Number of tubes in Pure Class A amps

Furthermore, does Triode mode mean Class A by definition, and, conversely, does Pentode mode mean AB? Is Ultralinear the same as Pentode? (eg Manley Stingray and TAD 1000 have Ultralinear/Triode modes.

>>Is the power output ability in pure Class A not a linear function of number of tubes?<<

It should be, but there are several "classes" of Class A.

Pure Class A1 in triode has an efficency of 25% output power compared to the actual heat dissapated in the tubes - so to get 50 watts of audio you need to "waste" 200 watts in heat. A KT88 has a maximum rated anode dissapation of 40 watts, so would need at least 5 tubes to be safe, and that's not possible in a push-pull amp, so 6 would be a minimum.

But some manufacturers care about reliability as well as advertised power output, so may use more tubes than absolutely needed so as to run the tubes at (say) only 75% or even 50% of their maximum ratings.

This lowers the temperature in each tube and promotes far longer life.

But using the tubes in ultalinear or pentode/beam tetrode mode will increase this efficiency to around 50%, so you will get about double the power with no more tubes.

Realise, not all manufacturer's are 100% accurate/truthful when they call an amp Class A, maybe it's Class A up to a certain point and shifts over into Class AB something when pushed hard. So you might get a higher power rating for a given number of tubes from them.

Caveat Emptor...

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>>does Triode mode mean Class A by definition<<

No, there is no connection. Triode mode normally refers to pentode/beam tetrode tubes (e.g. EL34/KT88) wired as triodes as disticnct from real triodes (e.g. 300B, 845) and they can run in any class, A, AB, B, or even C.

>>, conversely, does Pentode mode mean AB?<<

No, pentode means pentode operation of a pentode (e.g. EL34) tube. A pentode can also be run in any class.

> Is Ultralinear the same as Pentode? (eg Manley Stingray and TAD 1000 have Ultralinear/Triode modes<<

No, ultralinear is a circuit topology where the screens of the pentode or beam tetrode output tubes are connected to a tap on the output transformer, giving (it is claimed) the sound and characteristics of triode with the power of pentode. It is easy enough to even have a three way switch that can actually change from pentode/tetrode output to ultralinear to triode mode in a EL34/KT88 tubed amplifier.

Each variation will sound different, personally I only like triode mode, (or real triodes) but that's my preference.

Regards, Allen (Vacuum State)

That answered all my questions. That's why I love Audiogon. Instant experts on tap. Thanks, Allen!

So Ayon may indeed be accurate in saying that it is Pure Class A, both in Triode and Pentode? But the 4 tubes only seems a stretch?

>>So Ayon may indeed be accurate in saying that it is Pure Class A, both in Triode and Pentode? But the 4 tubes only seems a stretch?<<

Can't say unless you give me the exact reference of the data on the Ayon gear you refer to. I'm good at what I do, but I'm not a mind reader...

Regards, Allen (Vacuum State)

Allen

This has always been something of a mystery to me the exact difference within tubes, and I have never seen it written about in such a straight forward fashion. Really insightful in the best possible way!