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I have noticed while playing shows that my amp usually sounds best by the end of the show. Feels like at least an hour later i like the sound better?

In addition to what's going on in the tubes....

As an amp warms up, resistors heat up from being close to the tubes and/or from the amount of current they're expected to pass. Some of these get quite hot by design. As resistors heat up, their resistance changes, so when compared to a cold circuit, the warmed-up circuit has different resistor values here and there.

Any resistors that are in the path of juice-to-tube -- bias supply, cathode resistors, anything in the power supply -- will shift the tubes' bias points if they change temp significantly, and a shift in bias will affect the sound. It may take quite a while for all components to get to the balance point where they're no longer getting hotter; definitely more than a few seconds.

Likewise, the power transformer heats up to various amounts depending on amp & trans design, and this is going to change the PT's impedance and power supply performance, which is likely to affect the sound as well.

Filtercap to the modafockin rescue!

Thanks. This actually sounds like what is happening with my amp during warm-up.

If I remember from the last time I opened it up, my amp's tone stacks are situated right above my power tube sockets. You could probably toast bread with those EL34s... This is making sense since the change I'm percieving seems to mostly have to do with a flattening out of my EQ.

Any idea about the tube expansion business?

Also, over time in operation, tubes do get hotter. Very much so.

Given that heat, as thermal energy manifesting itself as excitation of free electrons is a principle operand in the operation of both pre and power amp tubes, there will be a difference in perfomance of the tube depending on how long you have been playing the amplifier.

Isn't education a wonderful thing!!??