Call Groove Tubes or Boogie. I bought a MKIII that had GT kt66's in it, and within 5 minutes of turning it on it blew a fuse. I called both Boogie and GT and thay both said that the 66's run too hot for the fixed bias and I'd need to get a bias mod done for them to operate properly. So I'd think it'd be the same for the .50 cal.
