Hello Fred,
Many decades ago in my infancy I picked up a Johnson KW Matchbox at a hamfest. I didn't really need it as I was just starting out in amateur radio but it just looked so cool, and it was made by Johnson, so I got it. Cheap. Years later when I actually pulled it off the shelf to use it I discovered that it had already been apparently modified for 160. The paperwork that came with the box included this xeroxed note from Johnson about adding 250pf capacitors across the tuning capacitor sections. Mine has two 500pf doorknob caps instead. Switched in by two vacuum relays. I've never actually gotten it to work decently on 160. With very long wires (400 to 500 feet) I'm able to tune it initially with the capacitors switched in but after a few seconds the SWR starts to climb slowly upwards. I replaced the caps and the SWR still wanders around so I'm thinking maybe some bad solder joints on the relays are causing it? Don't know and this problem got buried on my list of things to do so I haen't investigated it in awhile. So find some doorknob capacitors and vacuum relays and see if you can get yours to work. Maybe I'll see you on 160 if I can ever climb towards the top of my repair to-do list.
Mark - WA2FXM
The Johnson note specifies A high voltage Mica capacitor which will have something close to a NP0 curve with temperature shift.
My experience with those CRL doorknob caps is made of a ceramic dialectric that has a -700 to -1500 ppm/degree C shift with temperature.
Putting a pair of these in a tuned circuit with a high current will cause considerable heating, I suspect and when they get hot, these caps drop in value a significant amount. While they may be ok for a non-critical plate coupling circuit where the value is not a big deal, a tuner, by definition has a critical value for resonance and for matching. I would get rid of the door knobs and put some surplus vacuum caps in or some large transmitting micas and maybe it will work, though the L-C ratio cannot be optimal for 160, so losses will likely be high..
There are some nice 160 tuners on this forum history.
The Johnson Tuner is good for what it was designed to do, but they were not hinking of 160 meters back then.