Tube Guitar amp sounds different over a period of time

I don't know for sure , but I have the feeling that my Marshall JMP starts to sound different after 1/2 hour or so of playing.
What parts could cause that, if so ?

This probably an effect of thermal drift. Likely suspects are carbon comp resistors (though there may not be any in your amp, they are more likely carbon film), or ceramic caps which may have been used in the tone stack. These guys can change value by 20% or more with temperature shifts, so image your tone controls all moving by 20%.

By 1/2 hour the amp should be running at a constant temperature and the tone should stabilize.

Cheers,

K