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In Reply to: RE: LL1689AM/PP differing currents each primary posted by Andy Nehan on June 18, 2019 at 13:59:39
Andy, in my linestage which currently uses the 6N6PI in differential mode with cathode CCS and LL1689AM (not unlike what you intend), I have installed test points across a 1 ohm resistor, which allows me to actually measure and match the tubes. I have played with this for a while and with a bunch of tubes, but ended up finding that a little DC imbalance doesn't hurt much. Even though this tube is said to be prone to large variations between triodes internally, my (very) non scientific measures on a large sample dictate that many of them are just fair. In then end, I believe that the use of shunt regulation (using Kevin's kits) for the high voltage helps a great deal in improving the output stage and DC balance, and I can only recommend that you try it if it fits your design somehow.
Follow Ups:
Well I intend doing a similar thing, actually test points on the anodes (I have really nice, coloured and safe 2mm test points) as it makes measurement a bit easier as the transformer has approx. 1.2k anode to anode.
I look forward to getting it working for my son. He is with me this weekend so will test the constant current cathode option and also a bias voltage on the heaters, as I have found that also makes a difference (There is a Glass Audio article written by me 6/98 on this topic. Will report back.
" I believe that the use of shunt regulation (using Kevin's kits) for the high voltage helps a great deal in improving the output stage and DC balance"
While shunt reg. will certainly improve sound quality, it won't do squat for DC balance. That requires matching tubes. 6N6Ps are cheap. Buy a bunch and start matching them.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
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