Johnny saidThanks B! Appreciate the help and the insight on CCS/cathode in regards to the differential circuit… Really hope the regulator can handle the increased load because the 6N6P sounds really good, just don’t want to leave it in without knowing long term effect… when looking around I did have on a short list the 12bh7, 6cg7/6fq7, 6gu7, etc… to try,… all of which can be bought cheap… comparatively speaking of course, and have suitable load lines…
Best,
Well, I took my preamp out to my lab and set up to check the heater regulator’s input and output voltage for the stock 12AU7 load vs. putting in a pair of tubes that would draw more current and still be pin for pin compatible. The tubes were 12BH7 which draw 600 mA each for a total of 1.2A. The input to the regulator with the stock 12AU7 tubes was about 11-11.5V for 120 V AC line voltage. However with the 12BH7s, it dropped down to about 8-9V and the regulator was just dropping out for 120 V line voltage and the output was showing ripple break through. NOT!! So some of the tubes of interest like 6H30, 5687, and others which would work for loadline considerations just won’t work for heater current limits. What might work is a 6CG7 which is very similar to a 6SN7 and would draw about 900 mA which might just work and would worth a try. Someone had asked about a Western Electric 2C51. That one is OK for heater current but is not for loadline considerations. If I learn more or look into some more tubes, I will update this post.
BHK