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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