Friday, June 5, 2009

Push Pull 2A3 tube amplifier complete

From 2A3 Push Pull Tube Amplifier

After neglecting this project for over a year because of a variety of reasons, I finally got it working over the weekend. I addressed a couple of irritating initial issues namely:
  • Electrical hum due to a ground loop (shortened the ground return paths)
  • Mechanical hum in L1 of the LCLC power supply filter (replaced L1 (320 mH / 10 ohm Triad choke with a 10 ohm resistor resulting in an RCLC filter). Since the output stages is push pull, they can excelled common mode noise rejection and aren't bothered by what I simulated to be around 1V PP of hum. The input stage is quitened down by a rather severe RC stage dropping about 50 % of B+ (290V).
I'll add a schematic later on, but the circuit itself is exceedingly simple: Single ended D3A (in triode mode) driving a rathe expensive (and luckily for me, free) Electra-Print SE:PP interstage transformer powering a push pull pair of 2A3 triodes in cathode bias . Output transformers were scavenged from a dead Fisher 800C receiver and I'm estimating output power in the 6W / class A range with some A2 capability (though I could probably get this circuit to run about 15 W class AB with some minor mods + fixed bias).

No global feedback. Some minimal local feedback around the cathodes of the push pull pairs via 30uF capacitors (all I had at that moment). Bias is relatively conservative around -45 V and PK voltage of about 245V,

Power supply capacitors are around 50uF ASC motor run capacitors. Still need to bypass them .

Power transformer is a heavy duty low DCR (< 10 ohm) 240:110V transformer run backwards in step up mode.

Sound: I'll let others decide, but I think it's fair to say that, unlike some of my other "endeavors", this has been a successful high end project.

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