Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: markmalin on September 15, 2026, 02:04:51 pm
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Hi all
I just built up a chassis using a Hammond 290AX power transformer. The amp's working fine, voltages all fine, but there is an issue I can't figure out. Powering up the amp with nothing plugged in to the input, and a speaker attached to the output, I can hear an audible "buzz" vibration from the PT. This is not noise coming from the speaker, this is physical and audible from the transformer. I tried tightening down the 4 bolts that hold the laminations and bells together, and also re-did how I mounted it to the chassis as the chassis is non standard, and not "flat" (there is an arc to it).
Before I order another transformer, any thoughts? I do notice the bell is raised at one bolt, and there is some sort of spacer in there. I have not removed the bell as it would be necessary to unsolder and remove the OT. Holding the OT with my hand or pressing on it does not change the sound of the vibration.
Thanks for any thoughts regarding how to proceed.
Here is a link to an iPhone video of the vibration: https://youtube.com/shorts/oAb4lAGgCFY?feature=share (https://youtube.com/shorts/oAb4lAGgCFY?feature=share)
Humbly,
Mark
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All transformers buzz, some more than others. Looking at the very tight fit of the transformer in your complex chassis, my sense is that your installation could be the problem. Here's what I would do:
1. Operate the amp with the transformer physically removed from the chassis. Obviously you'll have to de-solder the wires, and wire in some extensions to your current circuit. Wagos will be your friend, but it will obviously be a pain. If it buzzes too much when unmounted, then you know the tranny is the problem. If not, it's the installation.
2. I think you need more clearance between your interior bell and the chassis. The bell should not be touching. It doesn't need much clearance, but "some." And the transformer itself should not be in contact except for the four bolts. Are you using washers as standoffs? They can contribute to the buzzing. I'd suggest using nuts instead. In similar situations, I've used short standoffs also.
Those transformers are generally very well made. If it does buzz, I bet a phone call to Hammond would get you a new unit.
Looks like quite a project!
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can you remove the mounting screws without unsoldering?
I had one that hummed with or without the end bells or mounting, it became my bench transformer
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can you remove the mounting screws without unsoldering?
I had one that hummed with or without the end bells or mounting, it became my bench transformer
Yes, I can remove the mounting screws without unsoldering. Are you suggesting that to see if it doesn't hum with the mounting screws removed?
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All transformers buzz, some more than others. Looking at the very tight fit of the transformer in your complex chassis, my sense is that your installation could be the problem. Here's what I would do:
1. Operate the amp with the transformer physically removed from the chassis. Obviously you'll have to de-solder the wires, and wire in some extensions to your current circuit. Wagos will be your friend, but it will obviously be a pain. If it buzzes too much when unmounted, then you know the tranny is the problem. If not, it's the installation.
2. I think you need more clearance between your interior bell and the chassis. The bell should not be touching. It doesn't need much clearance, but "some." And the transformer itself should not be in contact except for the four bolts. Are you using washers as standoffs? They can contribute to the buzzing. I'd suggest using nuts instead. In similar situations, I've used short standoffs also.
Those transformers are generally very well made. If it does buzz, I bet a phone call to Hammond would get you a new unit.
Looks like quite a project!
Thanks for the input. Actually yes, the inside bell is touching part of the chassis (actually in several places). I will try loosening the nuts and letting it sit on the bench (with the chassis elevated) and see if it's quiet then. I didn't realize the inner bell is not supposed to touch the chassis (only the bolts).
If that doesn't work, I will desolder it and try with it away from the chassis (using clip leads). It will be a bit of a faff desodering everything, but if it's bad, I'd have to do that anyway. With it actually physically out, I can get my file out and make the opening larger.
Looks like quite a project!
I've probably lost my mind...but yes, quite a project, indeed. Once it's all complete and running I can provide more pictures. It was a harebrained scheme, but it actually sounds pretty nice, save for the vibration.
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Ok, what I just did was to unbolt the 4 mounting nuts holding the transformer in. I let the transformer rest on my bench, then held up the chassis so it was not in contact with the chassis at all. The only connection to the "chassis" was via the wires attached to the circuit. In other words, there was NO place where the chassis contacted the metal of the transformer. Result: Same thing. Same sound. Is this indicative of a bad transformer, then? I often use Hammond transformers and have never had anything like this happen. Thing is - the OT, which is a Hammond, had an open wire going to the primary (so only 1/2 the primary was connected)! I just got a replacement for that and the amp works now. Now the OT... Go figure.
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Is this indicative of a bad transformer, then?
my vibrator worked fine for years as a bench PS, probably more like poor workmanship. if the windings are "loose" abound the laminates, the inductive kick could cause the hum.
does the PT get "too warm to touch"?
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Is this indicative of a bad transformer, then?
my vibrator worked fine for years as a bench PS, probably more like poor workmanship. if the windings are "loose" abound the laminates, the inductive kick could cause the hum.
does the PT get "too warm to touch"?
Not that I can tell, but I haven’t run the amp very long yet. I will let it run a while and check. Granted, this is a 1 watt amp using 4 12ax7’s and a 12au7, so it’s not working very hard. I’ll check out how hot it gets, though.