Champ +bias filament / hum cancel

[lain]

I just finished redoing a SF champ and it has a bit of hum—no guitar connected and volume cranked. Hum gets a lot louder with 330pf snubber cap removed. Filaments seem like a good way to start, and I thought +bias would be a good way to go—saw it in a book (though poorly explained).

On my SF Champ, one end of my 6.3V winding is grounded, and so is one of the pilot lamp terminals. So should I:

1) Unground the 6.3V lead and connect it to one side of the new 2-wire heater circuit, utilizing both 6.3V PT leads?

2) Unground the pilot lamp terminal and include both terminals parallel to the 2-wire circuit?

3) Unground all sockets in heater circuit.

4) Add one side of the new heater circuit to the B+ via 2W 220K, and ground that same side with .5W 27K (maybe near the power cord ground or preamp filter ground)?

Does this sound right?

[MWJB]

First things first, you need to be sure that the hum is coming from the filament circuit in the first place, for this to cure it...still, it can't do any harm to dc bias your heaters. So...

1) Yes.

2) Yes.

3) Yes.

...with all the above you are making sure that there is no ground reference for the heater circuit, other than the ground connection of the 27K load resistor. When you are happy that you have rewired the 2 wire heater circuit, check resistance to ground with an ohmmeter (before you wire up the 220K/27K voltage divider), should read open/no reading.

4) Yes, 27K will probably be fine if your plate to ground voltage on the 6V6 is <390vdc? If much higher than this then that 27K may need to be reduced...22K might work for very high plate voltage, like 420vdc+. At the end of the day you want more than 35vdc, but less than 40vdc.

[lain]

Thanks for the hum advice.

Thanks so much for the guidance. I wired it up as noted with a 22K load R and the circuit was at 40.6 V with the B+ around 415V. I switched to a 20K and the circuit came down to 36V. The amp is quieter indeed. Good enough for me. I still have a bit of hum however, some goes away when I touch the chassis with my body and ground it, and a bit lingers. Might be what I have to live with. Does anyone notice any grounding weirdness or others things that I don't see that might induce hum?

[MWJB]

I like to ground the preamp filter cap to the input jack ground, rather than at the PT bolt.

[lain]

My input jack isn't grounded anywhere other than the fastener nut/washer. The vol pot has a ground, and so does the first stage bypass cap. Do you run a wire from the preamp filter cap above the tag board and over to the input grounds?

[MWJB]

You can. But why not just mount the cap's "+" at the junction of the 100K 12AX7 plate resistors, mount the "-" terminal in one of your vacant eyelets (making sure it is isolated from anything it might have connected to in the past) & run a wire from there to the input jack ground?

[sig]

Filament Hum issue Grounded Heater Wire

Six year old blog caught my interest today as I'm working on an old Western Electric amp that has 6SL7, 6V6, 6X5 tube. From the PT one of the heater wires, (green) is grounded. Making connections to this heater wire, Pin 8 of the 6SL7 is grounded, Pin 2 of the 6V6 is grounded, pin 7 of the 6X5 is grounded, one side of the pilot lamp is grounded to chassis.

The other heater wire, also (green) from the PT is routed to Pin 7 of both the 6SL7 and 6V6, pin 2 of the 6X5, and the positive side of the pilot lamp.

I too am experiencing the same symptoms others list with hum and I'm convinced the heater wires running on ground are the source.

Please look at my schematic and let me know if I should attempt the sames steps to lift the grounded heater wires and run a 2 wires setup?