i just bought a valve amp ( this one http://www.aberdeen-music.com/forums/equipment/42538-fender-super-60-tube-amp-sale.html.) this is the first valve amp i've owned and was wondering if anyone has any tips on them? got a few from the previous owner such as to switch the power on for a couple of minutes before the standby, but when will i know when the valves need changing, how do i go about choosing valves, is it easy to change them myself, etc. any tips or information is much appreciated. thanks!