Q: The tubes in my guitar amp are glowing blue instead of sort of a yellow glow. Are the tubes bad?
A: That blue glow is just a side effect of a power tube — a fluorescent glow in the blue spectrum. The tube is fine! It actually indicates that the vacuum inside the tube is very good, which is what allows this phenomenon to occur. It will not have any affect on the performance or tone of the amp.
HOWEVER: If you notice a red glow on the metal plates of the tubes, turn off the amp immediately. This indicates that the tube is overheating and is red hot. It's likely that the tube bias in the amp is set incorrectly, but there may be some other problem causing the tube to overheat. Have the amp checked by a technician before you use it again.
"Slew" refers to how quickly a circuit's output voltage can change; it's usually expressed in volts per microsecond. When it comes to pro audio equipment, this spec is usually referred to as the slew rate and is most commonly associated with amplifiers, but in fact applies to many types of gear. In amplifiers, a low slew rate "softens" the attack of a signal, "smearing" transients and sounding "mushy." Since high frequencies change in amplitude the fastest, this is where slew rate is most critical. An amp with a higher slew rate will sound "tighter" and more dynamic to our ears.