Ode to a burned screen resistor, Deluxe Reverb RI

[nuke]

Working on a re-issue Deluxe Reverb with a burned screen grid resistor. Left quite a burn in the circuit board. Kind of a garden variety failure. But we always want to know why. Burn probably wouldn't have been so bad if the mains fuse were the 2 amp type as per the label, not the 3 amp fuse that was in it... But hey, guitar players.

The amp had some prior work, done neatly. But it included a "type 3" master volume. The external speaker jack was replaced with a push-pull pot, 100k and a 1.5k resistor, and about 7-inches of paired wires back to the 1.5K grid stoppers on the circuit board. Pushed in, the circuit was open, no MV, pull it and the MV is enabled. Ken Fischer's pages show a 1M-ohm pot and no switch.

The owner said he never liked it and didn't use it. Meaning, knob in, open circuit.

That means the pair of 7-inch long wires, tacked onto the grid circuit, just hanging in space like two hungry little antennae waiting to gobble up any EMI nearby. Thus, probably making the 6V6's more likely to oscillate.

I'm guessing root cause is likely power tube oscillation, induced by the excess wiring. Might just be happenstance, that a screen grid just shorted out. Or also the prone-to-fail Fender molded 1/4" speaker lead.

In any case, the MV pot is now gone, the 470-ohm screen grid resistors and the 1.5k grid stoppers are now relocated directly to the tube sockets as they were in the original BFDR era. I'm still digging out the rest of the burned PCB material. Since the board is lifted for work, re-capping the little e-lytics on the main board as the amp is now 20 years old. And, yes, the right amperage fuse is already in place.

[Stevem]

In my opinion every output section should have a HT even if it’s added and not externally accessible since they can really cut back on expensive damage.

I also prefer the PI section output coupling caps to be no more then 1.5 inches away from a output tube.

[nuke]

If the fuse is accessible to a guitar player, they will access it.

[Bobwith2Bs]

I have seen a surprising amount of Fender RI amps from the late 90's/early 2000's era that have leaky coupling caps before the power tubes and have caused bias drift that is higher than I will tolerate.

I have started routinely changing those caps as well as part of my service on these amps.