I don't know if anyone has tried this on a 5150 (Eyes might have though), but 3 of the 4 6L6 tube sockets on that thing will take EL34's (Ed's old tube).
Imagine a high peformance car, but with 3 wheels instead of 4. It'll do some amazing things, but won't last too long before needing new parts. This is esentially what you have here. Running on 3 tubes does make the amp sound very good, but you going need to have a supply of parts to keep it running.