300b lovers

Which I guess leaves the question why does the input stage have the topology it does. The quick-and-easy approach would be a Mullard-style long-tailed pair or differential stage, or maybe borrow from Williamson or Dynaco and direct couple a half-6SN7 input tube to a "concertina" or split-load inverter. It would certainly be cheaper, and is the approach of just about every Golden Age amplifier.

This is the intuition part. There is something wrong with the sound of Golden Age amplifiers ... hard to describe, and it’s not there in SE amps. Something to do with diminished low-level detail, subtly flattened tonality, and a lack of air and "shimmer". The folks at Sound Practices were confident that this "PP" coloration was inherent in push-pull itself, and that’s where I parted company with the common wisdom.

I became convinced the problem was the phase-splitter tube. For one thing, the three approaches to vacuum tube phase splitting (split-load inverter, long-tail pair, floating paraphase) sound quite different, and they all have varying levels of that "push-pull sound". So why not take a passive approach? Studio transformers have been around a long time, and if they are good enough, retain phase integrity through 20 kHz. Then the rest of the amplifier can simply be fully balanced, with none of the circuitry devoted to phase splitting, just amplification. Do one thing, and do it well.

Sure enough, even in the first version of the Amity in 1996, the coloration was gone. It didn’t sound push-pull, and it didn’t sound SE, either. It sounded like itself, and not like anything else. The rest of triode community went their own way, off in SE-land, and I did a lot of historical research for Glass Audio and Vacuum Tube Valley, while thinking of the next steps beyond the 2-stage Amity amplifier.

I’m sure this will sound wonderful - but I can’t stop and think about why not simply run 845 SE, what are the sonic factors driving the adoption and development of 300b push-pull?

I do understand the desire to avoid working with high voltages however.

The 845 is not happy at 500 volts. It is a (low-power) transmitter tube, and is designed to work from 800 volts (minimum) to well over 1000 volts.

Once you go over 500 volts, construction, and the parts required, are a whole different world. It requires ham-radio transmitter technique. Parts are air-spaced, wires DO NOT lay on each other, circuit boards are out of the question, and special-order high-voltage power and output transformers are required. Electrolytic caps have a hard upper limit of 550 volts, and 1 kV film caps are industrial parts, not audiophile specials. In short everything is different. Consult a 1950’s American Radio Relay League (ARRL) handbook to see what safe construction technique looks like. It is nothing like audiophile practice.

Sure, the builder can ignore safe construction technique and build it the regular way, but that’s a very serious safety and fire hazard. You do NOT want an amplifier exploding and then catching on fire. Transmitter technique takes us out of consumer electronics and into the realm of professional high-voltage equipment ... interlocked chassis doors, special start-up techniques, status lights, etc.

Yes, I see audio equipment at shows with hard-core Eimac transmitter tubes that light up the room. I would never allow anything like that in my house, unless it was in an outbuilding. The companies that build these high-voltage amplifiers have no track record of building ham or pro radio gear ... they’re just winging it, despite the curved glasswork and the pretty CNC chassis.

By contrast, the 300B lives in KT88 territory, with similar voltages and operating currents. Standard hifi building technique, but still not a plaything. The voltages in the B+ caps are quite lethal, so no poking fingers where they don’t belong.

Best thread on Audiogon in a long, long, long time.  Great information, and has me wishing I could attend the Seattle show.

@lynn_olson

The usage of 845,211,805,GM 70 and other high voltage transmission tubes have been manufactured and sold by very reputable and established brands for quite awhile now. They’ve been enjoyed by owners for years without reliability or hazard issues. So it appears they’ve addressed the real world problems you logically cautioned about.

I will say however that this new approach/design of utilizing 300b tubes in push-pull is highly interesting and frankly exciting. Certainly in terms of acquiring higher tube power (Thus expanding speaker choice flexibility). I’m really looking forward to reading listener feedback from those attending the Pacific Audio show.

Charles