Single-Ended Hum

[Sir Cuitous]

Iain - this is an old thread - you may get more looks by starting a new one. This thread was about one particular way to elevate the heater voltage with a DC bias voltage. It's is usually not necessary to do this and was in part an exercise for me to see how it worked.

You essentially have it right. However, you may get good results (assuming you have a reason to want to re-wire your filament) with other approaches. I would recommend first wiring your filaments in parallel and grounding the filament CT from your PT or, grounding through an artificial CT using two 100 ohm resistors. See if you get results this way first.

[lain]

Thanks

I just finished redoing a SF champ and it has a bit of hum—no guitar connected and volume cranked. Filaments seem like a good way to start. I thought +bias would be a good way to go—saw it in a G weber book. Would the false CT approach, as with the Princeton, work just as well, keeping filament hum from leaking to the plates?

[Sir Cuitous]

Yup, that's the book I saw it in. But first wire the twisted pair as descibed by MWJB and ground it with the artificial CT. Once that is working it may be quiet enough. Then if you want you can always remove the atificial CT and try elevating the voltage as further described. Otherwise, grounding the heater CT to the power tube cathode is more expedient and probably just as effective.

[Chuck H]

I re-did the filaments on a VC with a false CTR, twisted pair and soldered the CTR to the top of the 6V6 cathode resistor. Not a peep of hum. I know there are good reasons to go higher with the positive heater bias V, but with the false CTR and twisted pair the V on the 6V6 cathode resistor was enough to do the trick in my case so I didn't bother with the extra +V circuit.

Chuck

[tyru007]

My favorite method for Hum Free SE amps is adding an extra filter cap to the power supply and using either a 90 to 120 mA choke or 150R resistor to create a PI filter prior the plate voltage.

Tie the ground on the first filter cap to the transformer ground away from the preamp filter cap grounds.

Create an artificial center tap on the heaters and connect to the cathode of the power tube.

I've done this on three SE amps and each one is silent.