Famous Last Words #249:"Of course the top cap is the grid."
That reminded me of an old audio amp I had to service for a drive-in theatre..it was made by Quad-Eight, and had integrated circuit (IIRC) input stage, transistor driver stage and TUBES (valves for our non-U.S. readers) for the output!
Rated 350 watts into 4 ohms mono, it would easily drive a 1 ohm load (typical of a field of drive-in speakers) with an output close to 900 watts! (Yes, I did measure it, and the output was fairly clean, with only minor clipping. I also destroyed three sets of dummy loads in the process.)
It weighed around 40 lbs. and had this charming sticker on the top of the chassis:
"WARNING : Lethal voltages inside and on grid caps. To be serviced by qualified persons only."
I think I still have the schematic for the beast in my files (somewhere).