There is a common fault that keeps turning up with these amps, and also the Hot Rod and Deville amps. The output stage of these amps is very similar, if not identical.
As with many new valve amps (and this includes many of the ‘reissues’) they are not built in any way similarly to the originals. It would just be over the top expensive to build say, a Tremolux, or Super Reverb or whatever, with all the hand wiring that went into them at that time. So most of the components are mounted in the same way as they are in solid state amps; i.e. on printed circuit boards. As everything is always in exactly the same place everytime, you can get your friendly pick ‘n’ place robot to do nearly all the work. Whereas the originals were entirely handwired. That would cost you a lot of money today.
Anyway, there’s nothing fundamentally wrong with this. Often. However, now we get to the DeVilles, Hot Rods etc.
These amps use pcb’s like nearly everything else, but the way that the output valve bases are connected to the pcb is a big problem, and it’s not just Fender that does this, many new Marshall amps with valve output stages are also built similarly. The valve bases (sockets) are connected to the pcb by solid metal rails, the bases themselves being screwed to the metal chasis of the amp, and the pcb these rails are soldered into is fixed onto metal posts (plastic in the case of Marshall). The problem is that the pcb flexes in transport and in use, and your nice amp develops cracked joints where the valve base pins are soldered onto the pcb. So what?
Well, the last one I saw had blown the inside of a 6L6 output valve to bits, because a cracked joint to the grid had taken the bias off the valve completely. The output transformer had survived, but that was bit of luck really.
The first thing to do is resolder all the joints to the output bases. I use silver content solder, and I put plenty on. This strengthens the joints, but you can’t unfortunately strengthen the tracks, and these can break. There’s nothing you can do about this. It’s possible to clean back the tracks and run solder a little way along, but you can’t strengthen the adhesion to the board, so that’s probably time spent that isn’t very effective.
If you take the back off (Unplugged of course!!!!!!) the output valves are at the bottom near the middle, and you can see the rails and the soldered connections. Get a magnifying glass to it, and you’ll very likely see cracking round the joints, even on quite new amps.