[Amps] Ameritron AL-82 blowing zener diodes

[Joe Musto]

I have an Al-82 amp that is blowing the zener diodes in the bias 
circuit.  The first time it happened, ZD1 blew first then a few weeks 
later, ZD2 blew.  I replaced both at the same time and asked the guys at 
Ameritron what would cause them to blow.  They told me it is usually caused 
by a tube arcing over.  After I replaced the diodes the amp worked fine and 
put out full power.  Two days later, I noticed the tubes getting hotter 
than usual again and checked the idle current and it was high.  ZD2 blew 
again.  Has anyone seen this and/or have any ideas?  I have heard no 
crackling or snapping and I really baby the amp.  I run it SSB only at 
about 500W on 75M and 1000W on 160M.  SWR is low on both bands <1.5:1.

Thanks,

Joe N1FCJ