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

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RE: SUN SV 2A3 stereo SE amp .... MEASUREMENTS

Posted by Paul Joppa on April 2, 2015 at 18:49:52:
I did not locate Jeff's question to me, but having been invited here I'll take this opportunity to give my thoughts. (I'm an engineer, but not specifically electrical - worked mostly in acoustical engineering FWIW.)

At the operating point mentioned in this thread and shown in the curves provided by Caucasian Blackplate (250v/37mA/2500 ohms) the tube is operating at about 11% second harmonic distortion. As long as the reflected speaker impedance is greater than the nominal, distortion may be high but technically it is Class A - the tube is never cut off. But it is at the edge - if the impedance of the speaker dips below nominal (they all do!) then the operation will be Class AB, with the tube cut off for a portion of the cycle but less than 50% of the cycle.

The interesting thing to me is that the output power is 2.9 watts at the edge of grid current, whereas if it were really linear then 37mA would produce at most 1.7 watts. In addition to the extra power, the high second harmonic gives a psychoacoustic effect of greater loudness, often described as "more dynamic". The price paid is masking of the music by the distortion products. With enough distortion at full power, the distortion of real overload clipping is probably less obvious.

The average current at full power is nearly 50mA, 35% more than the 37mA at no signal. This is a reasonable explanation of the need for a tightly regulated and fast power supply - to control envelope modulation. Rapid power supply recovery is much less important when the tube is operated as RCA suggests, where the variation of average current is less than 10%.

At smaller signals, the distortion can be reduced by using a driver stage with similarly high levels of second harmonic, which is what you will get from a notoriously nonlinear tube like the 12AX7, with a resistor plate load. Of course, this distortion cancellation actually increases the higher harmonic distortion products.

Using direct coupling eliminates blocking from grid current into the coupling capacitor, but blocking from the cathode bypass capacitor is still present, and is increased by the large variation in average current. Balancing the blocking against the power supply regulation is a complex challenge, since the time scales are psychoacoustically important as well.