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EF Johnson Audio Amp

Amateur Radio > Vintage Transmitters

The E.F. Johnson, 250-33, is a 10 Watt audio amplifier intended to work with the Johnson Viking Kilowatt for AM operation when the Viking Pacemaker SSB transmitter is used as the exciter. A Johnson sales pamphlet states it will work with other Johnson AM transmitters.

Given the above, the 250-33 is now serving as the primary audio driver for the Viking 500's 811A modulator tubes. The 250-33 was modified to bypass its own audio clipping and processing to widen the audio spectrum resulting in very clean and strong audio supplied to the grids of the 811A's.

These modifications merely require constructing a new connecting cable between the power supply and the RF deck. The stock circuitry of the Viking 500 is essentially unchanged. Using the regular connecting cable, standard internal Johnson audio is restored.

The amplifier had not been touched in decades. Like the Viking Valiant found elsewhere in this site, the power cord had rotted away leaving strings of copper wire and rubber crumbs. New capacitors and resistors were installed as needed. Connecting through an audio output transformer to a 10 inch speaker, it produces very clean audio in the standard EF Johnson tone.

A plug for the rear jack was found thanks to tips from users of the Antiqueradios.com forums.

Rear view.

The schematic for the 250-33 Amplifer. The audio string has some similarities to those of the Viking Valiant and Viking 500.