> What would happen if you use a 100M resistor instead of the 33M resistor in the G7? My guess is it would work the same, as long as it's a high enough impedance...
Too low, lose bass response and get thermal noise. Too high, grid/gate-current causes noise and offset error.
Vacuum tubes can go into "run-away" with a too-big grid resistor. Grid current is a complex thing, but if it goes a little positive then the tube current, temperatures, and grid current tends to rise even more. If you fool with big grid resistors, be sure there is enough resistance in the plate-cathode circuit so the tube can't burn-up no matter what happens.
Common tubes are rated for 1Meg, some 5Meg, grid resistors. If you are working well below melt-down, you can get away with much more. Special tubes are made for super-low input current. I'd think that 22M or 33M would be safe for selected common tubes working 100V with 50K protection resistance, 100M seems high for common tubes but might work.
Try it and see. If you had 300V supply with 2K of DC resistance, you might burn-up tubes, but tube mikes usually run much lower voltages with high series resistance.
