Estimating Power Output For A Class A1 Amplifier From Valve Specifications
In class A, in order to produce a full sine wave, the tube must be exactly half-way on. Therefore the maximum current swing is +/- 50%. No actual amplifier will ever achieve this.
Obtain the following parameters:
- Pa (or P for pentodes) (in watts).
- Rp in triode (in ohms),
- load resistance (Rl) (in ohms) (which may vary depending on the design.)
Compute maximum theoretical power output by P = Pa / 2.
While the valve can be driven all the way to shut-off fairly easily, the maximum current will be limited by the internal resistance of the valve as the grid reaches the voltage of the cathode and is not impeding electron flow. For this reason, valves that have a low Rp can yield more power in class A1 than other valves with similar Pa ratings.
Next, compute P = P * (1 - Rp/Rl) to obtain an estimate of the maximum obtainable output power.
Deduct 10% from this figure to account for the relatively heavy distortion encountered as the valve approaches cut-off.
Read more about this topic: Single-ended Triode
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