40+ watts SET, cost is not a problem

@jond 

As usual a very descriptive and informative article by Andy Grove. In addition to audio/electrical engineering talent, he is a very adept writer.. Appreciate his distinctions between the 845 and 211 DHT tubes and why he decided upon the former in the Ankoru amplifier.

Charles

@charles1dad Very much agreed and glad you enjoyed it!

Most audiophiles who have heard tube amps have listened to "Golden Age" push-pull circuits that date from the Fifties, with a few modern tweaks like regulated supplies or a current source or two sprinkled through the design. A smaller number have heard low-power SETs in the 2 to 20-watt range.

Looking back, the "Golden Age" circuits were optimized for power above all else; there were no 100-watt transistor amps back then, so it had to be tubes, and high-voltage transmitter types were never sold as consumer products. So the tubes used then were 6L6, EL34, KT88, 6550 and other similar types. If you optimize for power, you can squeeze 60 watts out of a pair of 6550’s, or more if you’re really willing to hammer on them.

This requires Class AB operation with a relatively small Class A region (to increase efficiency) and using 20 dB or more of feedback to both linearize the pentodes and smooth out the crossover region. This is the real reason a Golden Age amp, or a modern copy, sounds quite different than a zero-feedback SET. The SET relies on the inherent linearity of the triode power tube, while the Golden Age amp relies on feedback to do the heavy lifting.

For most of the history of audio, high power and the highest quality sonics have been in opposition to each other, although the marketers always want to sell the most expensive amp ... which these days means an amp with banks of power pentodes sizzling away in the chassis.

Rest assured, any amp with arrays of devices operating in Class AB will NOT be the most linear amplifier, since every device will have a slightly different AB transition point. Perfect matches are impossible to maintain, even if they come out of the factory that way. Tubes drift as they age.

There’s a rule of thumb that if linearity is important, minimize the number of output devices. One device is obviously the lowest number, but SET output transformers have serious design challenges once the power gets much beyond 20 watts. A useful compromise are two devices running in push-pull, but biased into Class A, like SET amplifiers.

Unfortunately, our 60 to 100 watt Class AB amplifier now shrinks to 15 to 20 watts if re-biased and re-designed to run in Class A. Yes, you can build really big PP amps using transmitter tubes, but these are notoriously hard to drive, and safety considerations become very serious with B+ voltages of 1000 volts or more ... audiophile construction technique is NOT appropriate at these voltages.

To be blunt, if your speakers need more than 20 to 30 watts to fill the room with full dynamic sound, your choices for amplifiers narrow quite a bit. If tube amps are a must, there are Class AB PP pentode amps with 4, 6, or 8 output tubes. This is expensive and no fun to keep in balance. There’s good old Class AB transistor, using either bipolar or MOSFET output transistors. Or modern Class D amps, preferably with the latest ultrafast GANfet output devices.

@lynn_olson

but SET output transformers have serious design challenges once the power gets much beyond 20 watts.

In the link provided above, Andy Grove (Audio Note) acknowledged that in his article and explains his remedy. By many listener accounts he was successful with the 845 tube amplifier output transformers dilemma.

I wish the OP could sample your Black bird designed 300b push-pull with the TAD speakers he wants to drive with tube amplifier. 17x13 ft room, pretty much near field environment.

Charles

Hmm, well the Blackbird puts out an honest 22 watts, with half-second transient peaks probably around 60 watts. They power highly reactive electrostats with no issues, because of deep Class A biasing and zero feedback ... the load-lines for the PP 300B’s are nearly perfectly straight, which is ideal for reactive loads. With zero feedback and power supply isolation between drivers and outputs, the amplifier behaves like a limiter/compressor as it is pushed beyond 22 watts. (The power supplies are good for 200 watts per channel, plenty of reserves there. The voltage and current limiting is in the 300B’s.)

But for inefficient speakers with high-order (18 or 24 dB/octave) crossovers ... probably not. Speakers like that need an amp with a high damping factor and at least 200 watts of clean power. High damping factor typically requires high feedback, and high feedback also means hard clipping. Hard clipping sounds terrible and can destroy tweeters, so the amp should never be operated in the clipping region. So lots and lots of power, enough to never approach clipping.

Modern Class D amplifiers, particularly with GANfet switching transistors and well-designed PWM modulators, are probably optimal for these kinds of speakers. This class of amplifiers avoid those annoying Class AB colorations, and don’t have problems with bias drift as they warm up. And power reserves are large enough to avoid the clipping region.

Note there are two approaches with not that much in-between. Either moderate to low power (2 to 20 watts) with very gentle clipping and modest damping factor (between 3 and 4), accepting that clipping will happen but will have low audibility, OR 200 to 500 watts of power and clipping never happening. There’s also a handful of low-feedback low-power Class A transistor amps, which I guess are sort of a halfway house between the two.