They are also known as valves..
They have clear where you can see the filiments and rather easily accessable for replacement ..
You would most likely feel heat emitting from the amp if it were a tube amp..
The Peavey Backstage is call a transtube-emulating practice amp.
A tube amp is more delicate then a solid state amp.
The best info I could fine... Take note of the text overlay on the video.
good luck and keep pickin'