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I had my recently built ST35 amp packed in foam and well protected for a move to a new apartment. It had been working very well before the move with no apparent problems.

So last night I unpacked it and fired it up and immediately hear a buzzing sound coming from the power transformer. Not real loud but if no music is playing it's definitely loud enough to draw attention to it.

Any thoughts on what could be happening here? It was purchased several months ago from dynakitparts.com. I haven't measured anything yet but as far as I can tell the amp still works fine. I even tried swapping power cords but with no change.

Try tightening all of the bolts, both the bolts that mount it to the chassis
and the bolts that go through the transformer that hold the end bells on.

... tom

Maybe check your line voltage, as that could have changed.

One of the most common problem is DC on the AC line that causes transformer hum , Usaly a bad light dimmer or poor ground at the electric service can cause it or alow it to enter the house .

Well, I tightened up all of the bolts and screws.

Still buzzing.

I double checked the voltages: 666 VAC off the high voltage tap. 6.3 VAC off the heaters.

I just want to emphasize that's it's not loud. But it just bothers me that it didn't do this before I moved.

I'm not too worried about it as I only play it occasionally and it still sounds fantastic.

I don't think I qualify for dynakitpart's warranty as I'm not using it in a stock kit. It's only $65 to get another one so if it's bad then it's no problem replacing it. I've never had any problems with the other dynaco transformers from them. Still... it's a mystery.

In the olden days, Dynaco used to tar-dip the PA774s in order to keep them quiet. It worked very well, but these "tar baby" trannies are just plain butt-ugly, but then, when they are hidden under a top cage, it doesn't really matter.

One thing to check, is that the entire winding assembly isn't vibrating against the laminations. If it feels loose or wobbly, that may be the case.
Seen that happen before, with other power trannies

/ed B

Does it get any warmer than before? Could a cap going bad overload it some, making it buzz?

Some transformers, depending on who made them, just buzz. Never heard of a Dynaco buzz.

rmyauck may have the answer... is it being overloaded by a bum part?

Cheers!

It gets pretty hot but I don't think it's any hotter than it was before. I don't feel any vibration on it.

Now Dynaco power trannies and heat go hand-in-hand!

Cheers!

So I'm pretty sure the transformer is not bad. I've disconnected all of the secondaries and applied power to the primaries. Dead quiet.

As soon as I connect the filament leads I get buzzing.
(green+brown = A , green/white+brown/white = B)

I checked the wiring for any possible shorts and found none. Each leg is connected to ground via 100R3W resistor as in Shannon's circuit.

I even tried removing all of the tubes and I still get buzzing from the PA774.

It's really quite frustrating. Anybody have any thoughts? As far as I can tell everything measures as it should. Sound-wise the amp itself is quiet and sounds terrific. I just can't seem to shake this transformer buzz.

See if the bells pop off easily. They usually are basically glued on with varnish. And then examine the inside potting. There's a lot of 60Hz current flowing through there, and more potting/varnish/baking might be needed.

I'm not a transformer guru, so while I think that is your issue I'm not sure of the best or safest way to proceed. I have a noisy PA060 clone transformer that I plan to rehab this way. If I Google some good DIY info, I'll share it.

Shannon

I've taken off the bell covers but everything is taped up inside so that there's really nothing for me to do. I'm no tranny expert either.

I wonder if it could just be a bad batch as I've tried 2 PA-774's from the same place and the problem has been consistent.

The PA-774 on my latest get*set*go vibrates too. I thought it was
because I took the bolts out and reversed them, so the raw ends
with the nuts were in the back instead of the front. I guess I'll
take it apart and see what I can do. The bolts are soft steel, with
straight heads (not Phillips) so they are hard to tighten properly,
as they start to rip when you crank on them.

I'll see what I can figure out.

... tom

The buzzing I'm getting comes from the core of the transformer. I was getting it even after taking off the covers.