I was hoping someone could shed some light on this hum I have in a 1979 Bassman 10. The amp is all original as far as I can tell. I've done the typical stuff - cleaned and re-tensioned the tube sockets, changed tubes, cleaned the jacks, and checked for mechanical/microphonics with a chopstick. The hum appears randomly, and will go away if you switch the ground switch one way or the other. Unfortunately, it reappears on it's own, but again can be mitigated by flicking the switch the other way. It always returns.
FWIW, the filter caps will be replaced as soon as the replacements arrive. I figure they're to blame, but wanted to know if there were other opinions.
Humm noises are one of those things that can be generate do to almost anything that has do with you setup.
I found that find the guilty you have to start elimination the basic components and figure it out by a process of elimination.
Start with the guitar, cables etc.
If you narrow it down to the amp...well again you have to do a process of elimination with each component. Sounds to me that the switch that you mention has a faulty ground connection of some sort or needs to be cleaned or replaced.
Do you mean that the hum comes up slowly in both grounded AND floating modes? (Or does the ground switch perform some other fucntion?)
Do you have a capacitor between chassis and ground anywhere?
You need to reverse engineer the ground connections between the signal input, the signal output, the chassis and the power supply? That grounding switch could also be contaminated/corroded - a relatively quick check to replace/short out.
On the Bassman 10 (and most 'vintage' Fender amps), the 'Ground Switch' connects one of the incoming AC conductors to ground via a capacitor ie either the hot or neutral wire is selected. It's not a 'ground lift' switch.
This can be a dangerous thing, so the ground switch should be eliminated. It looks like the Bassman 10 has a 3 conductor power cord. If yours doesn't, it would be a good step to attach a new 3 conductor power cord and bypass the ground switch. Ground the chassis via a solder lug and toothed washer.
You can find instructions like this on the web. If you aren't comfortable working with the AC circuits in a tube amp, a pro should be able to do this quite cheaply- it's not a big job.
That cap/switch may well be bad and acting intermittent-grounding and un-grounding the amp.
At any rate, please (!) ground your amp securely. Those caps are commonly referred to as 'death caps'.
Bassman 10 Schematic
I actually typed some of my previous reply without really checking your amp schematic. Most of the bad problems with ground switches come with 2-prong plugs (or where somebody cuts off the ground prong, or plugs into a 2conductor extension cord, or...)
Actually, if your amp does have a good 3-conductor cord, you could simply remove the cap attached to the ground switch.
If you felt you needed to filter noise from the incoming AC line, then you could add two new Y2-rated caps- one from each side of the AC to ground.
Check out this page for a good explanation of caps on the AC side of the circuit.
If you are working on amps at all, it can save some grief if you build yourself a 'light bulb' current limiter.
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