We found this 6-tube amplifier in a 1947 publication. It is a Public Address amplifier with excellent performance using triodes on the output.
In the figure we have the typical configuration of a voltage regulator based on an operational amplifier....
This circuit makes use of a 741 single source operational amplifier to measure resistors on a...
Active with the output at the high level, this oscillator was found in November 1984 Radio...