The negative grid current issue is covered in our 432 amp manual. I will have
to consult it to see what Steve claimed was the cause. I remember reading it.
Apparently it is only on higher frequency that it happens. If I recall he
claimed it was harmless. I will look into this.
-----Original Message-----
From: km1h@jeremy.mv.com
To: Carel, pc5m <pc5m@xs4all.nl>; amps <amps@contesting.com>
Sent: Sat, Aug 24, 2013 7:59 am
Subject: Re: [Amps] 3cx800 flashover ?
Congratulations on the salvage work Carel.
I suspect the negative grid current is due to the tube being run above its
maximum rated frequency at HF voltages; I seem to remember Steve, K1FO (SK),
mention that several years ago during a VHF/UHF conference.
It may also be due to possible grid damage during the arcs and Id keep a
close eye on it including regular regettering with just filament voltage.
Disable the HV supply and consider giving it a good 36-48 hour run and hope
for an improvement.
Carl
KM1H
----- Original Message -----
From: "Carel, pc5m" <pc5m@xs4all.nl>
To: <amps@contesting.com>
Sent: Saturday, August 24, 2013 11:38 AM
Subject: Re: [Amps] 3cx800 flashover ?
> 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@xs4all.nl]
> Sent: woensdag 21 augustus 2013 19:19
> To: 'amps@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@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@xs4all.nl>
> To: amps@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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