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 1990s William Sheets, this automatic gain control for audio amplifiers uses...
In the figure we have the way to use the comparator LM339 and others of the same series with the...
This circuit is from an article in Radio Electronics magazine from August 1972. The circuit is...