by potatofarmer » Tue Jun 06, 2017 1:01 pm
I have an '81 135W Twin Reverb. They aren't really technically "ultralinear" but you have to measure the screen taps on the OT to find that out. Basically Fender just did this to protect the screens while they pushed the plate voltage higher (~520V), but keeping the tone closer to pentode-mode. They're definitely not going to get you Model T tone or headroom though. Taking the screen taps from the OT gets you better bass response, at least.
#1 best thing you can do is to remove the NFB loop - the screen taps on the OT give a little NFB anyway. I had mine on a switch for a while, but I never used it so I took it out completely. Plus with it removed, a PPIMV works great. At least clip out the bright cap on the existing master volume; no idea what they were thinking.
The bias balance control is essential for filtering power supply noise. Well, it doesn't really filter it - screens are very sensitive to noise, so the Model T uses a pair of chokes, while Fender just uses the opposite tubes to cancel it out. So don't take out the balance pot if you want a bias pot, just add one upstream. I put mine in the "line out" hole because who the hell is ever going to use that? Also the balance pot makes bias-vary tremolo impossible, or at least I can't figure out a way to pull it off and I've tried just about everything. I finally went with VibroChamp-style tremolo, injecting into the first stage. It works really well, but I never use tremolo anyway.
They also did something weird with the speaker jacks; plugging into the external jack puts the two jacks in series and connects to the 8-ohm tap on the OT. So you can use one 4-ohm load, two 4-ohm loads, or one 8-ohm load and still be matching. Kind of a complicated way of not mounting a third speaker jack, but it is really crowded in there so I guess it makes sense.
Pretty sure all this applies to any of the 70W or 135W Fenders from the late 70s-early 80s. Oh and the wattage numbers are pretty reasonable (as far as manufacturers' claims go) - my Twin puts out right around 115W clean.
Oh and the stock speakers mine came with were crap - Utahs probably only rated for 50W. I put in a pair of Swamp Thangs which seem to fit the sound really well but the huge increase in sensitivity made it much much louder.