Thank you Jeff,
I have posted this question across several reflectors today. Your reply made
sense, and worked! I had already begun a long drive to retrieve some finger
stock when your email came in.
Yesterday I was able to calm it by cleaning things up. Yet, after reading
your reply, I decided to take a good look at the tuning of the qro as a
whole.
I have nearly 15 dB gain with it at an output level of 1400 Watts, which
seemed fine, if not good to me. At this level my grid current was about 30
mils., also good I thought as gain was good and this was a very safe grid
current. However I noticed as I tightened up the Loading and the gap became
smaller, then retuning, I could realize the same output with a lower grid
current and about the same plate current? Was I tuning the qro better? I
guess so, I am now seeing about 24 mils. grid current at about 1400 Watts.
My input IPA is SWR happy too and no more arcing even when driven to 4kV by
800 mils!
Crazy results from this silly box.
Thanks again, Jeff.
73 Jeremy
http://www.oregoncoast.com/oaxaca
-----Original Message-----
From: jeff millar [mailto:wa1hco@adelphia.net]
Sent: Sunday, February 06, 2005 14:03 PM
To: Jeremy Alexander
Cc: amps@contesting.com
Subject: Re: [Amps] W7EME 144 mcs. 8877 problems
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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