First a standard disclaimer: If you have to ask, you probably shouldn't be working inside a tube amp. There is in excess of 500VDC in places on a live chassis and if you don't know EXACTLY what you're doing, you can DIE!!!! REALLY......... A good tech probably will charge about $40-50 for the job  (including parts), so if you're a little unsure, have it done professionally please.............The following procedure may seem overly complicated, but it's really necessary since in addition to using an ungrounded 2 wire plug, the old Fenders had the switch on one leg and the fuse on the other. If the neutural (white) wire were fused, should the fuse blow, you would still have lots of current, only it would travel through the green wire, not the white. Best have both fuse and switch on the hot (black-wire) leg.

Ok let's get started

1) Purchase a 18g/3 wire cord set (type SJO or SJT) about 8-10 ft long.

2) Solder a ring-type terminal onto the green wire of the cord set. Get
a couple of toothed lock washers of a size which can fit over one of the
studs holding the power transformer in place. Tin the exposed ends of the black and white wires. You can also solder the green wire to the ground on the back right rear transformer stud but make sure it's neat with no cold solder joints!

3) Clip out the old cordset from the "convenience outlet". Remove all
traces of solder and cordset wire from the outlet terminals...you'll need
the entire original insde diameter of the lug free for the rewiring.
Don't try to unscrew the lugs...not necessary nor easy. Use a solder
sucker and/or desoldering braid to remove the solder.

4) Remove the old cordset and strain relief by using pliers to compress
the longer diameter of the strain relief and pull it out. Save the strain
relief you will need it later.

5) Unsolder the white/yellowish jumper from the power switch (leaving one black power transformer primary lead attached to the other terminal). Remove the fuse from the holder and unsolder both leads of the fuseholder. Unsolder all 3 terminals of the ground switch. Keep the
wire...especially if it's the nice cotton-wrapped stuff...the longer
pieces are good to use for the rewiring coming up.

6) Remove the "Death Cap" (.047uF/600V) attached to the wiper of the
ground switch...lower right side lug. Clip the other end of the cap where
it goes through the hole in the chassis. You probably would have a hard
time actually desoldering it from the chassis, neatly clipping is ok.

7) Now it's time to rewire. Starting from the power switch and working
toward the AC cordset: Using 20-22g unstranded wire, neatly run a jumper from the unused lug of the power switch (the other lug has one power transformer primary attached) to the "ring" lug of the fuse holder. Keep wire runs neatly tucked inside the lip of the chassis.

8) Run another wire from the tip (innermost) lug of the fuseholder to one
upper terminal of the ground switch (probably should be the right-hand
side in order to make sure the other black PT primary will be long enough
for step 10.)

9) Run still another lead from the ground sw right side lug from step 8
to the lug on the convenience outlet with a brass-colored screw.

10) Attach the other power transformer primary (black) to the left side
lug of the grounding switch. Run a jumper from that (lefthand) lug to the convenience outlet lug with the silver-colored screw.

11) Strip about 4 inches of the power cordset outer insulation and neatly
remove any paper or fiber wrapping. Insert the cordset through its
chassis hole and attach the green wire ring terminal over the rear right
stud holding the power transformer. Use a lockwasher above and below the
ring terminal and top it all off with the original nut. Tighten securely.

12) If possible, twist the black and white cordset leads and solder the
black cordset lead to the convenience outlet terminal with the brass
screw, attach the white wire to the silver-screwed terminal. Make sure
you don't have too much exposed wire between the end of the insulation and
the solder terminal here...it's really pretty easy to short across to the
other 120V leg.

13) Secure the cordset to the chassis with the strain relief. If the new
wire is much thicker than the old, you may have to hog out the inside of
the strain relief with a rattail file.

14) Inspect all work, make sure wire ends are neatly clipped and all
residue removed from the chassis (it's all too easy to inadvertently
damage the insulation on the existing wiring by slipping with the
iron...you'll have to repair any insulation burns with heat-shrink
tubing), replace the fuse and speaker, hold your breath and power up.

Note: if you follow these instructions, you'll still have the appearance
of the ground switch but it won't be functioning. No problem as long as
the venue you're playing is wired to code. Always try to use a 3-prong
outlet, and if it ABSOLUTELY HAS to be a 2 prong outlet, use a "cheater"
adaptor on your cordset and DON'T under ANY circumstance defeat the
polarity of the cheater by filing down a prong (note that one prong is
wider than the other).

Here's the diagrams to help you through the process!