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Ordered 500R pots for the driver stage.

Posted 18th November 2009 at 09:27 AM by Bas Horneman

And while I was at it I saw some 0,22uF Epcos MKV from the same seller. The Epcos will do duty as coupling cap, as I've seen good reports (anecdotal) on them.

I asked Ian (gingertube) what the 470R resistors were for. Because I figured the CCS would take care of everything of the balance.

Here was his answer:
The 1K pot is to set balance in the diff amp. The resistance in the triode cathode degenerates the tube gm (Ia/Vgk). Sweeping the pot wiper from side to side varies the amount of degeneration on each of the 2 triodes until they match. This is the minimum distortion condition for the diff amp. As we don't want to throw away gm un-necessarily we don't want those resistance values too high - just high enough to be able to adjust for matching behavior.
For a random sampling of 12AX7 and ECC803S that I had, 1K was too much, 100R was'nt quite enough so I dropped the 470R on each side of the 1K pot which effectively makes that pot about 600 Ohms (about 300 Ohms each side of the wiper when set to the middle). If you can find a 500 Ohm pot then you can use that, in fact if you select a tube with matching triodes then you can get away with a 100 Ohm pot.
Hope this explains what those 470 Ohm resistors are doing.
I then asked how I should use the pot. To which Yves (Yvesm) answered:
Two ways:
Check for the lowest THD or simply for the same voltage at the plates of the triodes.
(Attach the voltmeter between plates and trim to read zero)
The Epcos was 1,90 on eBay. I did a search on Conrad. Their asking price was 16,58 But then again. Conrad's version looks way better. (3rd pic) O..yes and ofcourse mine look used.
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