This months tube is the queen of flea power triodes. The 71A.
The 71A is a directly heated triode developed for use in the output stages of audio amplifiers or radios. As mentioned above it is the queen of the flea power triodes delivering output in the sub 1 Watt range only.RCA data sheet lists a maximum obtainable output power of 790mW but closer to only half a Watt for typical operating conditions. The plate resistance is 1750 Ohms which narrows the selection of suitable output transformers. A minimum of 5k primary impedance should be used while I would rather use 7k or more. With its low amplification factor of 3 the tube still needs considerable voltage swing at the grid to reach maximum output power. In the range of up to 80V peak to peak. Nevertheless a very interesting tube and I have to admit that I never built anything with it. Could be a nice tube for a tweeter amplifier in a an active set up or a headphone amp. Of course could also be used in a full range amp with suitable speakers which would have to have a sensitivity well above 100dB.