[Amps] Rehabbing that dusty old 3-500z

[Brian Moran]

I'm 'bringing back' an old Heathkit sb-220 that sat around unused for a while (years), and it has some Eimac 3-500z's in it. Having read the archives over how to test, test, test, bring the HV up slowly, cook the tubes for a while, etc., I did a mult-hour 'burn in' at reduced voltage (10 hours) , then slowly brought up to 'normal voltages' over some hours, and left it on 'CW voltage' for about 6 hours.
   
  Tentatively keying down and testing at 2300v (CW mode) yielded reasonable output (900w) after gradually bring drive from zero to 90w. In the SSB mode (2800v), doing so yielded 1200w (into a dummy load).  Thought I saw a faint blue glow after an extended keydown into the dummy load. The first CW QSO, however, into a resonant antenna brought a brighter blue flash in one tube, popped breakers on the primary, and the that familiar cooking something electronic smell ('The smell of progress!'). 
   
  Subsequent reset of the breakers and slowly bringing everything back up on the variac, everything is well behaved again at idle, and at no-drive keydown, up to 30 seconds.
   
  I'm getting much more interested in the messages regarding 'positive bias at reduced plate voltage to show color on the plate, to rehab the tube'.... 
   
  Any reason I can't do this 'in situ' by putting the variac on the plate transformer only, and just keying (5v bias) with no drive?
   
  Any way other than destructive testing to figure out if the tube's going to arc again?
   
  Any other suggestions ( I have a couple of NEW 3-500zg's, but really want to see if these Eimacs can be put back into service )?
   
  Brian N9ADG