Is it a OK to leave tube pre-amps powede up 24/7

and says the power surge upon turn-on is harder on the tubes than 24/7 usage.

This really has to do with the design! The phenomena known as 'cathode stripping' really affects power tubes a whole lot more than small signal tubes!

I find that I get much longer life out of the tubes if they are only on when I'm using them. After 20-30 minutes they sound great and after about 2 hours they've gotten as far as they are going to go. Running a tube preamp 24/7 is going to shorten the effective tube life unless you work at home and have the stereo playing all the time.

I've yet to see a signal tube explode! For that matter its really really rare even for a power tube.

I've been servicing consumer electronics since 1974. I've yet to see a fire in **any** preamp, tube or solid state that was not caused by a lightning strike. Not saying it can't happen; I **am** saying that it likely will not be caused by a tube.

A tube power amplifier is a different matter. Power tubes can arc and cause associated resistors to heat up and fail (these days, responsible manufacturers use 'flame proof' resistor types in those locations).

But we're not talking about tube amps, we *are* talking about tube preamps.

Well, some  ARC tube preamps can power up a pair of small speakers if connected to the output to significant SPL levels.
That is to say that signal tubes there are driven rough almost just like power tubes.

I have a set of adapters to use my preamp with my speakers in case I don't have a power amp to use at the time. The preamp (MP-1) will play my speakers with about the same volume a type 45 SET. But the tubes aren't run that hard and last for years without any degradation of the circuit boards even after decades of use.

What you are saying here suggests that such is a different matter with ARC preamps. But I think if that were really a serious problem we would have heard about it by now! Preamps bursting into flame is a pretty unusual thing and the age of the Internet has been with us for 24 years!! And no such reports. So I think we can regard  this as a 'theoretical' (or perhaps more accurately as a Red Herring) rather than an 'actual' and only applying to certain ARCs, which, if true says more about them than tube preamps in general.

But I do find myself doubtful about this idea- I have friends that work at or have worked at ARC for many years (I live in the same town and knew them before they went to work at ARC) and none of them have reported anything like this (and they would have if they'd heard about it) so I just don't think this is a concern. 

There are only so many electrons in a vacuum tube.

There might be a limit to how many electrons can **flow** through a vacuum tube... I've seen some 6SN7s go over 50,000 hours in our gear.  Admittedly, that's an outsider number but some tubes have very long lives!

I'm much happier this way, but am I doing it any harm?

No.