CCS adjustment question

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Thermionic Idler
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#1 CCS adjustment question

Post by Thermionic Idler »

Considering the two CCS's I have on the anodes of this 6V6 driver stage:

Screenshot 2021-06-21 at 13.12.27.png
Screenshot 2021-06-21 at 13.12.27.png (105.6 KiB) Viewed 384 times

So far I've been adjusting them such that the *current* is the same for both 6V6's, however due to the inevitable variance between valves, this results in different plate voltages in a range from 225V to 270V (although since then I've fitted more accurately matched pairs).

Should I be adjusting them for matched plate voltages instead?
Deck: Garrard 301 - Audio Origami PU7 - Van Den Hul Black Beauty
Phono: Whest PS30RDT
Linestage: AMB α24/LCDuino
Power amps: 'Newton' 300B PP based on a design by Lynn Olson
Speakers: FF225WK 'Big Met' Metronomes c/o Cressy Snr
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Nick
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#2 Re: CCS adjustment question

Post by Nick »

I would ask the question "what do I want from the stage" and adjust for that. I assume that if the stage is already being sent a perfectly balanced signal, I would adjust for matched AC output from the two phases. Or adjust for lowest distortion which will related to the output being AC matched (but require close matching of the valves transfer characteristics).

On the back of that the question would be why would it matter if the voltages were the same, and would that matter more or less than if the current was the same. I would argue (as I did above) that neither matter in and of itself.

If you had a transformer, then matched current would be worthwhile to reduce standing flux, and if the two sets of primary windings were the same DCR then matching the voltages would be a useful analogue to the current to allow you to match the current. But that’s not the case here. But the transformer would have the advantage that the coupling of the windings would force the transfer characteristics of the two triodes to be closer in ways that matter.

Maybe the simple way would be to adjust for the least AC signal on the cathode resistor (without the cap as that makes it even worst and removes all AC coupling between the two valves), that would indicate AC current matching. But you would still need equal signals on the output, so maybe its not the best as it doesn't equate to the AC conditions on the anode unless the valves are matched.
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Thermionic Idler
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#3 Re: CCS adjustment question

Post by Thermionic Idler »

Many thanks Nick, that's a useful insight. I *think* the Analog Discovery can measure distortion...
Deck: Garrard 301 - Audio Origami PU7 - Van Den Hul Black Beauty
Phono: Whest PS30RDT
Linestage: AMB α24/LCDuino
Power amps: 'Newton' 300B PP based on a design by Lynn Olson
Speakers: FF225WK 'Big Met' Metronomes c/o Cressy Snr
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