Dozin wrote:If we could put a good list of mods for the Twin I'll add it to the web site. I need to revisit the Twin myself. My buddy just picked up a 74 twin and we need to do some work to it. So, I guess I'm back into it and this is good timing to get it done.
i've pretty much noted on the schem and layout over at gearheads the majority of changes. the thing to remember is that the babbs amp was pulled from a pool of twins at Front Street, and while jerry "might" have used it, we still have never gotten under the hood of his twins. also, for the most part jerry's twins were stock. that said, i have tested and played all of the mods. as a matter of fact i loved them all so much they are all still in use.
here is a list of knowns that he did have.
1) individual channel preamp taps
2) metal film plate load resistors
and these are very good mods that probably were done to "clean" things up on jerry's amp and ARE shown on the babbs amp.
1) vibrato leg snip - eliminates the vibrato and increases output. by snipping the leg you also elimnate any noise from the unused portion of the circuit.
2) cap material change of the 10pF cap, parallel with 3.3meg resistor, on the vibrato channels 3rd gain stage from ceramic to silver mica. ceramic can drift causing a more dramatic treble boost.
lastly, the alembicization input mod is NOT shown on the babbs amp, but is a good possibilty that it was in jerry's twins. it eliminates noise on the input by removing the 68k input resistors on input 1. input 2 stays stock. here is the possible clue for this mod, Jerry ALWAYS used input 1 on his twins whether it was the blackface or the silverface.
as for tubes, forget power tubes, once jerry had the preamp taps to an external power source his power tubes were removed. as for preamp tubes, it is well noted that they went to military surplus sales and bought all the tubes, caps, resistors etc,etc they could. so he probably had some american made RCA, Phillips, or GE tubes in the preamp slots. and being in the preamp section they could probably last a long long time. i would shoot for the cleanest, less noisy tubes you can find.
also, i would like to thank buck for all he has contributed. we collaborated quite a bit before posting what we discovered and while i cannot speak for him, i would say that we both enjoy the fact that people like "you all" are getting a chance to use the info. my amp sounds great and i hope you guys get the same feeling i do when you play yours.