Do preamp tubes burn in over time ?

[CA$H FLOW]

I just took out a Tung Sol RI from V1 in my amp cause it had a fizzy distortion when using high gain(which I like), and put a JJ in place of it. I'm now finding that the JJ sounds too trebly with the highs and doesn't really have a nice distortion character.
Do you think it will improve with a few hours of use ?
I do still have the original Sovtek preamp tubes and an extra Mullard RI 12ax7, so I may do some swapping to find something I really like. When I swap preamp tubes around do I need to turn the entire amp off or can I simply put it on standby ?

 

[Blue Strat]

Standby is ok for swapping unless you break a tube, which is possible, while inserting or removing...in which case you'll get a shock.

Tubes can change character over the first 10 or so hours of use.

 

[CA$H FLOW]

Standby is ok for swapping unless you break a tube, which is possible, while inserting or removing...in which case you'll get a shock.

Tubes can change character over the first 10 or so hours of use.

Thanks Blue Strat! I figured as much. I recall that the gain from the stock Sovteks seemed to improve a bit after the first week that I owned the amp.

 

[robertkoa]

Standby is ok for swapping unless you break a tube, which is possible, while inserting or removing...in which case you'll get a shock.

Tubes can change character over the first 10 or so hours of use.

I had a tech put in a JJ803s the one with the huge plate into my old H@K

Tubeman and it sounded a little fuzzy on the higher gain modes so after about two days I was going to bring it back and try something else- and I plugged it in one last time and BAM- the fuzz was gone.

It has a MUCH wider usable gain range( compared to the Groove tube )and very clear gain- surprising- this must have been what happened- the JJ803s is a very impressive sounding tube.