Class of operation for Tube Power Amps

dynamiclinearity

In fact any push pull amp solid state or tube is unlikely to be class A at very high powers although it may be what's called rich class AB . . .

My former monoblocks, CR Developments Artemis, were P/P Class A - all the way.

@steakster what do you mean by class A all the way? The amplifiers may actually have higher power in class A/B above what they are rated for in the spec sheet at class A.

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. 

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