Hum Help

[Daver]

I've got an amp here that I built about a year ago for a guy. It has developed a hum that I can't seem to nail. The amp is a 4X6V6 that has switchable cathode/grid bias. The preamp goes gain stage-volume-voltage divider-gain stage-tone controls-PI-power tubes. Nothing fancy. The wild card is that the owner has done some work on the amp himself including replacing the output transformer and choke and removing the negative feedback. The amp was originally quiet.
The hum seems to be affecting the second stage. If I ground the input or output of this stage the hum goes away. I've replaced the tubes and socket added sheilded wiring, rewired/re-routed the tone controls, put a grid stopper right on the tube socket, re-connected the negative feedback and checked coupling caps for leakage. The Bass control changes the level of hum from almost zero to noticeable. It seems to be 60 Hz.
The other odd thing is that if I touch the chassis near the tone controls the hum goes away. This is with me not touching anything else and nothing plugged into the amp. Even if I touch the plastic faceplate near the tone controls and not the metal of the chassis the hum goes away. I thought the replacemant OT was too close to this tube so I tried moving it around a bit. I can't move it too much do to the wire length. What little I could move it didn't make a difference. The owner isn't exactly sure when the hum developed or what, if any, change he made preceded it.
I'm sure I'm leaving out something I tried so ask any questions, please. Thanks, as always.
Dave

[guitician]

Check the chassis for ground to the 3rd prong of the line cord. It sound like your chassis is missing a ground.

[Daver]

I do have ground on the AC cord. I just checked it to make sure. Thanks!
Dave

[Daver]

Hmmm... Reconnecting the negative feedback does seem to help a lot. It didn't seem like it did before. Maybe it was because the owner was hovering over me the first time I tried it. I can still make the hum go away by touching the chassis or plastic faceplate.
Removing the cathode bypass cap on that second stage lowers the hum, but I think that comes from the lower gain. Subbing in another cap brings back the hum to previous levels.
Dave

[guitician]

Check that you have all ground points going to ground. Do the inputs get grounded when they don't have a jack inserted?

[Amp Kat]

Does it have Reverb and if so does turning the pot effect the hum ? Pulling preamp tubes is another good source of finding and eliminating it. You can also take the signal after the coupling cap and send it to the PI and bypass all the other stages and see where it starts. If you can't get the first gain stage hum free you won't get anything downsteam hum free either.

[Daver]

The input jack is grounded when nothing is plugged it. That and all other ground points measure about .2 ohms to the chassis.

I've checked for leaky coupling caps too.

Removing the input tube doesn't change anything. There is no reverb. The hum seems to be completely centered around the second stage. I did jump from the first stage to the PI and the hum goes away. Thanks for reminding me. I've tried so many things I forget to write them all down.

Thanks for the help!

Dave

[Daver]

Well, I've tried rewiring the grounds. I had a two point star before. One near the input for the preamp and one near the PT for the PI and power amp. Now it's one star near the PT. Still no help. If I put the amp in its cab with shield plate on the bottom the hum is less, but still noticeable. This, I guess, is the same as when I touch the chassis, although me touching it nearly stops it all together. Where can this hum get injected to only the second stage? I have sheilded wire from the input to the 1st stage, to and from the volume pot, from the voltage divider to the second stage and from the tone stack to the PI.
Dave

[trem]

I am battling a similar problem with a Fender Twin, it is driving me to an early grave. I have not solved my troubles yet, but here are some things I have tried:
1. Chop sticking until my fingers hurt.
2. Shielded cable to the inputs of the preamp tubes.
3. A jumper wire from the PT ground to every cathode ground, pot ground etc. etc. just to see if I could touch anything with the jumper that would make a difference.
4. Removing, going without, reinstalling the two 100 ohm resistors in the heater circuit. Even if your PT has a center tap, disconnect it and see if the "virtual tap" is any better.
5. Threatening the amp with violence, showing it a 120 to 220 step up tranny as possible torture, praying and perseverance. I am in this 'til the end.
Good Luck

[drewl]

I'm going through a similiar situation with an Ampeg M15.
Someone was in here a "mucked" it up.

[tubeswell]

Originally Posted by Daver

The hum seems to be completely centered around the second stage. I did jump from the first stage to the PI and the hum goes away.

