I recently bought a 470 amp head as-is, and it was blowing its main 5A fuse. Sound familiar? The issue could affect many of you 470 owners.
It turns out that C409 (2000MFD, 50V) is a capacitor that is wired into the final speaker output path, and it had shorted to ground. The temporary short blew the bridge rectifier in the power supply. And so the BR became a dead short to the secondary windings of the power transformer. Replacing the BR is quick, easy and cheap.

Now for the cause: In my amp (maybe other vintages are different?) there is very little clearance between the terminals on C409 and the metal case of the reverb chamber. Originally the factory glued a piece of insulating foam on this area of the chamber but over the years it rotted away, leaving lots of black particles floating around inside the head. All that was needed was for the screws on the cap to loosen up (they did) or for the head to take a small impact while powered up and “poof”, a short will result that I believe blew out the BR.
So, as soon as possible, pull your amps out of their wood cabinets and look at the condition of this insulator. You will know what to do. And reply to this tread, letting us all know what you found.

Here’s are two internal photos of the 470. The output capacitor (C409) is the blue one. In these photos I’ve already got an insulator covering one of the two screws on the top of the capacitor. In the second photo you can also see where I’ve attached an insulating pad to the side of the reverb chamber. The terminals of this capacitor come to within 1/8″ of that chamber. Although the silver power supply filter capacitor looks to be the same length, it is actually shorter and doesn’t get close to anything. The little square green item to the left of the PS filter cap is the original bridge rectifier. It was replaced with a new one.