Hum Help

[drewl]

"The Bass control changes the level of hum from almost zero to noticeable".

Do you have a scope? If so I'd start at the stack and work back toward the input until you find it.

Cheers,

Bill

The bass control is most likely passing or cutting the 60hz hum, that's why the volume changes.

[Chuck H]

If adding a ground, that is, touching the chassis or faceplate, reduces the hum, then it's a grounding issue. I would focus on that. Perhaps change where the bass pot or tone stack are grounded. Or look over the layout and try to find what may be grounded with the tone stack that could inject hum. Maybe layout is the issue. Consider what leads may be too close together around that area and perhaps change the lead dress.

Chuck

[satsloader]

"The bass control is most likely passing or cutting the 60hz hum, that's why the volume changes".

Exactamundo, so we know the noise/hum is at or before the bass pot. I probably should have stated that and not used the term "stack" ( tone stack ), as some might not include the pots in the term.. Thanx Drewl..

Bill

[Daver]

Gentlemen,
Yes, I understand that the bass pot is passing or cutting the hum. I realize the hum is at or before the tone stack. I'm down to the stage that drives it. I have moved that stages ground to several other points. I have also moved the tone stacks ground to several other places too. I agree that it sounds like a grounding issue.
To summarize, if I ground the input to that stage, the hum stops. Grounding the output of that stage, of course, stops it, but you can still see it on the scope at the plate. The wires going to that stages grid are shielded. The heater wires are twisted and away from the grid. I've tried removing the heater center tap and using two 100 ohm resistors to form a virtual center tap. That didn't help. There isn't any AC near the voltage divider before that stage. I'll try moving the filter cap ground to some other place, breaking the star for that stage and see what happens. Thanks for all the help!
Dave

[tubeswell]

Gentlemen,
To summarize, if I ground the input to that stage, the hum stops.

Hmmm... okay so it can't be coming from that stage itself can it? (because there is no signal on the grid and all other things being equal there is no hum when there is no signal on that grid). I am thinking that it has to be coming from the stage before.

Is the coupling cap going into that grid bad, or the plate resistor on the previous stage, or the previous stage's heater-to-cathode arrangement? Have you tired elevating the heaters? (Sorry if I missed that bit)

[Daver]

tubeswell,
If I remove the first tube or disconnect its coupling cap or simply have the volume control off, I still get the hum. It sure seems like it is happening in the second stage. The farther away I apply ground from the grid, the louder the hum. What I mean is if I directly ground the grid there's no hum. If I ground before the grid stopper, I get a little hum. If I ground at the junction of the voltage divider, I get more hum. I seems like it's getting into the grid, but there's no AC near it. I just tried moving around the filter cap ground for that stage and nothing changed. I've also replaced the filter cap, too. Thanks!
Dave

[satsloader]

"The farther away I apply ground from the grid, the louder the hum. What I mean is if I directly ground the grid there's no hum. If I ground before the grid stopper, I get a little hum. If I ground at the junction of the voltage divider, I get more hum".

Hi Dave,

Excellent description. Based on it, it's possible the hum is generated from the cathode of that stage.. Check the resistor's ground.

Bill

[tubeswell]

yes going by that description the minute signal that the grid is picking up is being amplified by that stage and if you stop the grid swing by grounding it that stage is quiet. What condition are the socket pin connections like? Are they nice and tight?

[mfratus2001]

I spent a while chasing a hum issue on an amp I had just made, and it acted just like what you are going through. It ended up being the filaments were not grounded at all, just floating.
There was no center-tap on the filament wiring, so I added two 100-Ohm resistors to ground (a virtual ground) and that solved the problem.

Check out his workmanship, too. He may have left something unsoldered, or misconnected something.

Mike.

[rudutch]

I spent a while chasing a hum issue on an amp I had just made, and it acted just like what you are going through. It ended up being the filaments were not grounded at all, just floating.
There was no center-tap on the filament wiring, so I added two 100-Ohm resistors to ground (a virtual ground) and that solved the problem.

Check out his workmanship, too. He may have left something unsoldered, or misconnected something.

Mike.

same thing, except I was was the problem.. a learning experience

[Daver]

Thanks guys.
I "think" I know a little more now. Looking at the scope, it appears that the hum is 120Hz and not 60Hz. Although the bass control affects it and it sounds very low, the scope doesn't lie. I don't see much of a hum signal at the grid. There also isn't much of anything on the plate. It would seem that this should be power supply noise. Each stage in the amp is decoupled and individually filtered. I have subbed out the filter cap for that stage. The heaters have a center tap. I've gone over any solder joint that wasn't originally made by me as well as many others. I have also replaced the tube socket. It's nice and tight. More troubleshooting.........
Dave

[rudutch]

found mine by running a cap to ground off the pilot light

[Diablo]

How have you grounded the pots? I always use a copper buss wire soldered to the back of all the pots, and then connected to the chassis at the PT bolt. The preamp board grounds get soldered to the buss wire.

[Daver]

diablo,
Any pot ground, if it has one, has a wire that goes to its respective filter cap negative. The pot cases are grounded to the chassis with a star washer. The whole amp is star grounded. Thanks!

Has anyone ever had an output transformer, properly connected and grounded, cause hum?

Dave

[mfratus2001]

If the two push-pull outputs are not balanced, you will get a lot of hum from the power supply. The way to tell is to pull the phase inverter tube (I'm assuming a long-tailed-pair or similar output driver), and if the hum is still there, that's it.

If the hum is gone, look closer to the input.

Reason (I believe) is that when there is ripple on the B+, the two out-of-phase tubes tend to cancel the ripple out. Pull one tube, and the hum will get worse. Fender's have a "bias balance" pot that, when turned, has a null point where the hum from the power supply is minimized.

It takes a REALLY well-filtered supply to avoid that.

Edit: To answer your question, is that a really, really out-of balance transformer would prevent the identical halves from balancing. But it would have to be so badly made that you would see more than, say, 30% or more difference in the winding resistances to the center tap with a DVM.

Mike.
Amp Repair Guy