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Anyone know if it's possible for that classic, annoying 120Hz hum to be caused by a cheap, no-name output transformer? I've built amps before, and have pretty good construction standards as far as grounding and wiring, but I just recently built two single-ended amps that both have exactly the same hum, and exactly the same cheap, no-name OT. I can't find the source of the hum anywhere else in either of them, so I guess I really just want someone to tell me it's possible for crummy OTs to cause it.
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Push pull amps have the delightful attribute of cancelling out the hum in the final stage. Power supply ripple hits the two phases with opposite polarity so it cancels. That is why ther can be 30 volts of ripple on the 500v rail and you won't hear it.
There is zero cancellation in a single ended amp, so the B+ has to be much cleaner for similar low hum levels as P-P. In your case, what is the same is not the cheap transformer, it is the same circuit - the single ender.
SOmewhere around here I have a service bulletin from Fender. In it it tells us service centers NOT to try getting rid of the hum in the Champ amps. It is not a defect, it is simply the natural amount of hum in these amps.
In a typical P-P amp, the plates of the power tubes - via the OT center tap - tap right off the first filter. If you did that in your SE amp, you are going to get hum. Try adding a stage of filtration. In other words, looking at the typical B+ string, take the plate power supply from the point where the screens usually come off.
You can measure the ripple with a scope or AC meter.
One other idea. Pull the output power tube and fire it up. Is there STILL hum? It is possible the power transformer magnetic field is coupling into your output transformer. The OT then acts more or less as another secondary of the power tranny, and can induce a few volts of 60Hz. That is why good practice has power and output transformers mounted with their iron laminations facing 90 degrees apart. Ever notice on Fender amps that the power transformer is mounted flat and the output transformer is mounted standing up? If it hums with the power tube pulled, that might be the case.Education is what you're left with after you have forgotten what you have learned. -
I'd say try choke type power supply, if you can find one large enough.Comment
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Sure, add a choke and an additional input cap. If you add a choke only, the voltage will be lower than with a cap input filter.Education is what you're left with after you have forgotten what you have learned.Comment
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Although not classic 120Hz or60Hz hum/buzz, or very loud, yes, absolutely.
Pull the power tubes out.
Plug a set of headphones into the speaker jack and put them on.
Turn the amp back on.
Undo the bolts or screws holding the OT to the chassis and rotate it around.
If you hear any difference, you found the OT coupling to the fields from the PT.Comment
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Not enough dumb questions in this thread...I'll fix that:
Is it possible for one tube (in a 2 power tube P-P amp) to oscillate ultrasonically while the other one doesn't and if that did happen...would you then hear hum?Comment
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I have not seen that happen but I can't say that it's not possible. When troubleshooting I think it's best to keep an open mind about things that could possibly go wrong. I willl tell you that hum will be generated if something happens in one side of a PP output stage to cause a significant imbalance. That something could be a bad tube, bad socket connection, OT problem, bias problem etcComment
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