This circuit is from a 1975 RCA tube manual. It appears elsewhere on the site. R5 depends on the voltage of the amplifier with which it is used and can be changed.
We found this preamplifier for recording heads in a 1980s documentation. The circuit can be used...
Found in Popular Electronics' 90s, this small megaphone is based on LM386. The circuit uses a high...
This circuit with a common FET generates signals around 1 MHz, depending on the crystal used. We...