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EL34 active-load (SRPP) parallel-feed amplifier
This is a little experiment I've been playing with. It uses EL34's in SRPP, or active-loaded, with a parallel feed (no DC) output transformer. Running on a B+ of 600-650V, it puts out around 8 watts.
I connected the lower EL34 as a triode, and the upper as a pentode. This arrangement provided low distortion and really good PSRR. Details forthcoming...
I tried both a Radio Shack 70V line transformer and a Hammond 125J as the OPT. The RS worked pretty well for a really cheap TX, but distorted pretty badly below 100Hz. The Hammond 125J did better, fairly decent at 20Hz up to a couple of watts.
I used a hybrid tube/SS rectifier to get ~650VDC from a 270-0-270 VAC transformer. This worked really well.
Schematic (PDF file, 31k)