on jerry's both channels were preamped, although one channel can be done and an rca hole probably would need to be enlarged. the "preamping a twin" over at dozin.com works, but is not how jerry's was. here is the layout for both channels. simply eliminate the other if you choose to tap only one, normal or vibrato.
this isolates each channel and shunts the power section out. isolating each channel and shunting the power section drops unused portions of the circuit thus reducing the noise. with the proper jack(switchcraft 13) and wired correctly.the normal channel essentially becomes an alembic f2b, while the vibroto channel is the the jerry version with reverb on it(and vibrato if the opto's leg is not snipped).
as for the input mod.
input 1 eliminates the input resistor and eliminates the noise associated with them when used exclusively. i have noticed a slight audible gain also, but others have not. input 2 stays stock wired(with the exception of the jack used) so that its jack can still be used. the mixing resistors both come back into play when both inputs are used simultaneously. so there is 3 scenarios coming from them.
most will see the 68k input resistor of input 1 still in circuit, and it is, but the path of least resistance is used thru the always closed switched jack of input 2. that switch opens when a plug is inserted. actually the preamp tap switched jack is the opposite of this. those boy's were mighty clever.
jeff, dozin's preamping a twin shows the vibrato jack being used, and it can and does work. but that is not how it was done. jerry didn't use the vibrato so using it's rca jack could work if you only preamp one of the channels. by rca jack i actually mean it's hole. with the 13 jack, you can use your amp as a combo with nothing plugged into the 13 jack and the power tubes inserted. and to preamp the same amp, with the proper wiring and 13 jack, simply plug your power amp into the tap and remove the tubes. the removal of the power tubes is to save the output transformer and WAS done on jerry's.
that all said, if one had a seperate power amp, speaker cab AND a combo. i would think that the power tubes could stay in when preamped as long as the combo's speaker stays also plugged into the combo. this would provide the load needed when not used and since the power amp side is shunted it would get no signal. this would make for a sweet backup if the external power amp and cab went down. and would be as quick of a change as removing the preamp plug. NOTE: i have never tried this, but would think it could be done.