by EWBrown » Sun Nov 18, 2012 9:23 pm
The 6B4Gs would be biased differently for Class A operation, than for Class AB1. In either case, the 15Watts plate dissipation should not be exceeded.
For Class A operation, the plate current through a single 6B4G would be approx 60 mA, at V P-K of 250VDC, and for AB1 operqtion, closer to 40mA idle, and 50 mA max, for each tube with a V P-K of 300VDC.. \The 80 and 100 mA specified in the Class AB1 ratings is for both tubes combined.
The B+ voltage for the PP 6B4G amp is 375 VDC, as compared to the usual 300 VDC for Class A SET operation.
Class A PP would work, and sound very good, it's just a lot more inefficient in power output vs DC power consumption. Also, the maximum RMS power output would be approx 6-7 Watts, vs 10-14 Watts for AB1 operation.
With the values given, for Class A operation, the voltage across the "cathode" resistor would be approx 45 VDC (60 mA X 750 ohms) and for AB1 PP operation, it would be approx 62 VDC (80 mA X 780 ohms).
In the Acrosound design, the cathode resistor is 850 ohms, so this would allow for using a higher B+ with slightly lower cathode current.
/ed B in NC
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