Rigs of this era use a hard to find 4 Watt wire wound resistor to adjust the grid drive. This is a design consideration in the Heathkit DX series and Apache transmitters and other classic rigs, not just Johnson equipment. In most cases, these components are operated at the limit of their power ratings. Modern AC line voltage is higher than the rated line of years gone by. This increases the low B+, exacerbating the existing power rating problem. Often when a well meaning ham uses solid state diode rectifiers in the low voltage supply, a series resistor in the diode string is not installed to simulate the dynamic resistance of the vacuum tube which it is replacing. This is frequently the straw that breaks the camel's back for the poor drive control resistor. It also increases transformer heating due to the higher peak charging current in the "improved" resistorless rectifier design.

This is a stand alone modification of any of the classic rigs. You should adjust the values and techniques to be appropriate for your rig. This will be applicable to the Viking One and 2, the Ranger, DX100, and probably many others. There is a fix on the AM window that uses a TV horizontal sweep transistor that is also very good. WA3DSP also uses a FET in his article on the Ranger update, which is the best comprehensive article I have ever seen anywhere on the Ranger.

See WA3DSP's article "Johnson Ranger Restoration and Upgrade": http://www.crompton.com/hamradio/JohnsonRanger/RangerRestoration.html

The following is an extract from my Johnson Valiant article that explains the idea behind this change. I am here explaining why this problem occurs, not just patching the problem.

WA1HLR recommends placing a 5K 5 to 10 Watt wire wound resistor in the LV B+ side of the drive control R51. I strongly recommend this along with the rectifier design included in the schematics to avoid excessive power supply voltage. He recommends another 5K in the ground return side. I found that 160 Meter drive could not be reduced to spec with this value. Maybe my tubes were newer and had more gain. Try a lower value if that is your problem. My rig arrived blowing fuses due to a bad drive pot shorting the B+ to ground. I did not have a 25K pot in my junk box. I used a 20K pot with 22K in the B+ end and no resistor in the ground end, and it worked fine for me. Drive was OK on 10 and 15 meters with this configuration with good exciter tubes. I super glued the extra resistor to the back of the pot as shown in the photos so it was not just flopping around trying to short out. Timtron is correct in pointing out this chronic problem; I just had to improvise with what I had in stock. If your drive pot has not yet failed, do this before it does. As an exercise, do the math. The pot is running at its full 4 Watt rating under ideal conditions in the stock Valiant. With high line voltage, it gets worse.

Click on thumbnail to enlarge.

This is an example of how to properly implement a solid state rectifier replacement. Check the B+ and bias voltage with a known good tube first. After you install the new solid state replacement, check the voltages again. If they are not the same, work on the resistor value until it matches or is just slightly lower. Yes, it makes heat in the resistors. But isn't that better than having the heat in the core of an unobtainium low voltage transformer? Also, add a “soft start” thermistor in the primary of the low voltage transformer AC winding. The B+ at power up will soar with the solid state rectifier, since it is “instant on” and the speech amp and VFO and exciter tubes are not warmed up and drawing rated current yet. This can stress the drive pot, capacitors, and other components.

Click on thumbnail to view MOV and thermistor.

Click on thumbnail to enlarge.

I hope you find this useful.