307A Push-Pull Amp

Posted on Feb 6, 2014

A single-ended tube headphone amp using the TTVJ Millett 307A. I`ve been breadboarding various SE and PP power amp circuits with the 307A for a good year or more, but just haven`t had the time until now to come up with anything I like. As far as I can tell, the 307A was only used in a WWII US army field radio. Even though the only datasheet I can find is from Western Electric,

307A Push-Pull Amp
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I`ve never actually seen a WE branded 307A (and I have a pretty good collection of them). I have 307A`s from Ken-Rad (the most common), and Sylvania, Raytheon (AKA RK-75), and National Union. At least in the pictures the WE tube looks exactly like the Sylvania tube, so I`m guessing that WE may have just re-branded Sylvania tubes (or vice-versa). Finding 307A`s can be a bit of a challenge. My stock came from England. I believe that most of the spares wound up in Europe since this is where the radio was deployed in the war. There are small quantities available at US tube dealers, and I know of at least one guy in Europe who seems to have a big stash. Anyway, back to amplifiers. I have run the 307A in pentode, ultralinear, and triode mode. I have not been impressed with pentode or UL operation - it works fine but nothing to write home about. But triode connected - in this case by connection G2 AND G3 to the plate - you get a very nice triode, somewhere between a 2A3 and a 300B in characteristics. I found some triode curves somewhere on the net. I`d like to credit whoever made these but I cannot figure out where they came from - if it was you, please let me know so I can ask permission (and forgiveness) to post it! The plate dissipation rating is 10 watts, but that seems to be a very conservative rating. I`ve been running them right at or slightly above that point for some time with no problems. This design is pretty...

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