I remember reading somewhere some stories about guitar enthusiasts younger than me getting a valve guitar amp for the first time. One of the things that doesn't always occur to them is that, yes, there are actually different types of valves.

One story was about one such guy who found his amp stopped working one day, so he pulled out one valve and went into his guitar shop and asked for "six of these". The store clerk was not being observant that day and sold him precisely that. Not surprisingly, his amp didn't work with the 6 new valves.

Wade.