This 1948 circuit uses miniature tubes and can be built into a very compact assembly. The circuit has 1.5 V heater tubes and the source with the indicated voltages is external. A 22.5V battery can be used.

This 1948 circuit uses miniature tubes and can be built into a very compact assembly. The circuit has 1.5 V heater tubes and the source with the indicated voltages is external. A 22.5V battery can be used.