Class of operation for Tube Power Amps

How to Tell the Difference: 

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. 
2. Check the power consumption: If the amplifier draws a lot of power relative to its output wattage, it's more likely to be Class A. 
3. Observe the heat: Class A amplifiers run hotter than Class AB amplifiers. 
4. Consider the design and circuit details:Class A amps often have simpler designs and fewer components. 
5. Consult the manufacturer's specifications:The manufacturer's documentation or website will usually specify the amplifier's class of operation. 

that’s simple!

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 It's not simple if you are looking to buy an amp and don't have access to it.

@gkelly Thanks for the reply. I was looking at 3 different tube amps and none of them address the class of operation in the description or specifications. I do know almost all solid state amplifiers do address this.

willywonka what are the 3 tube amps you're looking at?

gkelly nailed it.  

If a mono tube amp has just one tube or a stereo tube amp has just 2 tubes it isn't likely to be class a, it is absolutely class A. There is no other choice due to single ended circuit topology.

Other wise it's not absolute but the odds are any push/pull amp(two or more tubes per channel) is class AB. 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 meaning it stays class A to a power high enough it almost never goes out of class A into class B.

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.