I recently picked up a pair of allen 90's and now in a quandary. Do I refresh the existing circuit or make use of the ultralinear taps on the huge output transformer? I am still a novice and would value any feedback you have !!! Since I am planning on using these for my primary tube hifi system some of the questions I have: - Are there more advantages by incorporating the ultraliner (UL) taps? - What would be the best way to switch over to utilize the UL taps? - Would I need to drop the B+ voltage by about 100v (from 580v to 480v) to accomodate the anode and screens? - Does the bias voltage have to be changed from -35v to say -70v? The amps I have are similar to the schematic however they do not have a 5k pot in the the bias circuit and the bias circuit extends to what I think is the 6SN7 driver stage, right before the KT-88s (entering the 6SN7's by the ground and 510k resistors which ultimately bridge pins 1,4,6 and 3) These seem like they could be sweet amps but I cannot find any information let alone modification advice about them.