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In Reply to: RE: " A mu of 100 tube is usually used in a two stage DC 2A3 amp" posted by GSH on February 24, 2015 at 09:21:06
nt
Follow Ups:
See his last sentence, URL below, two stage DC amp.
GSH just never did his as best as it should be done, with Rk bypassing. Thats OK.
MOST 2 stage DC amps use a mu of 70 or higher ( eg mu 100) tube, ie : 1947 Robin/Lipman, 1960s Asano, 1970s Shishido and 1990's Garber, Uthus, and Fraker.
Jeff Medwin
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Read his last three paragraphs. He too never properly bypassed Rks, but he is talking about doing that now, in the referenced post. Thanks for looking. Regards.
Jeff Medwin
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Sure, thats fine, Rage is discussing his very recent experience - of going back to a mu of 100 12AX7 as a driver in a two stage DC amp, and liking it.
This pertains to what GSH and I were discussing, I see no problem, its on-topic.
Jeff
the 12ax7 is better than the 6ej7 pentode (triode strapped) for me right now.
I took some measurements and distortion is slightly lower with 12ax7!
hf rolloff is more but not that much. I will post measurements on the diy forum here pretty soon..
I am actually very surprised by this as I was swayed by the technically correct... and have learned a lot there... but the 12ax7 works IMO. MUCH better than expected.
You would probably need to run the pentode driver as a pentode w/feedback and measure again to see if the distortion from the driver stage can be lowered.
FWIW, the 7B4/2A3 combo at the standard 60mA operating points produced the best 1W THD% I measured when I had that circuit on a breadboard.
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