Advice on selecting a replacement transistor for a high voltage transformer
If you wanted to replace a rubber hose in some equipment and could not find the exact replacement what would you do?


Would you only look for one that has the same fittings?
Would you look for one with the same fittings and same internal pipe diameter?
Would you do some dimensional checking then decide whether product A or B was best, based on what pressure and flow rate and fluid type was being transported?


The latter one is what you should do because not understanding the application for your hose could mean it burts (because of over-pressure) or burts because of material incompatibilities (i.e. the fluid degrades the rubber used).

It's the same with transistors.