Class of operation for Tube Power Amps

@westcoastaudiophile 

my comment was related to “real” class A design, not hybrid (pseudo ab) A2/A3 with enabling output tubes grid currents! 

Class A is defined simply as the output device does not go into cutoff at any point of the waveform, regardless of output power, right up to clipping. Class A2 and A3 meet that definition; are not 'hybrid' or any such since you can run class A2 or A3 in a single-ended embodiment.

The only class A3 amp we had in the shop was an SET. Running a single 300b, it made 50% greater power with less distortion running zero feedback than a class A1 amp can using the same tube. 

IOW all three variants are "real"; none are AB in any respect. 

A2 and A3 require the driver of the power tube to maintain linearity when grid current is present. AB1, just so you know, does not require this, no grid current. AB2 does. Its my surmise, although I've not talked to the inventor (Jack Elliano) of A3 (for which he holds a patent) about this, but a class AB3 is likely also possible. 

just plug amp into ac power (or current) measuring unit, and dial the volume control.. class A power consumption is not sensitive to volume, while AB will consume significantly more with volume increase. 

@westcoastaudiophile This is only true if the amp is class A1. If A2 or A3 things might be different. 

Look at the number of output tubes: If the amp has only one or two output tubes, it's more likely to be Class A. If it has four or more, it's likely Class AB. 

This statement is misleading. We make tube amps with 20 power tubes and they are class A. Whether its class A or not depends on the operating point of the tube (plate Voltage and plate current) not the number of tubes!

Cathode bias circuits used to be quite common - these tube amps don’t have normal bias adjustments - maybe a "balance" pot at most (to help match the push/pull sides). These amps run hotter per Watt, and much further into class A territory. They’re not so common now; auto-bias circuits (Prima Luna, VAC, ARC) are the new way to achieve cooler running combined with "optimized" performance, and old school cathode bias eats up modern Russian / Chinese power tubes. It was a different story back when we had golden-era tubes aplenty - Mullards / RCA / Tung-Sol / Sylvania / etc - that you could abuse all day (every day) and they’d keep ticking for years.

@mulveling Just so you know this statement is 100% false. You can cathode bias a pair of power tubes to be AB and running a power tube cathode biased does not affect its life in class A, so long as the tube is operated within its specified parameters. So those tubes you're referring to above 'kept ticking' because they were not being abused.