Hey, I have a cpl heads needing work, and I have a couple questions. First, can anyone recommend a good tech to do the repairs in the Chicago/Joliet area, with a phone number if possible. Second, I have a Marshall model 4100, a 5881 based amp that I know has a bad power tube. I would like to know if it is possible to use KT66 tubes in it. I'm not sure if the amp can accomodate the actual size of the larger bottles, and also, the amp has a 1/2 power switch that goes from pentode to triode (I think) and I'm not sure if the KT66 are compatible with this. As always, thanks for taking the time to read and reply, and for sharing your wisdom!