I still have a few things to do to the Vibro Champ.
The first change is to rewire the filament supply to a twisted pair with artificial center tap arrangement.
You can see in this picture of the power transformer how Fender wired these puppies - simple and cost effective. The two filament wires are the green ones coming out of the transformer - it's a 6.3 volt AC output. There is no center-tap ground reference for the filaments. And it's not needed for the way it's wired.
Here's the deal. The pilot lamp and the 6V6 output tube, and the two 12AX7A tubes (one is the preamp, one is the vibrato/tremolo) have 6.3 volt filaments, and there are two points where each side of the two filament wires from the transformer connect.
Since there are two filament wires on the transformer, one of the wires goes to one side of the pilot lamp socket and then to one side of the filaments on each tube. You can see the one green wire going to the pilot lamp in the picture. Then there's another green wire coming off the pilot light from the point where the transformer wire is - that goes to one of the two filament pins on each tube. (Technically, it goes to two of the three pins on the 12AX7s - pins 4&5, but that's not critical to this explanation).
The other filament lead from the transformer goes to ground. You can see it twisted in with a yellow and orange lead that are soldered to the chassis (ground). On each tube, the second connection also has a lead that goes to ground right there at the tube, as does the pilot light socket - note the wire from the socket to the soldered ground near the transformer.
If you were to trace the circuit, then, one side goes to a tube pin, and the other sides go to ground. The circuit has continuity; the pilot light lights up as do the tube filaments.
So what's the problem? This is the simplest, and cheapest, way to wire a filament circuit. It works. However, because the chassis is used to carry one side of the circuit, the circuit is prone to noise and hum. Noise because of the chassis return, and hum because there is AC current in a single, poorly-dressed lead.
What we can do to improve the circuit and cut some of the hum is to convert it to twisted-pair wiring. From the earliest days of radio, it was discovered that twisting two AC leads together reduced hum, and made a circuit more resistant to noise pickup.
More expensive gear uses this type of wiring. It takes more wire and it also requires a center-tap ground reference in the circuit. But a transformer with a center tap costs more money. In an inexpensive amp, that's cost-prohibitive. However, we can retrofit a twisted-pair wiring scheme in our amp - and substitute for the center-tapped transformer.
Take the filament lead that is grounded up off the chassis and connect it to the other side of the pilot light that formerly went to ground.
Next: this is critical! Disconnect the grounded side of the pilot light and the tube filaments that went to ground. If we don't do this, our power transformer will be shorted and will go up in smoke!
Place two 100 ohm resistors from each side of the pilot light connections to ground. You can see them in the picture above. I put a ring terminal on one of the transformer mounting bolts and connected the resistors to that. These resistors form an artificial ground tap. One end on the pilot light, one end to ground.
Remove the old single wire going to the tube filaments. Then we twist up some 18 gauge wire and run it from the pilot light terminals to the filaments. We're just taking the two connections from the pilot light and running them in series to the other filaments.
With this arrangement, the 60Hz filament hum I could hear with my ear close to the speaker while the amp was idling was cut by about half. For the cost of two resistors and some wire, we've made a substantial improvement!