SET Asylum Single Ended Triodes (SETs), the ultimate tube lovers dream.

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Re: SET and OTL

Posted by kurt s on December 24, 2006 at 12:02:45

In Reply to: Re: SET and OTL posted by Paul Folbrecht on December 22, 2006 at 18:15:22:

"Thanks, Kurt. I was surprised to read that AS are not true class A.
Are you quite sure about that?"

Okay, let's do the math. The individual bias currents for each 6AS7 output triode section is set to 60 mA. There's 2 sections per 6AS7, and 4 6AS7's per phase on each channel. With the circlotron, you have two power supplies out of phase that can contribute to the overall bias, which doubles that again to a total of 16 times 0.06 amps = 0.96 amps. That's 0.96 amps of bias current available as a peak current source for class A operation on each channel. The peak output wattage into 8 ohms is thus (0.96 amps)^2 times 8 ohms = 7.4 W. For RMS class A output power, that's (0.96 amps * 0.707)^2 * 8 ohms = 3.7 Watts, a little under my estimated 5 Watts.

Okay, so how much power is being consumed by this output stage at idle? Well, the voltages at the 6AS7 plates are about 180 VDC each. So the total bias power consumption is 180 V times 0.96 A = 173 W per channel. And the power dissipation on each 6AS7 section is 0.06 A * 180 V = 10.8 W, which is about right for that tube.

If it were competely class A to 60W as claimed, the current would have to be biased to 3.88 ADC per channel, with a total power consumption of 698 Watts. For both monoblocks that adds up to 1396 Watts which is not even including the heater power and the input section. Heater power is an amazing 126 watts per monoblock just for the 6AS7's. That means each monoblock would idle at a power dissipation of about 824W, or 1648W for a pair. Now that's a lot of heat for 60 W/ch, and maybe a little too much for the circuit breaker for the outlet.

Also, for true class A operation, each 6AS7 triode section would have to be set to 0.24 A, with a plate dissipation of 43 W. That would quickly burn out the tube since it's max rating is about 12 W.

Scaling up to the MA-3, if there was 200W per channel class A watts, each monoblock would be idling at 2750 Watts. For two channels, that would be 5500 Watts. Even if you had a 20A outlet, that's not enough for the outlet's required draw of 46 Amps. As you can see, it's not going to work that these are true class A amps in the classic sense of the term. However, clever marketing words make this sound like class A when they write "Class A means the current is never completely cut off at the extremes." If that's how you want to play the game, then all amps are class A as there is always some trickle of leakage current at the extremes.

Kurt

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