Recently I turned on the amp and the Sovtek GZ34 sparked and blew, it may have had 400 hours. I replaced it with another new production Sovtek and that tube and another blew the 5 amp fuse. I decided to try one of my 1958 Mullard GZ34’s. The relay clicked, no sparks and the amp come to life as normal. I played the amp 3-4 days until I received 3 new JJ GZ34’s. I tried a new production JJ and the sparks and fuse blew as with the new Sovtek’s. I know the NOS tubes are much better built with better materials and all that… The question is why did the amp run on both new and NOS GZ34’s for two years and now only the NOS? Is there a bigger problem in the Amp that I have not found?
The specs to my amp: