From the AM Press Exchange, October 1981

 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).

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2 August 1997