Amplifier Orientation?

I've got a pair of ARC VT100 MkIIIs and some space issues. One thing I thought about was welding up a rack to hold them vertically, and stashing them behind my front end equipment, which is sitting on a couple Solidsteel racks. Its not like there is any reason to actually see them... I can't intuitively believe there would be an issue with operating an amp in a different orientation--it might even improve airflow around the amp. Just thought I'd see if anyone had any wisdom on this grand plan before I embark on this scheme...
I would strongly recommend agfainst it. The VT 100's run hot and need some room to breathe. Stacking them vertically will just heat them up even more. Keep as much room between the bottom of the VT 100 and the shelf as well as keeping them clear on top.
two issues to deal with if you are saying that you want to stand your amps on their ends assumiong that they do not have cooling fans: `1, standing them on their ends could diminish the flow of cool air, i.e. the hot air flow off the lower tubes would be used to "cool" the upper tubes. 2, is the framework of the amp sufficiently strong to support the weight of the downward shearing pull of the transformers? i'd give arc a call before i tried this.
Stacking shouldn't be a problem but I would remove the top and (if possible) bottom covers to improve air flow. You could probably disconnect the tiny (and noisy) stock fans and put a bigger (and slower) mother under them -even try locating the fan elsewhere and channeling the air through some 5-inch dryer venting hose(s). Come to think of it, with the covers removed, they might not even need forced air cooling. Give Len' a call and see what he has to say...
Power tubes should NEVER be run horizontally unless the amp / tubes were designed to do so. Not only does this create hot spots on the tube due to uneven cooling, the plates can sag over time and short out. Some tube manufacturers in the old days actually had slightly different manufacturing methods for vertical and horizontal oriented tubes.

Line level tubes are typically not a problem to turn sideways. Due to their lower plate voltages, current consumption and heat generated, one typically need not bother worrying about that.

I have no idea which, if any, orientation would be lower in microphonics. Sean
Do not change the orientation of the VT-100s... They should sit on their feet... You really do not want to have all of the heat rising to the circuit card that is on the top. One of the things that will happen is shortened tube life. With that in mind - the biggest "no-no" of this all is having 1/2 of the tubes in this amp operating base up.. Just a bad thing to do to tubes.. A quick consultation with the owner's manual will also confirm ARC's position on how the amp should be orientated.

Per Sean's post, the best way to run tubes is base down. Audio Research has made the choice to run the tubes horizontal, which is why the later units have fans to assist in the convection process and reduce issues with horizontal tubes.

The next problem you will have is that the amps are not on their feet. ARC has done a decent job of providing feet that actually do some good in terms of vibration isolation. You will find this out of you ever experiment with isolation tweeks and the VT-100..
Thanks all. Didn't seem intuitively like an issue, but I wondered, and the points you raise make more than a little sense. I didn't think about the effect on the upper circuit board, for example. I'd always seen base down tube configurations, but the ARC, as some noted, have the tubes positioned horizontally, so I wondered if orientation was a problem. That's why I figured I'd consult the experts.

Thanks again. Guess my floor will just be messy...
i was completely unaware that this amp ( or ANY for that matter ) ran the outputs horizontally. Hopefully, they've done their homework and this will not turn into a problem in the long run. Sean
Sean - ARC horizontal tubes have not been a long term problem for me (4 yrs)... but my amp has plenty of air space.