From jackf@polychoice.com Tue May 01 16:56:43 2018
Subject:Re: Leslie preamps + speakeasy preamp conversion

Well, back in the day when I had a Hammond B3, I used to let my guitar player plug it in to where my expression pedal went to. On the box inside the B3 there is an RCA jack, I put an RCA to 1/4 inch adapter on that and a short extension cable that ran just outside the B3 with a 1/4 jack for the guitar player to use for a few of our songs.

Other than that, any pre-amp should do as long as you know the pin outs to the 122.