Push-Pull Class A Amplifier

This stero amp will give you 4-8W/ch of zero feedback triode power for less than a pair of Chinese 300B's ...if you know where to shop. For those of you who are unfamiliar with this lil' tube here's the poop: The 5687 is a 9-pin miniature twin triode (a la 12ax7, 12au7, etc - but with different pin-out). Its specs are quite interesting: mu = 16, gm = 5400uS, rp = 3k, plate dissipation: 4.2W each unit or 7.5W total (3.75W x 2). After playing with the curves (so kindly reprinted in The Parts Connection Catalog) I realized that you could get some juice out of them (these tubes were apparently designed for high current pulse application - hence the high perveance).
Push-Pull Class A Amplifier - schematic

Parallel the two sections and you start to approach the rp of a 45. The higher mu and gm mean that its easier to drive. No way it could sound as good, tho'!? Without getting my hopes up - I decided to give it a go! At first I tried to squeeze as much power as possible out of the 5687's. I ran the a breadboard circuit from a 350V external ps at a standing dissipation of approx. 3.5W per triode section (315V @ 11mA). I used cathode bias for stability (and so i could use a larger grid resistance). At first I used 220k grid resistors but soon had to lower them to < 100k to avoid thermal runaway. This worked pretty well, even with 12ax7's driving this grid load. Just to test the theory, I decided to use one half of each 5687 as a cathode follower (direct coupled to output) to drive the other half (thus buffering the 12ax7 input stage). Measurements suggested no real improvement in drive signal linearity. Only noted the requisite decrease in output going from parallel output triodes to singles (not so significant given the output high load resistance (12k p-p). The input stage is comprised of a both halves of a 12AX7 dual hi-mu triode in a differential amp configuration with a 1ma constant current cathode sink. Once again, I'm using field-effect (constant-current) diodes for simplicity. The current source/diff amp approach was chosen for good power supply rejection. In this way I could simplify the power supply design and...

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