[Arran]

Mike;
That's a very common setup even with a linear pot tone control, which is one of the reasons why you should always replace the cap connected to to the center tap of the pot used in such setups. I've ended up with more then one set where the tone control pot burned up because that cap shorted to ground resulting in the full B+ flowing through it. One was so bad that it even burned up part of the Bakelite the resistive element was mounted to.
In the case of Philco if any of those caps on the tone control were to short it may take out the rectifier tube, field coil, output transformer, and even the power transformer since there isn't even a resistor in series with them. So basically what Philco did in this case is they substituted their three position switch ans caps for what would be the tone correction capacitor in another make of radio. Caps are cheap, inductors are expensive, change the caps.
Regards
Arran