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In Reply to: RE: Questions about 6BG6G amp build (schematic) Grommes 216BA posted by Eli Duttman on May 23, 2017 at 12:00:31
Thanks for that - actually I was studying my old (1960 original! - with voltages) Heathkit W5M manual that made me consider variations, such as higher filter choke value and the 5R4 rect. But then I'm deviating from the Grommes design a lot, taking chances w/ things I can't properly measure.
What about plugging 12FQ7 into 12AU7 sockets?
"What about plugging 12FQ7 into 12AU7 sockets?"
That will work, if and only if the 'U7s are wired for "12" V. heater power. OTOH, the 12BH7 is 100% pin compatible with the 12AU7 and, as long as the doubled heater draw is OK, drops right in. While (perhaps) not quite as linear as the triode found in 'SN7s, 'FQ7s ..., the 12BH7 is much more linear than the 12AU7. IMO, the only audio jobs the 'U7 triode is suited to are "concertina" phase splitter and cathode follower. Inside the bottle, local, NFB saves the day, in those applications.
Thanks for that info, I have lots of 12BH7, and the other tubes. But not so sure about that much increased heater draw.
12FQ7s are getting quite scarce. It would be kind of you to let the guys who need them have first crack at them.
Besides, the heater winding in the amp is a 6.3 volt winding - so a very minor rewiring is all that's needed to plug in 6CG7/6FQ7s.
12FQ7 isn't a direct plug in for 12AU7 anyway. It can't run on 6V, no connection to pin 9.
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