Mc501 and Powerguard--Have you activated it??

Today I was litening to a live Grateful Dead show from the early '90's--"Without a Net". I am using a pair of Mc501's with a c220 pre and a pair of JM Lab Alto Utopias. After 25 minutes of driving pretty hard and really loud(volume level 67 on pre), the left amp's Powerguard was activated.
When I got up and felt the amps--WOW!! They were HOT,HOT,Smokin' HOT! To the point that I could not touch them for more than about 2 seconds.
The good thing is that they were doing their job by shutting down, however I NEVER heard the JM Utopias sound as though the amp was clipping. They never sounded strained.
I was wondering if anyone else with Mc501 amps had similar Powerguard experiences. How hard do you drive till they shut down?
BTW: the amp "came back" after about 2 minutes.
I hooked up some little cheapo book shelf speakers to a Mac 602 (600 wpc) and cranked it to see if they would explode. The power guard kicked in just like it was supposed to and no harm was done.

It really is a nice feature.
Actually Cajunpepe, what you triggered was the conventional thermal cutout circuit, not the Power Guard per se. In addition to the actual Power Guard circuit, McIntosh frequently ties both this thermal cutout circuit and a DC-offset protection relay (on direct-coupled amps) to the "Power Guard" indicator light. The operation of the actual Power Guard circuit is noticed when the indicator lamp flashes with the musical peaks, when the power meters are around a full-scale reading, not when the amp mutes itself for an extended period. In any of these conditions, damage to the amplifier (should be) very unlikely, and you won't notice any audible clipping. But when the amp is driven very hard into the actual Power Guard operation, you will hear some marked dynamic compression.

Mac solid-state amps, like any conventional Class B circuit, will generate the most heat when the average power output is about 1/3 of maximum. When the designers decice exactly how much heat-sink area to allow for cooling the output stage, there's some educated guesswork based on the peak/average power ratio of musical signals, and the amplifier's anticipated operating conditions, in terms of power output and ambient temperature.

Sounds like in the case of the MC501, McIntosh cut it a bit close for party use . . .
What binding posts are you using? If you have the speakers hooked to an impedance than higher you will overheat and the sentry will shut it down.
Are your speaker cables grounded properly, the only time this happened to me was when I connected a spade upside down and part of the ICONN the spade was screwed into was touching the mounting plate (MIT Cables). Otherwise I don't think I've come close to full power, even at 60+ on my volume.
Same speakers as you mentioned with a Mac 500 tube pre along with 501's. I was at a friend's home during the summer, an all day event and the speakers were at pretty well concert level and no issues what so ever and when I actually touched the top of the 501's they were just warm. I actually owned this specific pair prior and played them very hard at times with Eggleston Andra's and never had any issues you are describing.

Something is up, I would check your connections first assuring that you do have the right tabs hooked-up as per your speaker load, also check to insure everything is secured and not touching anything.

Do you have these into some type of power conditioner, both mine and my friends plugged directly into designated lines direct into the wall.

By the way my friend now has moved-up the line to the Nova's and I was over at his place a week ago having a listen, no issues what so ever and we were cranking the system. They are set-up in a very large open concept and the main floor area of aprox. 3,000 sq. ft. just to give you an idea.