This was explained to me by Charles at SwAmp, before he had some personal problems that made him shut down his shop. Thanks Charles, I use this mod alot!!

Follow the circuit from the hot on the power cord, back to the neutral on the power cord. 

Hot (black) goes first to the accessory socket hot, but since it’s a two wire accessory socket, which one is that? It’s the smaller blade opening.  Ok, from there, it goes thru the FUSE.  Remember, the incoming hot must be connected to the end of the fuseholder, not the ring.  The ring is the output of the fuseholder, and our hot wire will then go thru the ON/OFF SWITCH.  From the output of the on/off switch the hot wire will split in two directions.  One connection will go to the black or in put of the transformer.  The other will go to one side of a NEW CENTEROFF GROUND SWITCH. (more on this later).  If the transformer has two black wires, either one will do, and the unused one will then become our return wire.  That wire could be white also.  The return wire will then go to the other switch connection on the ground switch, and then to the accessory socket, and finally to the white wire on the power cord (neutral).  Green wire to chassis is all that’s left. 

Lets recap the order starting with black:

Black (hot) input wire, accessory socket, fuse, on/off switch ground switch AND transformer.  Then, transformer return wire ground switch, accessory socket white (neutral) wire on power cord and Green wire to chassis. 

Lets revisit the ground switch for a moment. 

You really need to put a new ground switch in there too.  The hot and neutral get connected to the ends and the cap gets connected to the middle.  In that way, the normal position for the switch is centered.  However, should you find yourself in a situation that has a two wire outlet with an adapter, you can still use the switch.  Keep the ground switch AFTER the fuse, that way if the cap fails, it will blow the fuse. 

Folks call that cap the “death cap” but I gotta say, I’ve never actually seen on that failed.  Could be though, so that’s why it’s after the fuse. 

Notice also that the accessory outlet is not fused.  That’s because we don’t know how much current the accessories use!  Keep is mind, the cord on the amp is an 18/3, more than adequate for even a Twin Reverb, but don’t overdo it with accessories. 

Also remember FUSES DO NOT PROTECT THE AMP!  Right.  Fuses are there to protect the upstream or source of the electrical power.  The idea here is that the fuse will blow, not taking the fuse or breaker in the distribution box with it.  Ie, the 2.5 amp fuse will go and not the 15 or amp fuse in the wall panel.  Now it could be, that if sized just right, the fuse could also protect the amp, and that’s why it’s sized as it is.