2204 preamp hum

[MikeT]

I built a 2204 from a combination of a kit and parts I had. I installed DC filiments. The amp is good but hum from v1 is nuts. I used shielded wire for the grids on v1a and v1b, and grounded the shields at the front only. The hum only occurs when the volume is increased. When I firmly hold the wire to the grid of v1b with needlenose pliers, the hum goes away and the sound is as it should be. Anyone know what this condition is and how I can fix it?

[Chuck H]

Probably a wiring error in the filament supply. Can you draw a schematic based on the actual amp in question? start by pulling V1. If the hum stops the problem can be isolated to that tube or it's circuitry.

[MikeT]

I brought it to a buddy of mine and it's in his bench. He's real good and very equipped. He seems to think it's a ground loop but I'll ask him to check out the filament wiring on the first tube. It's a work in progress right now, but thanks for the idea.

Mike

[Chuck H]

One easy way to see if it's the filament would be to disconnect the filament leads for that tube and power them instead with a 6V lantern battery. If the hum stops then you need to troubleshoot why the amps filament supply is injecting hum on the signal path.

[Gtr_tech]

Those can be quieted down be redoing the gnd scheme. Get the preamp gnd off the chassis (buss wire along all the pots) and move it to a quiter location (first or second filter cap gnd)....wherever it works best.