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.

Locked resonant relay (CB8840E)
This circuit uses a resonant reed relay to recognize the frequency of an input signal. Adaptations...
Constant Current Source II (CB19950E)
The operational used is uncommon today, but equivalents can be used. Unlike the previous one, this...
110 V or 220 V Voltage Indicator (CB16899E)
This simple circuit uses a zener diode and two LEDs to indicate whether the voltage of a power grid...