[Amps] 3cx800 flashover ?

[Carel, pc5m]

Tnx Peter/Carl/Bill,

Tube is now on with heather/blower, let's see what happens next Friday .... 
73's Carel, pc5m

-----Original Message-----
From: peter chadwick [mailto:g8on at fsmail.net] 
Sent: woensdag 21 augustus 2013 18:08
To: Carel pc5m
Subject: RE: [Amps] 3cx800 flashover ?

Try running it with filament volts only for 48 hours - apparently the getters on the ceramic tubes are activated by cathode heat.

73

Peter G3RZP


========================================
 Message Received: Aug 21 2013, 05:03 PM
 From: "Carel, pc5m" <pc5m at xs4all.nl>
 To: amps at contesting.com
 Cc: 
 Subject: [Amps] 3cx800 flashover ?
 
 Hi, I need some advice of the group if this 3cx800 is beyond revitalization.
 
  
 
 Working on an old SSB electronics 70cm PA with a single 3cx800 in grounded  grid configuration. Original bias/control circuit was replaced with g3sek  triode board. PA has been out of service for maybe 10 years or so.
 
  
 
 When applying heather and Ua and cut-off bias all fine, no flashes, no  currents
 
  
 
 Gradually lowering the kathode zenervoltage towards 8V the anode current is  increasing normally till approx 50mA at that point I hear internal tube  arcing (I think) and upon lowering the cathode voltge even more the over  current protection suddenly jumps in (set at 800mA or so).
 
  
 
 Question: is it normal to get internal flashover only at non-zero anode  current ? (at 0 mA no flashovers!)  Or do I have maybe another issue ? low  frequency oscillations ? 
 
  
 
 I did replace the active zener config for a brute force string of diodes (8  in a row or so), same phenomena .
 
  
 
 Anything I could do more before trashing this 3cx800 ?
 
  
 
 Tnx, Carel pc5m.
 
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