put the power section back to stock and to make sure that it's had a proper cap job. That way, you're starting from square one and you can work the trouble shooting knowing that you have fresh caps and a stock layout which matches up to known drawings and voltages. Working on it as is...it's a crap shoot.

I'm already restoring back to stock, piece by piece. Right now, it's at two tubes, but stock output layout (fixed bias pentode). Among the OLD problems was a nearly fused socket, so two sockets are getting rewired anyway - probably all four, in the end.

Thinking about it, it was biased at -48v, and I think the stock recommendation is -52v. -48v is kind of hot, but it shouldn't be hot enough to cherry a plate. :confused: At any rate, I'm going to mess with it more tonight, swap in a virgin pair of tubes and rebias it before turning off the standby. It may be something simple... I hope!

And yes, it WILL be getting a full cap job, once everything else is done. And probably new coupling caps too. I really want to get it back in fighting condition! I went shopping for steel amps last week, and the shop had a used Showman head in a 1x12" cabinet that SMOKED everything else I tried. It sounded completely effortless at any sane volume.

The only way you're going to know for sure is to put the power section back to stock and to make sure that it's had a proper cap job. That way, you're starting from square one and you can work the trouble shooting knowing that you have fresh caps and a stock layout which matches up to known drawings and voltages. Working on it as is...it's a crap shoot.

+1 again.....

Once again, I was barking up the wrong tree. I swapped the position of the tubes, and the one that cherried before cherried again, in the new position. Swapped it out for another tube, and now everything seems fine! Just a dying tube, I guess. There's still some pops as the caps re-form, but it's sounding okay otherwise. It still needs a cap job, and a new speaker, but it's working!