Audio Conversion for Johnson Viking I and II Transmitters

  Shown above is a schematic that gives you improved gain and audio quality. Remove the two 6AU6 audio tubes and the associated circuitry.
 Remove the first 7 pin socket (1st audio stage). Punch a hole for a nine pin socket.  The second 7 pin socket remains, as the 6C4 is a 7 pin tube.
  Rewire the first three stages as shown in the schematic above.  Replace the driver transformer 		with a larger one.  You now have two audio stages, 12AX7a drives 12AX7b. The 12AX7B drives the 6C4 driver which delivers extra drive to the 807 modulators.  

(For improved low frequency response increase the interstage coupling caps to .05 uF, the cathode bypass caps to 20 or 50 uF, and the two 8 uF decoupling caps to 20 or 50 uF. - WB3HUZ).
From the AM Press Exchange, October 1981

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Ed Patrucci, KS3K

  These modifications do not require any changing of sockets in the audio speech amplifier section. The 6AH6 is a plug-in replacement for the original 6AU6, first audio amplifier.

Although the driver tube now uses a 6C4, which is a seven pin tube, the socket will need to be rewired, except for the filaments. Be sure to compare the schematic above to the original schematic because some components have been changed, others removed and others remain the same.

 After the mods have been made, adjust the resting current of the modulators to 60 milliamps by adjusting the variable tap of R13.

For proper audio equalization, use an unamplified crystal microphone such as an Astatic D-104.

(Note 1: Better regulation of the screen voltage for the 807 modulators, and thus, more peak audio and less distortion, can be obtained by connecting the screens to the low voltage supply).