Thursday, February 13, 2014
Burned Up With Cement Resistors
In order to operate the solid state devices these amps contain, it is necessary to supply -15 and +15 volts. Fender did, of course use a dedicated winding which is good because some amps like the Valveking piggy back their low voltage supply off the filament winding, and a shorted tube can wreak havoc on those solid state devices if it burps and backfires.
However, Fender chose a brute force solution to reduce the voltage down to the required level and that was the pair of cement resistors you see. In many cases such as this HR Deluxe, the circuit board can get some significant burning and in some cases the solder can melt.
I got this fix from Andy Fuchs, the New Jersey Amp Wizard, including the picture of the Arcol resistors.
It consists of getting a couple 15w 470 ohm Arcol resistors at about $3 each from Mouser. They are then mounted to the chassis with 4-40 screws and nuts, the original cement resistors are carefully removed so as not to damage the pads (easy to do), and the resistors are wired up as if they were on the circuit board.
After reassembling, these resistors transfer any heat generated right in the handy dandy heat sink of the chassis, and are barely warm to the touch.