[AMPS] new to me al-811h amp

[Scott Bullock]

I just picked up a used Ameritron AL-811H amp yesterday for real short money, it was advertised as broken. It arced everytime you turned it on. I pulled it all apart and found a burnt grid resistor on one of the tube sockets. Upon further inspection of the tubes, it was noted that the plates weren't straight anymore, and the tops and bottoms of them curled in towards the grid. The tube that had the blown grid resistor had a obvious plate to grid short. 


These tubes have white ceramic bottoms with no external markings on them except for a "P" on the glass. There is some handwritten marking on the inside of the tube, but just numbers. Do these sound like the Svetlana 811a or some Chinese clone?


Without having the extended experience with 811a tubes as many of you do (it's only my 3rd amp), what would cause the plates to curl like this-just excessive heat, improper loading, improper cooling? 


One of the things that I looked at on the 811h amp when I saw this was that it only has one area for air outflow, and it looks to favor the capacitors in the power supply. The tubes really don't see all that much air around them. On my sb200 (2) I had always put a muffin fan on top of the amp to pull the hot air out and keep the internal temperature down in contests. What's the general consensus on installing a screened vent over the rf section to vent air there?


Thanks,

Scott

KA1CLX

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