Screen and Grid Resistor Replacement
The below pic shows a close up of a recent screen and grid resistor replacement on a '68 Bandmaster Reverb. As discussed on the previous page, screen and grid resistors are mounted on the output tube sockets and take quite a beating from excessive heat, vibration and/or from a previous tube which failed from a screen short. Notice on BF and SF Fenders, the green heater wires are routed right above the screen resistors. Fender amps typically used carbon composition screen resistors. When carbon composition resistors fail at the high screen voltages used on tube amps, they literally burn up. They will let off a puff of smoke and actually ignite into a 1" flame for several seconds. Worse case, the flame burns the above green heater wires and can cause the heater transformer secondary winding to short. Hopefully the amp's fuse will blow before the heater windings burn out. If the heater wires have been burned or even scorched, they should be replaced. Therefore, I used metal film flameproof 2% screen and grid resistors. When flameproof resistors fail they will smoke, but they will not flame out. I would argue that when re-tubing your amp with an expensive set of NOS tubes, have the screen resistors checked. If they are original, have them replaced.