Hello, maybe someone here can help me out.

I just bought a Kustom Tube 12A amp used for $40. I thought that was a decent
price and it sounded OK to me. I figured I would replace the stock tube eventually
with a tungsol 12ax7, I have read that doing so will improve the sound.

When I got it home, I unscrewed the back plate and discovered that no tube
was present in the socket. The amp still worked fine without it.

Can most amps bypass the preamp or is the Kustom tube in there just to call it
tube hybrid but doesn't really effect the sound?

Really confused now, thanks in advance for any help.

Its a solid state amp with a preamp tube in the preamp section, not a tube amp. Because its a hybrid is why it still works I assume. Goes to show just how much difference that one tube makes lol.
Agile AL3000
Douglas WRL90
J&D Strat
Squier Tele
Sammick TR2
Douglas Draco
Peavey JSX
Bugera V5

well TS. If I were you I'd get a new preamp tube and stick it in there and see what happens. If you are not sure what to buy let us know. It should be in your manual. Just get whatever brand but match the spec otherwise. Is there a tube socket? NOT having something plugged in there could be dangerous.
It might sound better with a tube. I used to have a Valvestate 8080 and the amp would play as soon as you turned it on, but the tone changed when the tube warmed up.
Jeepers. Hybrid technology really IS a myth... Who'd of thunk it?

Is what some junkies refer to heroin as. For me it's just loads of wires and good sturdy housing, but just as addictive.