TL;DR: They work fine if you have an adapter that swaps pins 9 and 1. Details below.

There had been some discussion previously about using the EL83 tube in place of the 6P15P/EL84 power tube in the UFO amps. And the consensus was bad things happen if you plug one in. A quick check of the datasheet confirms that the EL84 has the screen on pin 9 and no connection on pin 1 while EL83 has that reversed. So they are pretty much guaranteed not to be interchangeable, but they should be adaptable. And even better the EL83 does not have the suppressor grid connected to the cathode so it is possible the Hazen Grid Mod will work with them.

While I was waiting for my amps I bought some EL83s and a  couple EL83 to EL84 adapters from ebay. And sure enough, the EL83 tested as a EL84/6BQ5 on my tester. The problem was it also showed shorts, on several tubes and both adapters, all of which tested fine. And a quick check with the multimeter showed that instead of swapping the connections the adapters connected pin 1 on top to both 9 and 1 on  the bottom and 9 on top to both 1 and 9 on the bottom. Which would be fine for an actual EL84, which has no connection on pin 1, but a 6P15P has pin 1 connected to pin 6 which is the screen. So it is possible the amp may do something on pin 1 and I worried the ebay adapters would cause problems. So I set out to make my own adapters that worked they way I wanted. It was going to be a chore, but good for my solder skills, and a good way to pass the time while I was waiting for amps. Either fortunately or unfortunately after making a couple that kinda sorta worked on the tube tester I gave up and had a guy I know other another site to make the adapters for me. And they work great. In fairness to me, I'm much more of a "software guy" and have nothing but admiration for you guys that are handy with soldering irons and/or power tools.

I've had the amps for about a week, and they sound fantastic. So today I tried the EL83s for the first time. And they work. I'll have more to say about the sound after some runnin and controlled listening, but out of the box they sound fine.  So those of you who are interested in rolling a new power tube and have solder skills (or are interested in developing them) and way too much time on your hands here is the perfect holiday project.

thanks for sharing, james. very interesting. what makes this forum great is the willingness to experiment by many. this is how we all learn. this is how we get better.

kudos!