Ampeg V9 SVT filament transformer hum and vibration

[jvm]

I have a Ampeg V9 SVT with a vibrating filament tranny. Amp has been completely re-tubed and capped. Hum was unaffected by the tubes and cap job. I can feel the filament tranny vibrating with my hand on it. The transformer is somewhat rusty but I've seen worse. Filament voltage is at 5.7 VAC. How does one go about troubleshooting a vibrating transformer?

[Jazz P Bass]

That is what they do.

There is an AC voltage running through it.
Although the manufacturing process typically minimizes it.

So I would imagine the question is 'how do you go about alleviating (or minimizing) the vibration?'.

You may be better off simply buying a new one.

[The Dude]

It's vibrating- got it, but is it noisy or, other than you feeling a vibration, what is the actual problem? Is the vibration audible or too noisy? If it's just that you're feeling a vibration, I wouldn't worry about it.

[g1]

Is it stock? I don't have a schematic handy but I don't think Ampeg often used separate filament xformers. So I'd be wondering if it's up to the job. The 5.7VAC seems low, maybe it can't handle the current which would probably add to the vibration. With higher modern line voltage, I'd expect the heater voltage to be higher, rather than lower.

[The Dude]

This schematic shows a separate filament x-former.



I agree. I'd also expect the voltage to be higher. (OP) You may want to measure your AC power in to see if it might be low.

[Enzo]

Yes, the SVTs had separate heater transformers, and the V9 is nothing more than an SVT with a guitar preamp instead of a bass one.

5.9v seems low, but shouldn't be a problem. But it also might indicate a problem. How much does the voltage rebound with all the tubes removed?

[jvm]

Thanks for chiming in everyone. Yes, the filament transformer is stock, and the hum is audible. With all tubes removed, filament voltage is at 6.5 VAC but I also hear something arcing but can't see it. It arced 4 times so I pulled the plug. The amp works great with the re-tube and cap job but the hum was there before and is unchanged. There was no arcing before removing the tubes.

[guitician]

The sockets are probably arcing from pin to pin along a carbon path that was created in the insulating material. Or some other insulation has broken down somewhere. Look at the circuit in almost complete darkness and the arc may reveal itself.

[jvm]

I've tried to find this arc with complete darkness. I don't see anything at all. I've turned the chassis every possible direction. It sounds like it is coming from the power tranny. I unhooked the filament transformer primary and still heard the arcing.

[guitician]

I would say that testing that transformers isolation resistance is the next step then.

[g1]

I've tried to find this arc with complete darkness. I don't see anything at all.

Did you check out the hum balance pot?

[jvm]

I did test the hum balance pot, g1. It tests and functions fine.

[jvm]

I put the amp back together and the arcing stopped? Amp works fine and the filament transformer hum isn't nearly as loud as the cooling fan. The owner of the amp has opted to use it as is until he is forced to replace the transformers. Any reasons why I should try to discourage this?

[nsubulysses]

Well, that all sounds good, but in my experience when the problem stops and you don't actually fix something there is about a 50/50 chance it will come back with the same problem . If you don't fix it and send it out you may be doing free work in the future if you have a warrantee on your "repair."

If you have good communication with the customer you can probably explain this and avoid a possible bad scenario in the future.

[jvm]

I agree and have tried to convince him to fix it now while I have it on the bench. I've made it quite clear that there is no warranty on the repair if it leaves with a known problem.