John, what hardware you are using...Tan or Green HV boards

Any of this should not burn out the MJE521. If it burns out you have a issue on the board. I will say throw that away and try a new one.

If any of those you use lower the 220 res. on the flyback what value resistor you haveon the base to 521. What Voltage you have on idle on the .47 after the 4007 diode.

Adrian