Hum, not heater, not filter

[jeff]

 What would cause hum aside from heater or filter hum? I recently built a new amp. It's pretty cool. Basically a bassman with the first two tubes in series. Problem is I'm getting a hum. I don't think it's the filtering. With the first tube out I can turn the volume all the way up, no hum. With the tube in and the gain down I get a bad hum as I turn the MV up. Now I know what you're thinking, Hum from the heater. That's what I thought too so I got a 6V DC wall wart and disconnected the 6V ac heater wire and used the DC but the hum's still there. I can't think of what else to try. Please help.

[billcreller]

I know nothing about a Bassman design, but pre-amp tubes picking up noise has plagued me a few times.  The last time I had the problem, I switched to a metal tube, since it was an octal type, and it did solve the problem.  Maybe a tube shield would help ?

[67polara]

The amp I built is dead quiet with the lights off and the computer off but turn any of those on, hum city.  I have single coils on my guitar also when my friend comes over with his humbuckers all is good.  Now that they make you use the useless compact bulbs, This is what I have to put up with.  Didn't do it with regular bulbs.

Tony 

[Willabe]

Now that they make you use the useless compact bulbs, This is what I have to put up with.  Didn't do it with regular bulbs.

Like as in state law?                 Where are you at and when did that happen?


                        Thanks,     Brad                   

[67polara]

California can not sell any above 60 watts.

[HotBluePlates]

I don't know the schematic of your amp, as a Bassman doesn't have a gain control. But no hum with the first tube out means the hum is coming from something before the second tube.

My first suspect would be the input jack. If the shorting switch of the jack doesn't close fully or properly, you'll hear noise/hum with nothing plugged in. If you plug in a guitar and turn the volume all the way down, it should kill this cause of noise.

Do you have a filter cap feeding just that first tube? If so, suspect wiring between the filter cap negative and ground, especially the loop back to the grounded side of the first tube cathode resistors.

You don't have any strange sources of noise near the amp when testing do you? A really long-shot idea is too-long grid wires on the first tube, along with a nearby source of noise such as a computer monitor or flourescent lighting. The idea is the first tube wiring could be picking up noise, and the gain of the fist tube is needed to make the noise signal big enough to be objectionable. But these noise sources are more like a buzz than a smooth hum. Anyway, I'd suspect input jack wiring/grounding or power supply/ground wiring first.

[jeff]

I don't know the schematic of your amp, as a Bassman doesn't have a gain control.

I installed a master volume so what I called gain would be the Bassman's original volume control.

Do you have a filter cap feeding just that first tube? If so, suspect wiring between the filter cap negative and ground, especially the loop back to the grounded side of the first tube cathode resistors.

Yes. But I have the ground of the first filter cap going to the ground of the next going to ground. I'll try giving each filter cap it's own wire to a star ground.

My first suspect would be the input jack. If the shorting switch of the jack doesn't close fully or properly, you'll hear noise/hum with nothing plugged in. If you plug in a guitar and turn the volume all the way down, it should kill this cause of noise.

I don't think that it is the jack only because to me this would indicate the hum is on the grid. The hum doesn't get louder as i turn the volume(gain in this case) up. I suspect if the jack was picking up a hum as i turned it up the hum would get louder.

[38Super]

Now I know what you're thinking, Hum from the heater. That's what I thought too so I got a 6V DC wall wart and disconnected the 6V ac heater wire and used the DC but the hum's still there. I can't think of what else to try. Please help.

Just to be sure, have you looked at the 6VDC on a scope or checked AC on voltage out?  Wall warts are cheap and notoriously noisy in my experience.

I also get a lot of conditional hum when I am near fluorescent lights.  When I am plugged in, when I turn away from light source, noise goes down significantly.

cheers,

rob

[jeff]

 Good Idea. I'll check if I'm dealing with pure DC. There is a florecent light above the bench but I always turn it off when checking for hum. I can't think of anything else that would cause hum on the bench.

Let's just say it is AC heater noise I'm hearing. With the first tube out I hear no hum. When I put the tube in I hear it. I was thinking if it was heater noise the hum would get louder as I turned the volume up. Now that I think about it more, maybe not. If a tube is picking up ac and you ground the grid will it still hum?

[phsyconoodler]

A bassman does not like it's filter caps grounded near the transformer.I hand-wired a bassman reissue and grounded the filter caps all over the place and it hummed.When I just grounded them onto the filter cap can like they did in 1959,the hum completely went away.
  I suggest it's your grounding method.

[Merlin]

Do the heaters have a ground reference? Maybe you forgot to ground the heater centre tap?

[jeff]

Yeah 2 100 ohm resistors, no center tap. I'll try grounding the first tube's cap in different places. Every thing else is good as far as hum, without the first tube in.

Thanks
  Jeff

[jeff]

 I think I got it. I connected the cap and cathode resistors for the first tube to ground at the input jack. Seems to be better now.