File:Four tube 500W transmitter 1922.jpg

English: A 500 watt amateur vacuum tube AM and CW radio transmitter built around 1921 by Frederick Koenig, 2BAK, of Tarrytown, New York. This is an early tube transmitter; "hard" vacuum tubes suitable for transmitting only became available to amateurs in 1919 after World War 1. The transmitter uses a Heising-Colpitts oscillator circuit with the 4 tubes in parallel for CW (telegraphy) operation. For AM telephony two of the tubes are used as modulators. With 600V on the plates the transmitter produced about 1 A antenna current.

The four triode tubes on the front panel are mounted upside down; this common early practice was to prevent the fragile hairpin-shaped filaments from sagging when they got hot and touching the grid. Under the tubes, the dial on left is the feedback capacitor, on the right is the grid capacitor, both 1.5 pF. The multiposition switches up the center are the feedback, plate and antenna taps on the tank coil. The 3 meters in the center are (l-r) plate current of the two oscillator tubes, filament voltage, and plate current of the two modulator tubes. The 6 single-throw knife switches at bottom are for: CW, ICW, AM (phone), test buzzer, filament current, and plate DC. A relay attached to a push-to-talk switch on the microphone turned on the plate voltage. The filaments are kept heated with a 1 ohm resistor in the filament lead when not transmitting.

Alterations to image: Removed aliasing artifacts (crosshatched lines) introduced by scanning of halftone photo, using Gimp FFT filter