You are correct, I explained that incorrectly. The amp is switched on when I detect the 27V on the output jack. I meant it simply being plugged up for a period of time would cause the problem to happen when switched on. Initially I thought that it was happening after a period of being on and actively using it. Once it fails the only way to get it back is to leave it unplugged for several hours. I'm not sure how long it takes but overnight works.
So if left off over night I can come in and plug up the amp and then turn it on to test it and there will be 0V on the output jack.
The following two scenarios are repeatable.
1. I can play the amp for 5 to 6 mins and it will work fine and then the hum starts. Retest and the 27V is on the output jack.
2. I don't use the amp but leave it plugged up and then come back after several minutes and retest and the 27V is on the output jack.