Re: [Amps] W7EME 144 mcs. 8877 problems

[jeff millar <wa1hco@adelphia.net>]

Jeremy...

I've seen this behavior with light output loading, which causes the Q of 
the output network to soar and results in very high circulating 
currents.  All the plate network circulating current flow through the 
grid ring.  The high currents then burn the contact points between the 
finger stock and the grid ring.  Once the contact points begin to 
overheat, it snowballs into serious problems.

The solution is to replace the fingers, polish the grid ring, ensure the 
finger pressure is high, use as many fingers as possible and always tune 
up from the more heavily loaded side of the operation...or use very 
light drive while hunting for the correct operating point.

jeff, wa1hco

Jeremy Alexander wrote:

> Aloha to all:
>
> My QRO is having fits. I have an 8877 on 144 Mcs, the classic w6po design.
> It is showing some arcing at the control grid ring to chassis ground. This
> should be a low-inductance coupling shouldn't it? Instead of a continuous
> finger stock, it has four, three fingered pieces at each 90 degrees around
> the grid ring. Arcing is occurring between the fingers and grid ring. Were
> the fingers attach to the chassis, things look clean. I assume this "less
> than great" (my opinion) configuration is to allow more airflow through the
> chimney aperture? I think I can hear faint arcing sounds inside the cabinet
> causing these small pitting in the grid ring.
>
> Also about the same time my input tuning is not consistent. I use an
> external input tuning network, two series inductances and a shunt capacitor.
> I sometimes need to retune the cap to get coupling with the QRO?
>
> I see all of this as a changing input impedance due to the changing grid to
> ground resistance???
>
> Have any of you encountered these symptoms before? I am getting a lot of
> mail back saying this ground to grid arrangement is not suitable at 144
> mcs., however people seem to still be building these the same way!? It looks
> too easy to simply finger all the way around the grid ring and pressurize
> the cabinet in a more appropriate matter, with chimney on top of the valve?
> OR, Is my tube tired again already? Is it shorted between the control grid
> and cathode, only under load somehow? Or, another problem?
>
> Any help is welcome.
>
> 73 Jeremy
> http://www.oregoncoast.com/oaxaca
>
>
>
>
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