Preamp

What do they do?

Preamp tubes greatly increase the input voltage from your electric guitar. The signal from your electric guitar pickups is way too small to drive a speaker.

How can sound be affected by them?

a.) The amount that the input voltage is increased can be different from tube to tube. A bigger increase can give you more overdrive distortion and a smaller increase can allow for a cleaner sound.

b.) The amount that certain frequencies are increased can be different from tube to tube. One tube might increase high frequencies more than another, making it sound brighter.

Can they be easily changed?

Yes. Just let the tubes cool down, pull the old one out and put the new one in.