Jack,

You probably know this, but just to be sure.....
The tensioner doesn't work the same way it does on your late model white block. On a red block, in oder for the tensioner to apply more force against the belt, you have to loosen the bolt. Only then can the spring push the roller against the belt harder. Then, once the bolt is retightened, the tension is "fixed", so any stretching of the belt over time will make it looser.

As someone else noted, the tensioner might not swivel easily on the bolt, even when the bolt is loosened. That's why I like to tap on the bolt head a couple times with a light weight hammer when the bolt is loose to encourage the tensioner to move.