Crystal oscillator with a 6A6 (T437E)

We found this circuit with the 6A6 double triode in a 1938 Radio Handbook. The power source is not provided.

Erase oscillator (CB7338E)
Suggested by Mullard in a1969 documentation, this circuit generates the erasure signal for tape...
Low noise preamplifier (CB8942E)
This low noise microphone preamplifier circuit was found in American documentation from the 90s....
Optical transmitter (CB20695E)
This transmitter for modulated radiation can be used as an optical data shield. The maximum...