Semi starved plate tube dilemma
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Gnobuddy
diyAudio Member

Join Date: Mar 2016
Quote:
 Originally Posted by Zebb Actually these aren't mine, they're simplified theoretical circuits from Merlin Blencowe's article on Triodes at Low Voltages.
Yes, I got that from your first post.
Quote:
 Originally Posted by Zebb My dilemma is with biasing. Fig. 4 has an impedance buffer opamp that is powered by a single sided +12v supply, which means, I suppose, that the signal idle would have to be biased to +6v as well as the +2.6v for grid biasing.
Trust me, I understand the circuit, and there is no dilemma!

The op-amp isn't biased to +6V plus +2.6V. It's just biased to +2.6V. The voltage is indicated on the schematic you posted.
Quote:
 Originally Posted by Zebb But my circuit would be fed by opamps with a bipolar supply of +/-15v each side, so the signal idle will be at 0v, which must affect the biasing.
Firstly, the TL071 is a single op-amp; so you can run that one op-amp from a single supply rail: use your 0V and +15V rails. Simplicity itself. Other op-amps elsewhere in your circuit can still be run off the +15/0/-15 bipolar rails.

Secondly, if you do run that TL071 from a bipolar supply, that changes nothing whatsoever. Use R3 to set the op-amps (+) input at +2.6V, and its output will follow. The op-amp doesn't care if its negative rail is 0V, or -15V!

If you still don't see it: the op-amp is wired as a unity-gain voltage follower. The output voltage is identical to the voltage on the (+) input - the non-inverting input. Set that to +2.6V, and the output will also be at +2.6V.

This will be true as long as the op-amp has enough power supply voltage for its output to settle at +2.6V. The op-amp doesn't care if you feed it +/- 12V, +12V and -15V, or +12V and 0V.

Note that the 12AU7 will still be wired as in Blencowe's figure, between 0V rail and +15V rail. (NOT to the -15V rail.)

Quote:
 Originally Posted by Zebb My other dilemma is that I could derive a tube HT from the 30v available from my PSU (+15v to -15v). But would that have any advantage as it would make grid biasing even more complicated.
This is a bad idea, unless you want to throw out all of Blencowe's hard work, and start over doing your own research.

So, keep it simple: forget about the -15 V rail as far as the 12AU7 is concerned. Wire the 12AU7 between 0V and +15V, as Blencowe showed. Add the series resistor to let the heater work on +15V. And that solves the entire problem. No more dilemmas, except what snack to munch on while you solder the thing up.

-Gnobuddy

 11th October 2019, 01:17 AM #12 PRR   diyAudio Member     Join Date: Jun 2003 Location: Maine USA Fig 4 in post #1, the opamp can just as well be fed +/-15V as +12/0V.
 11th October 2019, 11:06 AM #13 Zebb   diyAudio Member   Join Date: Oct 2009 Location: London Great... I'll breadbord it and see how it sounds. Thanks for the advice.

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