[bradpdx]

I'll add a couple more things:

1. In older amps with original (30+ year old) resistors, the resistor values have often crept up. They do this even more when heated up, in my experience. If the resistors are new, they exhibit less of this behavior and are probably closer to specified values regardless. The old resistors are becoming chemically unstable.

This is one likely reason that some older amps appear to have more gain - the plate resistors have crept up 20% or so. If you have an old amp that changes a lot as it warms up, you may have a lot of old (plate) resistors. Change them for new resistors and the amp should stabilize, just like Leo intended.

2. If you wish to leave your amp on for long periods of time, take advantage of the standby switch. That turns on only the tube filaments and keeps the chassis warm without wearing out the tubes.

3. Cranking a tube amp doesn't usually cause the whole chassis to become much hotter, only a bit hotter. The only elements generating more heat are the power tubes, and they are exposed to the open air with no heat sinks connecting them to the chassis. Thus the difference in chassis temperature after an amp has been "cranked" is really pretty small.

This is very different from transistor power amps, because the small physical size of the devices (and greater temperature sensitivity) necessitates the use of heat sinks that frequently conduct all the heat through chassis components.