elevated heater center tap

[lowell]

Got something weird happenin' here. I tried elevating a center tap in a 2x EL84 cathode biased power amp. Vk is 16v fyi. The hum actually seems louder when elevated verses when grounding the center tap. How could this be?

[defaced]

How did you elevate it; can you provide a schematic? Typical arrangements have a decoupling capacitor in the voltage divider that if omitted, can allow hum to enter the system. Also, 16v isn't a whole lot, you may just be seeing some strange non-linear relationship between coupling an h/k voltage. I'm usually in the 40 to 60v range.

[pdf64]

A Vk of 16V seems way high for cathode biased EL84? Pete.

[Amp Kat]

Sounds like maybe he has a 16 volt zener back biased to the Cathode bypass cap and the diode could be backwards ? If not then measure from ground to each leg for DC elevation voltage but agree elevation should be 50 to 100 volts with the divider.

[lowell]

Nevermind guys. I had the reverb tank unplugged so that was causing weird results. However, that being said, there was no improvement in hum when the heater was elevated to the cathode... and yes Vk is 16v in this amp.. it's a Dr. Z Maz Junior.

[kg]

sounds like he runs some screen voltage!

[tubeswell]

Hi lowell - have you tried a voltage divider from the B+ (which will need the bottom leg of the divider decoupled as defaced said)? You could combine it with the bleeder resistor circuit and humdinger it as well

[lowell]

Very nice idea with the bleeder resistors!

[defaced]

Won't pulling it from the plate node subject the elevation from all of the bumps and bruises the B+ will take? Usually elevation is pulled from a more stable node like the screens. 1M and 1K with 33u (I think that's what it is anyway, might be 22u) is what I currently have in my amp.

Also, with 40-60v of elevation, I don't think the hum balance pot is going to do anything, but I've been wrong at least once in my life

[tubeswell]

Won't pulling it from the plate node subject the elevation from all of the bumps and bruises the B+ will take?

The top leg of the voltage divider and the cap from the knee to ground make an RC filter, so you are taking the heater elevation from a Pii filter (which is the same as if you were taking if from the screen node).

(As for the humdinger? likely unnecessary but WTF?)

[Enzo]

Keep in mind this is not in the signal path. It doesn;t matter if the B+ moves around, as long as the heaters always remain more positive than the cathodes. Being more positive is all it takes to prevent the electron migration. SO if the amount of more positive varies, so what. Think of it as a 20 foot high fence, and you can only jump 10 feet high. You are kept out equally well by a fence of a steady 20 foot altitude as by one that varies from 20 to 25 feet.