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!

SPIRIT II - SPECIFICATIONS

Amplifier Ayon Spirit II
Class of Operation Triode or Pentode mode
Tube Complement 4 x KT88, 4 x 12AU7
Load Impedance 4 & 8 Ohms
Bandwidth 15Hz-60kHz
Output Power / Pentode mode 2 x 60 Watt
Output Power / Triode mode 2 x 40 Watt
Peak Output Power / Pentode mode 2 x 75 Watt
Nominal voltage gain 40dB
Damping factor 1.02
Frequency Response 27 Hz – 42 kHz/ +/- 0.5 dB
Input sensitivity for full power 1V
Input Impedance at 1 kHz 100K Ohm
S/N ratio at full power 80 dB (2mV)
Hum 0,003 V
NFB 0dB
Volume Control Potentiometer
Remote Control Yes
Inputs - Output 4x Line, 1 x Direct IN, 1 x Pre out
Polarity invert +
THD at 1 Watt <1%
Power Consumption 145 Watt
Dimensions (WxDxH) cm 46x34x26 cm
Dimensions (WxDxH) cm (shipping) 57x47x35 cm
Weight 75 lb
Shipping weight 85 lb
Specifications subject to change without notice

This is where they allude to Class A operation:
Pure Class A, Zero Feedback

Is this enough, Allen?

I see no reference to Class A operation. Zero NFB does NOT imply Class A operation, they are two very different things.

And if it was pure ClassA, then the power consuption would need to be MUCH higher than 145 watts. As I said, 4 times as much as the triode output power - which would then need to be 40 x 2 x 4=360 watts. But it only uses 145 watts.

Regards, Allen (Vacuum State)