Class-A versus Class-AB...did you know?

[Old Tele man]

did you know? -- Class-A is not the only class of power tube operation where plate dissipation goes DOWN as power output is increased, it also occurs with Class-AB!

[VacuumVoodoo]

Originally Posted by Old Tele man

did you know? -- Class-A is not the only class of power tube operation where plate dissipation goes DOWN as power output is increased, it also occurs with Class-AB!

It was supposed to be a secret...

[Old Tele man]

...consider the 55W Class-AB1 example shown in the data sheets:

IDLE = 26.1W (DC), ie: 58mA each tube at 450Vp.

MAX = 55.0W (AC), ie: 210mA(rms) plate current swing and 262V(rms) plate voltage swing.

...but, since the tube doesn't have to "dissipate" AC-power that is "coupled" out through the OT to the speaker load, how much power does each tube dissipate as it alternates between max-signal output (27.5W each tube) and cutoff (no power)?

...well, average current is: 111mA = (58mA + 210mA/4) and average voltage is 188V = 450V-262V, so the average power is 20.8 watts

...which is 20% LESS than at idle! the secret's out!

[VacuumVoodoo]

Actually, that secret has been out for the past 50+ years. Crowhursts articles (1956?) on designing tube amps explain this well too. I'm glad you raised this point.

[John Phillips]

Originally Posted by Old Tele man

...consider the 55W Class-AB1 example shown in the data sheets:

IDLE = 26.1W (DC), ie: 58mA each tube at 450Vp.

MAX = 55.0W (AC), ie: 210mA(rms) plate current swing and 262V(rms) plate voltage swing.

...but, since the tube doesn't have to "dissipate" AC-power that is "coupled" out through the OT to the speaker load, how much power does each tube dissipate as it alternates between max-signal output (27.5W each tube) and cutoff (no power)?

...well, average current is: 111mA = (58mA + 210mA/4) and average voltage is 188V = 450V-262V, so the average power is 20.8 watts

...which 20% LESS than at idle! secret's out!

What happens when the amp is pushed into distortion (a condition almost none of these analyses - that I've ever seen anyway - seem to consider)?

My theory and math is a little rusty to work it out, but this I can say...

Crank up your Class AB Marshall full tilt and watch what happens to the tubes. If that isn't greater tube dissipation, I don't know what is .

(And those are a fairly - or should be - cool-biased Class AB too.)


Not saying you're wrong, just making a crude-and-dirty real-world observation...

[VacuumVoodoo]

You are correct in your observation, John. OTMs example and other calculations in standard texts deal with undistorted sinusoidal signal. When the amp enters non-linear operation and/or OT saturates it's a different story. For a simple analysis assume a full voltage swing square wave i.e. forcing the amp to deliver it's peak power continuosly.

[Old Tele man]

...but, the really interesting point is how that 20.8W average power value is so darn close to our "70%-of-Ppd" value of 21W (ie: 70% of 30W)!

...and, how they get 55W output at a plate efficiency of 65%(!) and at an idle wattage of 87%-of-Ppd!

[VacuumVoodoo]

Originally Posted by Old Tele man

...but, the really interesting point is how that 20.8W average power value is so darn close to our "70%-of-Ppd" value of 21W (ie: 70% of 30W)!

Numerology? Kabala?

...and, how they get 55W output at a plate efficiency of 65%(!) and at an idle wattage of 87%-of-Ppd!

2x6L6GC: 450V plate supply under load (480V idle), Raa=5.5K, Ua(pk)~385V: 52W is what I'm getting at 5%THD and no NFB.

[Old Tele man]

...very close! actual numbers are:

PLATE LOAD (Zoo,Zo,Zo')
a) Zoo = 5600, thus Zo = 1400 ohms;
b) Zo = Zoo/4 = 5600/4 = 1400 ohms
c) Zo' = dVp(rms)/dIp(rms) = 261.9V(rms) / 210mA(rms) = 1247 ohms (loaded by rp), not 1400 ohms!

PLATE I & V "SWINGS"
a) dVp = 261.9V(rms) = 370.4V(pk), so Po = 110W/2 = 55.0W
b) dIp = 210mA(rms) = 297mA(pk), so Po = 55.0W

PLATE EFFICIENCY
eff = (PI/4)*(370.4V / 450V) = 0.647 ~ 65%

PLATE CONDUCTION ANGLE
CA = 2*ACOS[ -Ipq / dIp(pk) ]
CA = 2*ACOS[ -58mA/297mA ] = 202.5 degrees (barely above Class-B, which explains high efficiency!)

THD = 1.8% (data sheet number)

[conundrum]

Blasphemy!

What's next?!

Are you gonna tell us that the ever so overlooked screen voltage is more important than our mighty plate voltage!??

Sacrilage!



:AOK

[John Phillips]

Or that just because the tubes are running at 100% of their max. dissipation at idle, that has nothing to do with whether it's Class A or not...

[Old Tele man]

Originally Posted by John Phillips

Or that just because the tubes are running at 100% of their max. dissipation at idle, that has nothing to do with whether it's Class A or not...

...exactly, John!

...I was just showing that a 2 x 6L6GC high-power (55W) Class-AB1 amp, when operated almost at Class-B (180-deg/202.5-deg) with high 65% plate efficiency, could be idled 'very hot' at 87%-of-maximum plate dissipation (26.1W/30W) and still achieve quite low (1.8% THD) distortion...data found in any RCA, GE, Tung-Sol, Sylvania, Svetlana, etc. 6L6GC data sheets.

...87% ain't 100%, but it *IS* quite away from our 70% "rule-of-thumb"

...moreover, that 87% value represents "almost Class-B" not "almost Class-A" operation...those are my points.