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I Never Met A 2A3 Amp That I Didn't Like
Article By Editor Joe Roberts
From Sound Practices Issue 15

  Nothing in audio is a sure deal but if I were forced to come up with a foolproof recommendation to stake my name on, I would have to go with the classic no-brainer: "It's hard to build a bad 2A3 amplifier."

There might be some bad ones somewhere but I have to say that every 2A3 amp I ever had the pleasure to hear was at least pretty good and some were downright amazing.

My introduction to triode amps came with restoring a few Brook 2A3 amps of the early 1950s, which I didn't get to hear until they were certified antiques in the early 1980s. Even in the late 1940s / early 1950s when they were current models, the Brook designs were a throwback to prewar technologies (see Sound Practices issue 2), valued by a few purists who knew that those newfangled pentodes were nothing but a sell out.

The Brook 10C wasn't even a pure Class A design or anything. It used a sliding bias adjustment scheme to switch the push-pull 2A3s way into AB2 when the signal peaked. Yet, despite this elaborate and somewhat dubious conception, the 10C still sounded very very good. Real, real good, in fact.

In an attempt to come up with a simpler amp than the Brook 10C, I built the "Recording Amplifier" shown at right (minus the mic preamps). Glory, this was a mighty fine sounding amp and I'd build another one in a second. The "triode sound" is still really there with PP topologies and... Man, you can really get some low end out of a pair of these tubes. After all the 2A3s were chosen to run a record cut ring head because of their good damping behavior even sans feedback.

Yeah, somehow PP 2A3s fell through the cracks in this age of SE mania, but, on the other hand, maybe a single 2A3 amp is about as good as it gets. My first exposure to SE amplifiers was a shot in the dark. I remember looking over a SE 2A3 circuit in the 1940 Radiotron Designer's Handbook (third edition), provided as an example for calculating the response of the circuit and thinking "Hmmm?"

With a few voltage adjustments, I figured I could scrape a single amp out of my "Recording Amplifier" by hooking the 6J7 straight into the grid of one 2A3. I didn't have any SE transformers back then. I just ran the DC through the primary of my vintage push-pull UTC LS-55 transformer.

When I turned this Frankenstein on I was floored. It sounded way better than it should have. It was a looser, more open, more organic sound than the PP 2A3, and although the bass wasn't as tight as the PP amp, it was very expressive. This experiment was obviously taking me somewhere I thought I might like to be.

Now, after years of study and experience, I might comment on how the 6J7/2A3 combo is reminiscent of the classic WECO Model 91 pentode driver/300B circuit and blabber on in an Ultra-Fl "dress-to-impress" patois about esoteric bullshit that doesn't matter anyway. Seems like everybody is a triode amp expert today, but back then I didn't know anything but what [was hearing. That's all I needed to know. The rest is history.

One of the things that I know now that I didn't back then was that Nobu Shishido, one of the ‘crazy Japanese guys' who revived the triode amp wrote the first article about SE in the modern era in a Japanese experimenter mag about a 2A3 "Loftin-White" he originally built from dumpster TVs while an MBA student in Oregon during the late 60s. He didn't "go single-ended" to be hip. It was a cheap and easy way for a poor tinkerer to get some music playing in the dorm. Anyway, that amp ruined old Nobu for life.

The last SE 2A3 amp I built was inspired by JC Morrisons' degenerate Loftin-White lash-up called the Micro 3.5 from Sound Practices issue 6. The custom features of my implementation included a bank of Western Electric oil caps and a tasteful pink-and-white speckle over gray primer paint scheme. There were only a few lonely parts underneath that big, ugly "budbox" chassis, making it an air box beyond anything ever dreamed up by the mass marketeers.

Also, I used indirectly-heated 6A5-Gs instead of directly-heated 2A3s, which sounded just like real DHT 2A3s to me but there was zero perceptible background noise even on a horn. It was a weirdo, one-off creation and no big looker, perhaps, but this amp kicked ass and took names, I swear!

I gave the 6A5 amp to a Korean friend of mine, who eventually lent it to a hi-fi magazine editor back home. The editor has since refused to return it, probably because it sounds better than almost anything you can buy1 No surprise there. Look, it showed up in the pages of a Korean audio mag".

Nobu Shishido probably loves the 2A3 as much as any man who ever walked the earth, but he has moved on in his own technical/aesthetic quest to exploring the grid circuits of transmitting tubes and the practical work of restoring old opera 78s. He downplays the 2A3 with a been there, done that attitude — "Sure the Loftin-White was a great amp but if it was that great why isn't everybody listening to Loftin-Whites?"

I'd be the last person to argue against audio diversity and the forward march of progress,  but I do believe that everybody should be required by law to enjoy the pleasure of a good SE 2A3 amplifier for a while. There is great reference value in knowing how much you can get with so little.

Fortunately, in this day and age, you can buy super-quality parts to put together a 2A3 of your own, or buy USA-made 2A3 amps, your choice of finished or kit form. For the last 60 years, you had to really work to hear 2A3s, but now it's probably easier than it's ever been. That is a strange historical fact and the more I think about heavy historical facts like that, the more I want to stop thinking, spark up a 2A3 myself, and just listen, if you know what I mean.