Push-Pull Class A Amplifier

Description: This stereo 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 tube, the 5687 is a 9-pin miniature twin triode (similar to 12AX7, 12AU7, etc., but with a different pin-out). Its specs are quite interesting: mu = 16, gm = 5400µS, rp = 3k, plate dissipation: 4.2W each unit or 7.5W total (3.75W x 2). After analyzing the curves (reprinted in The Parts Connection Catalog), it was noted that some power could be extracted from them, as these tubes were designed for high current pulse applications, hence the high perveance.

Parallel operation of the two sections approaches the rp of a 45. The higher mu and gm facilitate easier driving. Initial attempts focused on maximizing power output from the 5687s. A breadboard circuit was powered by a 350V external power supply, operating at a standing dissipation of approximately 3.5W per triode section (315V @ 11mA). Cathode bias was employed for stability, allowing the use of larger grid resistances. Initially, 220k grid resistors were used but were later reduced to less than 100k to prevent thermal runaway. This configuration performed adequately, even with 12AX7s driving the grid load.

To explore the theory further, one half of each 5687 was configured as a cathode follower (directly coupled to the output) to drive the other half, effectively buffering the 12AX7 input stage. Measurements indicated no significant improvement in drive signal linearity; however, a decrease in output was noted when transitioning from parallel output triodes to singles, which was not substantial given the high load resistance (12k p-p).

The input stage consists of both halves of a 12AX7 dual hi-mu triode arranged in a differential amplifier configuration with a 1mA constant current cathode sink. Field-effect (constant-current) diodes were utilized for simplicity. This current source/differential amplifier approach was selected to enhance power supply rejection, thereby simplifying the power supply design and filtering requirements. The constant current diode cathode is returned to a -18V supply (derived from a full-wave rectified small 12V transformer) instead of ground to mitigate non-linearities in the pinch-off region (Vpo ≈ 1.5V). The stage gain is approximately 20 times into a 100k following grid.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).

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 filtering requirements. The constant current diode cathode is returned to a -18V supply (full-wave rectified a small 12V xfmr) instead of ground in order to avoid the non-linearities about the pinch-off region (Vpo ~= 1.5V).

The stage gain is about 20 times into a 100k following grid.