A silverface super reverb walked into the shop today…..Underneath the chassis I found very old, factory-installed electrolytic capacitors. Probably at least 40 + 5 years old. These have got to be replaced!

Plus, instead of a Super Reverb spec’d output transformer the amp has a Twin Reverb output transformer. Who the heck bolted that on there?

A lot of things happens, and doesn’t happen, to guitar amplifiers over the years. Sometimes, mods happen, and routine maintenance doesn’t.

I hope to get this amp into great shape. It has potential to be a wonderful guitar amp..just needs some TLC.

Today I had a 100 Watt JCM2000 in the shop for repairs.

One of the power tubes wasn’t glowing – this means that the tube is not heating up to its operating point. A tube functions by heating a piece of metal so that it emits electrons. In order to do that you need a fair amount of current (~1A for a EL34, 6L6, etc) to flow through the tubes heater (or filament) circuit. This Marshall had a poorly soldered printed circuit board that was causing the problem. Instead of a resistance of an ohm or so, there was 2500 ohms in-series with the tube socket – in other words, the solder joint had a resistance of 2.5K !

Now this baby is cranking, loud!!

This is a 50W JCM 900 combo amp, a 2×12 cab. Two Celestion speakers of course!

The owner reports that the volume is dropping in and out…

All four volume pots in this amp are bad. Cleaning them did not help. Going to put in 4 new volume pots. The amp is about 12 years old, so we’re going to replace the power supply capacitors as well as the output-tube bias capacitors. The large blue cans on the underside of the chassis house 4 of the main power supply filter capacitors. The top photo shows why replacing the volume pots is somewhat time-consuming: The pots are soldered to the preamp circuit board. In order to de-solder the pots (so you can remove them) you have to access the underside of the preamp board.