I used to have one of these amp a LONG time ago (got it through a poilce auction for $50 in 1974). I never used the vibrato, I just yanked out the 12AX7 and left everything else as original. I should have performed one more step (see next paragraphs for explanation):
If you just want to disable the vibrato, the easiest procedure would be to pull out the 12AX7 in the vibrato circuit, and disconnect the top end of the 50K-RA "intensity" pot.
This effectively removes the circiut from the rest of the amp, and still allows using the "vibrato" input stage as an extra "Normal" input stage, but with the addition of having the "middle" tone control. This also allows an easy restoration later, if you should need it. Unless you're playing 1960s surf or hot-rod rock, or emulating Duane Eddy or The Ventures, vibrato has gone the way of the dinosaur...
If you wanted a "middle" tone control on the "Normal" input, take out the 50K pot, install a 10K pot in its place, and then and use it to replace the 6.8K resistor in the bottom of the tone stack. This will make vibrato restoration a bit more difficult.
Re the OPT:
The Triode unit is probably a "step up" already:
http://www.magneticcomponents.net/40-18013_Stock.pdf
If you want better low end response, than any good 1.9 to 2.2K OPT with the appropriate (at least 100W RMS) power handling capability should be good to go. Though this could involve some extra chassis drilling to fit the new mounting holes pattern. The Hammond should be fine for this application. But make sure that it will fit on the chassis, before taking the plunge, it is big and heavy (around 14 pounds).
TRANSFORMER, OUTPUT, PUSH-PULL, HAMMOND, 120 WATT
P-T1650T
Designed for push-pull tube output circuits.
Enclosed (shielded), 4 slot, above chassis Type "X" mounting.
Frequency response 30 Hz to 30 Khz. at full rated power (+/- 1 db max. - ref. 1 Khz) minimum.
Insulated flexible leads 8" min.
Includes 40% screen taps for Ultra-Linear operation (if desired).
Typical applications - Push-Pull: triode, Ultra-Linear pentode, pentode and tetrode connected audio output.
Specifications
Audio Watts (RMS): 120
Primary Impedance (Ohms): 1,900 CT
Maximum DC per side (mA): 403
Secondary Impedance (Ohms): 4/8/16
Mounting centers: 3" x 3.31", vertical mount
Weight: 14.0 lbs.
Suggested tube types: 6L6GC, 5881, EL34, 6550B, KT88 (push-pull parallel, 4 or 6 tubes).
I'd also replace the old solid state rectifiers with something newer, like UF5408s would be good, and just use two in series instead of three - you could probably even get away with just using one per leg. The silicon rectifiers of the 1960s were not nearly as reliable and hefty as those available today. UF4007s might work, but I'd go with the heftier 5408s just to play it safe, at little extra cost.
/ed B