Hot Power Transformer, Budda SD30-II

[analoguedemon]

I opened this amp to figure out why, once every couple of weeks or so, this amp suddenly cuts out for a few minutes at a time. The symptom is amp sounding great, then no sound (power lamp stays lit), then a few minutes later the sound comes back. The problem is so infrequent that I thought I would just leave it open on my bench until it happened, and then I could check at least the power supply and filament voltages to start diagnosing. I suspected the problem is the filament supply, because when the amp recovers from the fault, it slowly comes back to life as if one or any of the tubes were just re-lit from cold. During the fault, I can disconnect the effects loop and pedal jack and nothing changes.

The first week I left the amp upside down and open (tubes down, like in the chassis except no lid) and never observed the fault. A few days ago, I turned it upside down and left the amp closed against the benchtop, to simulate the heat dissipation situation when the amp is mounted in the chassis. I still have not observed the fault.

I replaced all the tubes already, and also cleaned the sockets (this made a huge beneficial difference in tone that I was not expecting).

But I have noticed that the power transformer is *scorching* hot. I can keep my hand on it for only few seconds before the pain is too much. There is no smell. I know the reputation that "Budda run's hot", but is this true about the power transformer?

If I want to replace this power transformer, where can I find one? I have had no luck consulting the Google, or the manufacturer websites of Budda, Peavey, or Mercury Magnetics.

Thanks for any advice you guys may have,
John

[Enzo]

Hi, welcome.

First, a lot of power transformers run really hot. I don't worry about it unless there are other reasons.

A transformer is just wire wrapped around an iron core, ther is no circuitry in it, so it isn't the sort of thing I'd expect to intermit much. Of course anything is possible.

It seems unlikely you will find mention o fyour particular transformer on anyone's web site. if you are convinced you need a new one, pick up the phone and CALL the parts department at Peavey and ask them about the part. They have thousands and thousands of different parts, and don't remotely try to list them online.

You suggest the heaters may be involved since the amp fades back in after quitting, but usually when you lose heater power, the amp would also fade out rather than just go silent. Dos that happen?

I have not seen the inside of one of these, but if the heaters do seem involved, try to trigger the fault. First, with the amp together, when it cuts out, try balling up your fist and whacking the top. Or with the chassis out, use a rubber mallet to whack the end of the chassis. This can sometimes expose loose connections. whacking may cause the amp to come back on, or it could also trigger the cutout to start. Either way we then know it is a connection. If there are any ribbon cables and ribbon connectors between boards inside, try using something insulated to push gently side to side on them. Such connectors are weak points in many amps.

[analoguedemon]

Hello Enzo,
Thanks for reading my post. Your observation that the amp should fade out, not just on, also if it is really a filament problem is a good one. Honestly, I am not sure I know the answer for sure. Most of the times I have the problem it is with a band, with a lot of other sound going on, and unfortunately I just do not have a good observation about it. After it cuts out, of course, I am paying a lot of attention to it and I definitely notice the fade back in. Because the problem usually occurs in a band situation, I am suspicious that the problem is heat related too because that is when the amp works the hardest. Actually, you just triggered another useful thought - I might hook up a THD Hotplate to it so I can run the amp wide open and push it hard to see if that triggers the fault.

This amp does not have any obvious weak connection points like a ribbon cable. It is about half hand wired, and the other half is a very simple looking circuit board, with large, through hole parts. Quite honestly, it is a beautiful construction and pretty much bombproof. When I first saw inside of it, I knew right away why this amp sounds so great.

I am far from convinced that the power transformer is the problem, because you are right: why intermittent? It just runs way hotter than my homebrew amps or any other amp that I know.

I think I might have observed once a flash in the rectifier tube kicking off the fault. I was not totally sure I saw it; it's kind of like a fuzzy dream 8-(. I have replaced the tube, but I wonder if some other arcing or problem with the rectifier tube (it is a 5U4GB) could somehow be suddenly depleting the reservoir and, after a few minutes, comes back.

I will go fetch my rubber mallet tomorrow and practice hitting very lightly first...

John