My understanding is tube amplifiers can be ran in Class A or Class A/B operation just like Solid State amps. MANY tube amps do not say what class they are running. If they don't say this in the specifications do you just assume the are Class A/B. How can you tell?
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.
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.
Observe the heat: Class A amplifiers run hotter than Class AB amplifiers.
Consider the design and circuit details:Class A amps often have simpler designs and fewer components.
Consult the manufacturer's specifications:The manufacturer's documentation or website will usually specify the amplifier's class of operation.
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.
@westcoastaudiophileIt's not simple if you are looking to buy an amp and don't have access to it.
@gkellyThanks 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.
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.
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.
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.
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.
@steaksterwhat 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.
@westcoastaudiophileThis 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.
@mulvelingJust 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.
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.
+1 wrm57 - I also own an Aric Audio Transcend "Push Pull" EL34, and it runs exclusively in Class A. It also is the finest tube amplifier I've ever owned, of the 6 I've owned over the years.
Aric Kimball of Aric Audio makes some extraordinarily good preamps and amps, using the highest quality components available. He'll also make custom changes if one is so inclined, and upgrade tubes to NOS tubes if they are available.....and I find his products to be rather affordable compared to much of the competition.
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