I want to drop between 70 to 120 volts of B+ in my 5E3-style homebrew. I started with five 16V/5W zeners in series between the PT center tap and ground. Worked fine, dropped about 70V with no apparent issues. I then tried three 39V/5W zeners in series to drop even more voltage with fewer parts. After a few minutes with the power on, I started observing some smoke in the area of the standby switch. Couldnt tell exactly where it was originating. Left the power on for another minute until the smoke intensified. Then I used alligator clips to bypass one of the 39V zeners, giving me two in series, and the smoke returned. Switched back to the 16V zeners and no more smoke. What about the 39V zeners could be causing the issue?