Re: Valve type (Marshall AVT50)
Should be a quick and easy replacement, as it's only a preamp tube, not a Power Tube.
I replaced mine several times experimenting with different tubes by simpy powering down, waiting for the tube to cool enough to handle, pop the new tube in, power up. Repeat as necessary.
The AVT series is a hybrid amp. Meaning it is both tube and solid state. It has a tube pre-amp section, and a solid state Power Section.
This makes tube replacements much easier, as it is a single 12ax7/ecc83.