He changed the parts or upgraded them to make it worth the tube change from the 811 to 572B tubes. Basically he made it into a 572B Ameritron clone. Alot of people did the tube swap with thinking there was going to be a big power jump but they dont realize there had to also be some curcuit changes
If you also look at the Dentron Clipperton L amplifier it also runs 4 x 572B tubes and is rated also at 1300 watts. The 572B is an outstanding reliable tube if not beat to pi$$. Its no 3-500Z powerhouse tube but none the less reliable when ran within its limits.
Im actually glad to see someone did the tube swapover the correct way and did the mods and upgrades to make it even worth doing! Although I to would love to know what meter he is using to test this power output even on a true peak reading meter these power readings shouldnt be possible I say this because I own an amplifier with 572B tubes and know what I see at driving them with 60 watts.
Perhaps he was getting those numbers on 11m where we all know the natural laws of physics and electrical engineering simply do not apply. :laugh: