Bill
Fortunately the mode switch was set in the AM (or CW) position when this happened. The schematic I downloaded from BAMA shows a 12k ohm resistor that is in series between the grid of the 4-400 and the -150 volt bias supply when in the AM or CW position... plus a about 800 ohms of additional resistance from the AM/CW bias adjustment pot.
This limited the current through the grid current meter shunt, the RFC on the grid of the 4-400, and a lot of other things (including the bias supply itself) to around 30-40 mA.
If you had the mode switch in SSB position [
], the path from the grid of the 4-400 to the negative bias supply (and everything connected to the bias supply) is limited by only the RFC, the 6 ohm meter shunt, and a couple of hundred ohms of resistance from the SSB bias adjustment pot.