[Amps] Tube testing?

[Gary Smith]

In amp that uses  3cx800a7's, is there any way to determine if one of a pair (or more) has gone south other than by simply replacing it to "make sure?"

What I did (and this may be wrong as all get-out) was take a Raytek laser thermometer and read the temp of the tube on the exposed plate area.  

What I found is there was only  a 7 degree F. difference between the two, 128 deg and 135 deg.  Not very scientific but... at least I know there is something goin on inside that sealed envelope to indicate to me that the electrons are swapping places at a pretty good clip to generate lots or RF.

If there might be a better way other than pay many $$$.$$ to have them checked by the manufacturer, I would like to hear about that method.

73
Gary 
WA6FGI