bias elevated heater voltage

[Broz]

Is there any reason not to connect the 6.3 v heater center tap ( I have one of Moose's PT's that has this center tap) to the bias supply? I have read that elevating the heater supply like this can help reduce hum. Thanks,
Broz

[mhuss]

The idea behind 'heater elevation' is to make the heaters more *positive* than the cathode, and therefore the (negative) bias supply would actually hinder more than help.

--mark h

[Broz]

Thanks Mark
Broz

[lf353]

Looking at the schematics I don't see the fil voltage elevated above ground. I think this would help with hum with the preamp and PI design of the amps.

Question does anyone know if there was a reason this was not done.

[Broz]

Well. I guess you would have to come up with the positive (not the negative bias supply like my dumb question above) dc for elevation. Could do a voltage divider off B+ I guess.
Broz

[lf353]

If the output stage is cathode bias you can use the cap bypassed resistor to tube cathode node to raise the fils above ground. this has been around a long time as best I can tell.

[mhuss]

KOC regularly uses a voltage divider (something like 330k/100k+10u) from B+ to lift the heaters in his TUT designs.

--mark h

[Moose]

Question does anyone know if there was a reason this was not done.

File it under the "If it ain't broke" category.

Elevating the heaters can sometimes help isolate them, but what you're isolating them from is other grounds, which generally are at 0V.

A very carefully laid out ground scheme will try and isolate things like large draws from the poweramp by keeping them physically away from, and physically closer to the star point than, a preamp ground. If you have good ground discipline, and you're not getting any noise from the heater circuit, then it might not be necessary to spend the effort and include the extra components.

This is just a guess, so take it as such.

All that being said, I tend to lift my heater ground as a matter of course. Either via reference to a cathode in cathode biased amps, or via a voltage divider off B+ to create a reference in the 20-40V range.

[chief mushroom cloud]

interesting that 'some' followers have >100Vh-k and no one addresses that, and they seem to work ok and prob 99.9% amps w/ no elevated heaters don't hum much at all (none of mine do)

I find more 'hum' from a sketchy grounding scheme than any other source

and terrible PT-choke-OT layout (inductive coupling)

[FUCHSAUDIO]

I've achieved the quietest results with a DC lifted AC line (if not doing true DC heaters).

The basic idea is to minimize the DC voltage differential between the cathode of a preamp tube and it's heater. 1-M over a 2.2-K (half watts fine) off a 400-V supply (approximate) with a small (1-UF or even smaller is fine) filter works good. Don't make the cap too big, as it would take time for the network to pull up to proper voltage over time, do to the added capacitance.

You want ideally, to have the 6.3V right around the average cathode voltage of 1.5 or 1.8 volts or so. A hum balance pot, with it's wiper tied to the lift circuit, will do nicely.