I had a similar problem once - turned out to be a dodgy CC Plate resistor that crumbled to pieces when I took it out. (one of the leads came right off the body of the resistor - must've been making poor contact). When I put a new resistor in the problem was fixed. (Not saying that's your problem tho'.)

[Daver]

Thanks for the show of support! I, too, have threatened the amp with great harm. Swearing makes me feel a bit better, but doesn't cure the amp.

tubeswell, I have tried replacing the plate resistor on that stage. It's an RN65 metal film.

The hunt continues.....

Dave

[tubeswell]

Originally Posted by Daver

The other odd thing is that if I touch the chassis near the tone controls the hum goes away. This is with me not touching anything else and nothing plugged into the amp.

Grid/Signal wires close to the chassis picking up microphonics?? (I had this problem in my recent bassmanish clone at first, and putting shielded cable in there did resolve that issue.)

[Daver]

tubeswell,
I do have shielded wire going to the second gain stage. I've also added a grid stopper to it with no help. I'm sure I'm overlooking something, but I'll be damned if I can find it. Thanks.
Dave

[satsloader]

"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

[drewl]

Originally Posted by satsloader

"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]

Originally Posted by Daver

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]

Originally Posted by 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.

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

[R.G.]

Originally Posted by 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

*Exactly* where does the CT of the PT connect to the negative of the first filter cap, and *exactly* how does that point get to (a) chassis ground and (b) signal ground?

Also, are the central axes of the "E" of the power and output transformers and the filter choke all oriented at mutual right angles?

[Daver]

mfratus2001,
Removing the PI tube stops the hum. Dead silence. I've also tried different output tubes, too. Thanks.

R.G.,
The filter for the power tube plates and screens is a 50/50 uf JJ can cap. The negative has a 2" long wire going to a ground lug on one of the PT bolts. The PT center tap is also grounded at that point. The center tap is fused, so the wire from the transformer runs to the back of the chassis to the fuse holder, then to ground. I've also bypassed the fuse holder and attached the CT directly to the negative of the can cap.
The OT and PT have their lams 90 degrees opposed to one enother. They are separated by about 4". The choke is in between them and oriented in the same plane as the OT. Are you thinking there is power supply hum coming from the output getting amplified by the second stage? Thanks for taking the time to help!

The only thing that seems to help is adding negative feedback around the power amp. More feedback, less hum.

Dave

[Enzo]

Well, more feedback means less gain in the stage, so sure.

[mfratus2001]

If there's too much of anything, feedback will reduce it.
I don't think that is a problem area.

Try removing the tube ahead of the phase inverter, and if there is no change, than the PI is your problem.

What are the voltages on the phase inverter pins? Should be (from Fenderish designs) about 240 on plates, about 75 on grids, and 45 on cathodes that are connected together. The phase inverter stage is a problem in some amps. Since the cathode is at 45VDC, when you add signal to it, the voltage between the cathode and filament can exceed the tube's max specs.

This is such a problem on some amps that they float the center tap of the filaments, not at ground, but at some percentage of the B+, like 60-100V. They then bypass the voltage divider to ground with a fairly large cap.

If you haven't already, try a different tube for the phase inverter.

Try disconnecting the center tap of the filaments from ground and see if it rises to some high voltage. I have seen tubes develop a high resistance between some electrode and the filament, causing the whole amp to act strange.

When subjected to running into an open circuit from having no speaker connected, an output tube will often short or arc from plate to filament. That puts all tubes at risk for developing internal paths to the filament across the mica insulator.

Mike.

[Daver]

Mike,
If I remove the tube before the phase inverter (the second stage where the hum seems to be coming from) the hum goes away. I have tried replacing the three preamp tubes, power tubes and rectifier tube.
The PI is a LTP. The supply B+ for the PI is 345 volts. The plates are at 206 and 200. The cathode is 24 volts and the grids are at 17 volts.
I've tried floating the filaments on the output tube cathode voltage and making a virtual center tap with two 100 ohm resistors.

Enzo,
I should have mentioned again in my previous post that grounding the input or output of the second stage kills the hum, not just feedback around the power amp.

Thanks for the help, guys.

Dave