Simply rectifying the existing CT HV winding for negative gets us all the way.
Using this approach I've used this several times to supply tail resisters of two stage
diff amps & the negative bias for the PP output stage, This vers has a slow start mode built in.
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Back biasing in an audio amplifier in which the pre-stages consume little current compared to the power stage, and with the negative voltage only applied to the control grid(s) of the power stage, is almost a form of cathode bias since the negative voltage depends mostly on the current that the power stage is drawing.
Agree.
For me it IS cathode bias, just with ground taken at a different point upstream, so by definition some part of the string is "negative" (relative to the "new funky ground" that is)..
For me it IS cathode bias, just with ground taken at a different point upstream, so by definition some part of the string is "negative" (relative to the "new funky ground" that is)..
Is it back bias? The key of the approach I showed was to put the smoothing components on the negative rail, and ground the supply after the second choke. 90V is dropped over the last choke (field coil), and a resistance divider delivers -45V to grid of the 2A3.
Most back bias solutions appear to be harvesting negative volts from the ripple before it is smoothed, using a diode.
Here is a similar example from a Geloso where the negative bias is extracted from the smoothing for the DC filament supply.
I see now there are different variants of 'back bias'. Some use an AC supply, or one with significant ripple, and a diode to derive a negative supply. The link from JStewart show two variants, using a field coil on the positive or ground, deriving the supply from being elevated from chassis ground.
Rectify the 6.3V and then use a DC-DC step up converter. Just make sure that you get one that is not having common ground, they are only a few dollars on eBait. Suggest you also set up protection in case negative bias fails. (I've got a bunch of them that I bought for making a tube tester that never eventuated, other stuff intervened and now no longer a need for a tube tester.)
Thanks - I got that! So the little circuit I see, like the RCA design mentioned previously, (or some derivative) will do for units having a center tapped secondary.
and Kodabmx - thanks! I'm probably going to go that route on the amp w/o center tap HV secondary.
Great idea - I never knew such a thing existed. Now, I wonder if they have a PSK-3D-30 model?
Bad word choices, right up to saying the opposite of what I wanted to say. I understand cathode bias, done right on all 4 tubes individually, you'll never have to worry... I'd say an illegitimate circuit would be one that does something really undesirable as a side effect, and at the same time that detail isnt all that easy to see.
That's interesting; 5W would limit the current to 250 mA, pushing the zener to the edge...