I am restoring an "old" drake TR-6, I just noticed a 100 ohm resistor
connected between pin 1 and pin 7 of
the three 6JB6 finals. I understand these two pins are both connected
internally to the screen grid, so

Looking to the schematic diagram, that resistors are in series to the screen, so originally they could not be shorted by the tube.

Are you really sure that the resitors are connected between pin 1 and pin 7? They often use the unsed socket pins as standing posts. So, if the resistor would instead be connected between pin 1 and pin 9, then all would be explained (pin 9 is unused).

Does the wire taking voltage to the screen go directly to pin 1 or pin 7?

Another explanation is that the previous owner, wanting to have a little more power, moved the resistors from pin 9 to pin 7, so effectively shorting them. In this way he could easily restore the original configuration by just resoldering the resistors and the screen feed wire.

73

Antonio I0JX