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]

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]

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