Viking II modifications
This is my Viking II that I have modified for better audio and RF linearity. I began by completely bypassing the stock audio circuitry and replacing it with a high quality broadcast transformer. It is a Hammond 841 which is a "line to grid" step up. I changed the Key jack on the front to a stereo 1/4" jack which allows me to feed a balanced audio signal to the transformer. I then directly coupled the secondary of the transformer to the grids of the 807's, via a pair of 47K resistors. This is such a simple way of getting great sounding audio out of the rig. No active circuitry prior to the 807's. You can see the grey wires running from the front jack and then directly to the 807's. There is no feedback around the audio stages.
The second change I made, was to move the PA stage screen supply from the modulated side of the modulation transformer, directly to the B+ supply. This got rid of the little kink in the trapazoid pattern which I was seeing in the stock configuration. I also added 15uF of C to the center tap connection of the modulation transformer, so that there is a little reserve right at the transformer for any audio peaks.
Here is a shot of the bottom of the rig, showing the little Hammond transformer I used to couple the balanced audio directly to the 807's.
Here is the PA screen supply connected to the modulated side of the mod transformer, (stock) and below is what the trapazoid looks like with the supply on the B+.
I also changed the 5R4 rectifiers to solid state plug in modules which brought the B+ up to about 630VDC from 575VDC. This is okay as the main supply cap is 1KV as are the caps I added. One other thing to note. I found that the PA is most linear when run at only 180mA current instead of the 230mA that the manual recommends. There doesn't seem to be enough grid drive to to make it linear at 230mA. (unless my meter is out of calibration) All of these modifications have made my Johnson Viking II my favorite boatanchor rig....and now onto some other projects.
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