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!!