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In Reply to: Best plate load for 2A3 posted by Alpha Al on January 5, 2012 at 15:31:10:
As I have said before, the load impedance only makes sense in the context of the plate to cathode voltage and the plate current.
As it happens, I have recently examined a number of operating points and loads for the 2A3, which I'll summarize here for those who might be interested.
First, note that the maximum quiescent plate to cathode voltage is 300 volts, maximum current is 60mA, and maximum plate dissipation is 15 watts. Two interesting points are 250v/60mA (the one in the RCA specifications) and 300v/50mA. Since I did the numbers for 300v/40mA as well I'll include that point.
Second, I use a reference load which would give equal current clipping on positive and negative half-cycles if the distortion were zero. In the real world, this gives a small but non-zero current at the low-current end of the cycle, which is where the plate curves get the most compressed and nonlinear. For the WE 300B and similarly linear tubes, you can expect about 3%THD, almost all second harmonic, at this condition.
At 300v, 40mA, the reference load is about 5300 ohms
At 300v, 50mA, the reference load is about 4000 ohms
At 250v, 60mA, the reference load is about 2500 ohms
If you use a load that is 70% of the reference, you would expect 20% more power, at 5% second harmonic. If the load is 50% of the reference, then you might expect 50% more power, at 10% second harmonic. These numbers are extracted from my analysis of the WE published operating points, and they assume a perfect sine wave at the grid, with fixed bias, a purely resistive load, and zero losses in the output transformer.
Less linear tubes will fail to show the power increase, and fail to provide the low distortion at the reference load.
For those who are conservative about dissipation, you can scale the voltages and currents with reasonable accuracy while keeping the load impedance - for example, 240v at 40mA is equivalent to 300v at 50mA and both have a reference load of 4000 ohms. (Power dissipation is 9.6 watts, about the golden ratio to 15 watts. Jeff will approve. :^)
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Follow Ups
- RE: Best plate load for 2A3 - Paul Joppa 20:01:01 01/05/12 (2)
- Almost the same as the Bugle 45 OP points - tweakydee 08:53:13 01/16/12 (0)
- Saved for future reference. Thank you Paul. nt. - RC Daniel 02:10:47 01/08/12 (0)
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