Hi, I have randall RT503H And would really appreciate your help on this, What is the preamp tubes schematics for this head, I want to use the power amp section with an external preamp, bypassing the head's preamp section using the 4 cable method, the thing is i need to know whcih of the 5 preamp tubes are the phase inverter and the FX preamp tubes, just to remove the other 3 tubes for the Ch.1,Ch.2,Ch.3,,,just not to end up with dead preamp tubes in the head (will this also may hurt my amp)?? Much Thanks!!!!!!