Class of operation for Tube Power Amps

ALL SET consumer tube amps are class A.

MOST PP tube amps are AB, but there are (or were) purist class A PP designs, VAC's Renaissance designs, for example, were PP 300B class A. I did a search for new PP Class A amps and found only kits or inexpensive units.

Class A designs will (probably) use up output tubes more quickly.

Each 135lb VAC Renaissance 140 monoblock used 8(!) 300Bs per channel and made any other source of room  heating unnecessary. Sometime in the '90s I heard a system at Sound by Singer with a pair of those driving JM Labs Grand Utopias making glorious music.

I used the stereo VAC Ren 70/70 for many years but eventually the cost of replacing a matched octet of 300Bs - and the near impossibility of moving the amp - caused me to trade it in.  I still have the little brother, the Ren 30/30 but it's in need of TLC.

If you want a class pure class A unit look for a good condition used VAC 300B.  I met Kevin Hayes at Sound by Singer and he said (in the 90's) that this was, probably, his best amplifier sonically but it was not really commercially viable as he couldn't make much margin on a 30 watt PP amp built to his standards.

1. 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. 

Interesting comment but we make tube mono block amps that have 4 output tubes and they are class A operation.

So the answer is to ask the manufacturer.

Happy Listening,

Not always will the number of output tubes dictate if it’s class A or A/B there are a few parallel class A amps with more than one output tube per channel. I know a couple of parallel class A SET’s for example.

I think doing a quick search on the amp in question on the internet will tell you quickly what class type it is. Most amp makers that are using a different class design i.e. parallel, etc. will state that as a selling feature.

Heat won’t really tell you much in a tube amp as tubes need heaters to function and they make more heat than the class type, usually. 

As mentioned above, SET is pure class A "by definition", though you can have SET with paralleled output tubes which - from the outside - is hard to distinguish from a push-pull configuration. 

Most tube amps are push-pull AB. The reality is that running power tubes in full class A necessitates either drastically shortening tube life or cutting power output way, way down. Neither of which is conducive to sales. And on the flip side, though I have no hard evidence - if "feels" to me that most of the sonic compromise attributed to Class AB solid state (versus Class A SS) is mitigated in the PP tube domain - maybe due to the larger amounts of "benign" low order distortion (masking effect) and/or lower amounds of GNF - contributing to masking the AB crossover distortion (or whatever).

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. Lots of Mullard power tubes out there with visible horrid scorch markings on their plates, that still measure & play fine lol. 

Tube pres and phono stages are pretty much all class A - power is not an issue there. There’s no reason not to do class A. 

This would be a great question for @atmasphere, my guess idle power consumption! 😎

Mike