Hi.
I have a Fender Bassman 100 with this odd problem.
It has a really loud mains hum when switched on, I initially removed the preamp tubes and this brought the noise down a lot, so initally suspected some kind of grounding problem,
If it's been on for a while, there is a pop and the hum disappears
I've left it running for hours and there is the occasional pop.
Other than that it sounds fine, clean signal despite the hum.
I've done quite a lot to it already - new filter caps, one of the power supply dropper resistors was way out so replaced that, new electrolytics in bias circuit and on preamp cathodes.
New control and screen grid resistors and a couple of plate resistors on the normal channel (didn't do anything)
The story behind it is that it has been standing for a very long time and the mains transformer doesn't look that great - the point where the wires come out of the mains transformer - the transformer case has a crack and it looks like kinda melted or something has leaked out of it.
Apparently it worked before then started red plating, after which it was put aside for 15 years.
I'm baffled by the fact that it is 'fixing itself'
Any ideas gratefully appreciated.
Comments
These things can be an absolute nightmare to find...
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
I've never seen an amp with so many messy grounding points - and that waxed eyelet board - yuck.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
The red herring was that although there was no hum in the power amp section, as soon as you turned up the the master and channel volume, the hum would come back, and the tone of it would change depending on the settings of the of the master vol and preamp volume - I think these tonal changes are the big clue that it's the heater circuit.
This led me to spend ages going through the preamp looking for what I presumed was an 'earth loop' including delving into the more esoteric aspects of the Fender Bassman, such as why the eyelet board doesn't seem to insulate very well.
Hope this helps someone in the future.