[Amps] 3cx800 flashover ?

[Carl]

Try running filament only for at least 24 hours.

Then run idle current for several hours at a safe level of no arcing..

Finally inject a little RF at a time and hope the tube has been regettered 
and doesnt have other problems due to hard use at 70 cm.

It may (or not) still be useable at HF.

Carl
KM1H



----- Original Message ----- 
From: "Carel, pc5m" <pc5m at xs4all.nl>
To: <amps at contesting.com>
Sent: Wednesday, August 21, 2013 12:03 PM
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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