This circuit is from an old Japanese publication of 1962. The circuit uses two batteries, one for the filaments of the tubes and the other one of high voltage for the plate circuits.
The sensor of this circuit is a large surface diode. We believe that a small solar cell can also...
The sensor in this circuit is a BA315 diode. Other diodes can be experimented, but this is one...
This active crossover filter was obtained from the component manual used. Below is the performance...