[MrFixr55]

Hi Arran, 

No, I did mean the 6F6.  The original tone control switch varied the amount of capacity between the 6F6 plate and cathode.  My proposal was to replace that with a single cap (which must be done to prevent parasitic oscillation) between the 6F6 plate and cathode (which is at chassis ground potential), have no switch or pot in series with that cap, and vary the tone by varying the capacity across the 7C6 (or similar) 1st AF stage  This is what is done in more modern sets such as the 41-255 that the schematic was pulled from.  The coloring on the schematic was to illustrate the 3 different paths.  The green path is the audio path which is left undisturbed.  The yellow path is the cap added to the 1st AF Plate to one leg of the tone control.  This is much less stressful on the tone pot than varying the resistance between ground and a cap to the 6F6 plate, and is the standard tone control setup in more modern sets.  The wiper is at chassis ground potential.  The "Blue" path shorts out a cap that is in the input circuit whose effect is governed also by the setting on the loudness control.  The extra bonus is that when reviewing the schematic for the 37-620, the parts in blue actually exist in that set and connect to one contact on the tone switch, making the mod that I am proposing to be a perfect fit for this set.

Compare my "Mod" to the original schematic for the 37-620.

Re the "Vintage" dim bulb tester, the bulb appears to be modern. However, remember that modern is anywhere from about 1930 on. You can start with a low wattage bulb and when you think that you are OK, you can go to a higher wattage bulb. Some folks make dim bulb testers with multiple sockets and switches. A poor man's Variac.

Do resecure the wires on the socket, it seems to be not completely under the screw. A little electrical RTV over the connections on the cleat type socket and replacement of the vintage power cord if bad will make it safer.  No guts, no glory.

BAD JOHN!!!