Hi all, I've been wondering about this for a while, and finally decided to look into altering the sound of my JCM900 4100 head. Please bare with me here, I'm not very well versed with the inner workings of valve amps so I may not be able to do what I want to do.. And all work will be done by a professional tech, don't worry!! So ideally, I'd like to change the sound of preamp overdrive on channel 'B', without changing the sound of the clean channel... can I do that? The reason being that I've always found the overdrive to be a bit too fuzzy, too gritty, where I'd prefer something a bit smoother, creamier, without loosing the sustain you get from pushing the gain up. I almost exclusively use the preamp for overdrive, not the poweramp, so figured this would be the logical place to start. So I guess my questions are... 1. Is there an alternative to the stock 12AX7 that would be a good match for the sort of sound I'd like to move towards? 2. Does changing a preamp valve require rebiasing like changing power valves does, or can I just drop a new one in? 3. Can I even do this?! Thanks for your help!