This 6W amplifier with 4 tubes was indicated in an American documentation from 1947 to operate with recorders. The circuit uses a source with a transformer.
Found in a 1966 documentation, this “coin finder” uses components from the era operating at 2.1...
This circuit is from a 1968 publication by General Electric. The circuit can be applied to any...
This is a typical point scale or moving bar circuit with the LM3915. We found it in old American...