1M linear pot for humdinger in lightly modded Harmony H415?

[Carlsoti]

Is there any reason I shouldn't use a 1M linear pot for a humdinger?

This will be going into a Harmony H415 that picked up quite a bit of hum after removing C1 and replacing R5 with a 1M mf resistor on ch1, then adding a .022 cap between R4 and R7 on Ch2.

The 1M is the only linear pot I have on hand, and everything is closed for the holiday.

[Dave]

Humdingers are usually hundreds of ohms, not millions.
You could put a 100r resistor on each leg of the pot to the wiper and that would give you a 190r (math in my head) or so humdinger which would probably work a lot better.


Dave

[sluckey]

Is there any reason I shouldn't use a 1M linear pot for a humdinger?

Yes. The humdinger or artificial center tap is all about providing a low impedance ground reference to the filament circuit in order to lower heater hum. Using a 1M pot totally defeats that purpose.

BTW, one side of that filament string is connected to ground. You will have to float that in order for the humdinger or artificial center tap to work. I suggest you just use two 100Ω resistors to create that ground reference unless you just want to have another know to twist.

[pdf64]

No point in a humdinger.
Just balance the heater circuit with a pair of 100 to 220R, and DC elevate the balance node to the output valve cathodes.

Without elevation, those unbypassed preamp cathodes will be liable to picking up heater buzz signal contamination.

[Carlsoti]

The humdinger or artificial center tap is all about providing a low impedance ground reference to the filament circuit in order to lower heater hum. Using a 1M pot totally defeats that purpose.

BTW, one side of that filament string is connected to ground. You will have to float that in order for the humdinger or artificial center tap to work. I suggest you just use two 100Ω resistors to create that ground reference unless you just want to have another know to twist.

No point in a humdinger.
Just balance the heater circuit with a pair of 100 to 220R, and DC elevate the balance node to the output valve cathodes.

Without elevation, those unbypassed preamp cathodes will be liable to picking up heater buzz signal contamination.

I knew there shouldn't be any current flowing to the chassis from the heater string, but didn't know if the low values typically seen in ACTs was a holdover from the good ol' days or for some other reason.

I had planned on cutting the ground wire from pin 9 of V1 and re-grounding pin 3, in addition to landing the wiper of the humdinger pot on the top of the power tube's cathode resistor. Lately, I've been hell-bent on making amps "studio-quiet," and I think the humdinger should help that as long as the issue isn't something else.
Aside from high voltages across the board and the newly prominent noise floor, the amp seems to be in good working order. This will all wait for some other day, I guess.

[kagliostro]

A value about 470 / 500 k is what I prefer

If you have under hand a 1M pot provvisory you can try to put a 500k in parallel to the pot and connect wiper to ground or to a resistor voltage divider from B+ (don't forget to add an e-cap) or if you are in a cathode biased amp you  can connect it to the cathode of the power/powers tube

Franco