5F1 buzz problem

[Jelle]

I have a somewhat unexplainable (to me) problem with a Champ I recently built. Apart from the hum from the inadequate powersupply filtering, there's a buzz that starts at about 5 on the volume and disappears after 10.

Checked the solder joints in the preamp section and played with wire routing: eventually it turned out that the problem comes from the wire that runs from the junction of the two 68k gridstoppers from the inputs to pin 2 of the 12AX7. If I just barely touch that wire (or the gridstoppers themselves) with my finger just after it comes from the board (not closer to the tubesocket) the buzz between 5 and 10 disappears, making the amp absolutely quiet (except ps hum), but making it buzz louder at volume settings 10 - 12, where it was quiet before Even touching the eyeletboard running over this wire causes this effect.

I've experimented with some impovisational shielding, but to no effect. Any insights or should I just move the gridstoppers to the pin on the tubesocket? It happens after the gridstoppers, so eliminating the wire altogether would seem a solution, although I would like to keep the wiring original....

[muchxs]

My Champs are silent, no noise. I use a stock layout.

Originally Posted by Jelle

I have a somewhat unexplainable (to me) problem with a Champ I recently built. Apart from the hum from the inadequate powersupply filtering,

Nothing prevents you from changing that. Hint: Build the power supply like a tweed Deluxe.

Originally Posted by Jelle

there's a buzz that starts at about 5 on the volume and disappears after 10.

We'll attempt to explain the unexplainable:

There are two likely sources of hum in tweed Champs, the problem is compounded by the small chassis. There's only so much room to move noise sources away from the signal path.

One possible hum source is from the filaments. Either use a transforner with a center tapped filament winding and ground the CT or create and artificial CT w/ 2x 1/2 watt 100 ohm resistors, one from each leg of the filament winding to the ground. Hang them off the pilot light lugs, it's a straight shot at the "-" end of the first filter cap from there.

"Stood up" filaments in tweeds are nonsense. Keep them flat to the chassis and tucked away under the chassis lip.

Your AC wiring flies to the exact center of the chassis and connects to the switch on the back of your volume pot. It's essential to keep the AC away from the signal path although your options are limited. A big ol' loop in the AC wiring will "broadcast" hum inside your amp.

All ground points must be solidly grounded.

Originally Posted by Jelle

Checked the solder joints in the preamp section and played with wire routing: eventually it turned out that the problem comes from the wire that runs from the junction of the two 68k gridstoppers from the inputs to pin 2 of the 12AX7. If I just barely touch that wire (or the gridstoppers themselves) with my finger just after it comes from the board (not closer to the tubesocket) the buzz between 5 and 10 disappears, making the amp absolutely quiet (except ps hum), but making it buzz louder at volume settings 10 - 12, where it was quiet before Even touching the eyeletboard running over this wire causes this effect.

Yeah, but it's like touching the strings on your guitar while standing under flourescent lights. You're acting as a big ol' capacitor and giving the hum a different signal path.

Eliminate the hum at its source. The source is on the other end of the chassis.

Originally Posted by Jelle

I've experimented with some impovisational shielding, but to no effect. Any insights or should I just move the gridstoppers to the pin on the tubesocket? It happens after the gridstoppers, so eliminating the wire altogether would seem a solution, although I would like to keep the wiring original....

Attached pic is one of mine. It does not hum.

[Jelle]

Thanks for your reply!

I'm actually fine with the hum from the powersupply; I have another Champ with a beefed up powersupply that is absolutely quiet, but also misses some of the 'bounce' this one has.

What I find somewhat annoying is the higher pitched buzz at certain volume settings. I'll play around with AC wiring a bit, but I had the heaters flat at first, but with the buzz stood them up; no difference. I'll move the AC cable around somewhat and check the grounds to see if that helps. Btw, I have an artificial CT for the heaters on the cathode.
What baffles me is that at certain volume settings the buzz disappears when I put my finger on the wire, but returns when I turn up the volume, which is quiet without touching the wire. If it picks up buzz from the AC, it should amplify this relative to the volume setting, or am I wrong here (which wouldn't be the first time...).