[Amps] AL-80B Trouble

[Mike Schlamp]

    Hi all,
       
      I am having my first problem with my AL-80B and I need some  help in isolating the trouble, along with some signal tracing data that is not  covered in the manual.  Below is a  synopsis of what is happening. 
       
      AMP: 3yr old AL-80B, Amperex 3-500ZG Installed, 110Vac  Primary, does not have the QSK kit installed.
       
      SYMPTOM:  With the standby  switch in standby, band-switch set for 80M, AC power applied to Amp, the meters  read ~2700Vdc (normal is ~3000Vdc), 150ma Ip, and the Ig is showing a negative  draw.  The Ig meter buries itself to the  left.  After 2-3 minutes in this  condition, the plate is glowing reddish orange.   With the standby switch placed into operate, RF applied from the  exciter, the exciter complains of High SWR and there is no RF power being produced  by the Amp.  With the band-switch placed  in any other position, the same symptoms apply.
       
      ASSUMPTION:  The  3-500ZG has a short, and or the tube is not receiving the proper bias voltage  during off periods .  The glowing while  in standby is due to the current draw across the plate; dissipating ~400W. 
       
      HELP: Is my assumption too far fetched?  If the tube is shorted, what could have  happened to cause this, considering the tube is only 3yrs of age?  My fear is if I just replace the tube, I will  end up back to square one over time.  Is  there any trouble shooting guide regarding the Amp?  I am looking for a document or notes  indicating voltages, along with tips of what to look for.
       
      Thank you,
       
      Mike
      W5CUL