An EL84 can safely put out 12 1/2 watts per tube,times four would be 50 watts.If the .50 Cal actually lives up to its claim or not,I cant say for sure.Many amp makers do exagerate or "round off" the output,for instance,its not uncommon to claim 50 watts for an amp that is actually putting out 45 watts.I have never measured a .50 Cal's output,but the 50watt claim is not far fetched at all.I would be more skeptical of the class A claim,many amps claim class A and arent.