• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

2A3 input sensitivity

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If you use the "original" circuit as a reference and the cathode resistor is bypassed with a cap, then the input sensitivity of the amp will be approximately 1VRMS for 3.5 Watts out at 5% THD+N.

Many factors come into play here, but that is a good estimate based on a few 2A3 amps I have built using a 6SL7 tube as a driver.
 
It is difficult to address your question without a schematic, and I am not attempting to disagree with TDH+N; but, in my estimation, it is difficult to obtain much more than 3 Watts out of a 2A3 into a reasonable OPT impedance...especially in your typical Loftin-White circuit.

Assuming a plate load equal to 200% of the 6SL7's R(p), and a fully bypassed R(k), I'm guesstimating that 1.3 vac input will give you approx 3 Watts into a 3k5 load.
 
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