[Amps] 3cx800 flashover ?

[Carel, pc5m]

Hi, small report on my 3cx800 project...

After more than 24hrs heather on (didn’t dear to let it running unattended overnight...) first test did show indeed improved vacuum ! flashovers started at higher idle current. So let it heat some hours with reduced idle current (50mA or so, dissipating around 150W, considerable more than heather power). Apparently that did the trick !. After some RF tweaking I just got stable 700W key down output (@ 432MHz) with approx. 20W drive, so good enough as a QRP backup during next contest :-) ... and no sign of flashovers.

Don’t know if those tube are known to exhibit reverse grid current ? See approx 20mA of reverse grid current at full output. 

BTW, How do you have to calculate grid dissipation when having reverse grid current ? I don't think DC grid current * DC cathode-grid voltage is anymore valid in that case ? 

73's Carel, pc5m


-----Original Message-----
From: Carel, pc5m [mailto:pc5m at xs4all.nl] 
Sent: woensdag 21 augustus 2013 19:19
To: 'amps at contesting.com'
Subject: RE: [Amps] 3cx800 flashover ?

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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