IMHO, Just grounding the grids was the wrong thing to do. What if a tube flashes over? Now that could be some interesting Fireworks. If, I were really determined to make a Mod of that type, maybe a fast blow fuse rated at around 25-30% greater than the Maximum rated Plate Current? In my days I have seen lots of Blown Plate Chokes, Rectifier Bridges, and Power Transformers just because of the Mod described here. Also, what good is the extra 30 to 50 watts going to do for you anyway? Just turn the drive up a bit and be done with it. With 600 Watts, and it is the first 500 to 600 that really counts. Figure what the difference to a receiving station would be to add just that little bit extra. In quick summary, those resistors were placed there for a reason. The Al-811 is known for arcing Plate and Load Bread Slicers. And if I were going to do any kind of mod on one at all, it would be 2 things. 1. Better Plate and Load Caps in the Pi Network and 2. Replace those 811's with 572B's ......
73 de Wade/KJ4WS
Maybe you should take the time to read the details about the mod. This mod is recommended by the designer of the amp. http://www.w8ji.com/al811_important_modifications_changes.htm
What do you mean by better plate and load caps? Do you have a recommendation?
Next time I need to replace the tubes I'll consider buying the 572's.