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.
What I know is 300B and 2A3 didn't produced for audio application at first... It mainly use for regulator application... just like 6as7 6080 5998 6336 6528....