On Wed, 27 Aug 1997 07:40:18 +0000 w8jitom@postoffice.worldnet.att.net
writes:
>> From: wrt@eskimo.com (Bill Turner)
To: <amps@contesting.com>
>> Date: Wed, 27 Aug 97 00:37:58 +0000
SNIP
>Tubes like the 8877, 3CX3000, 3CX5000 and so on are almost always
>totally stable without any precaution in a good layout.
>
>Other tubes, like the 3CX800 and 3-500Z are in between.
I always find a suppressor necessary on the 3-500Z but with the correct
one it remains stable long term.
The LK-800A ( 3 3CX800's) that I recently converted for a customer did
not require suppressors. Even with an air variable Tune cap and a less
than ideal Q it ran up to 3KW out without any excessive heating in the
tank. The vacuum relay got pretty toasty after 60 seconds key down!
>
>Short of owning a network analyzer, the only way to tell if you
>will have problems is to try it. If you do have parasitics, you
>should easily be able to correct them.
>
>The key is using a large continuous groundplane and
>surround the grid ring with finger stock bolted directly to
>the groundplane. No leads from the output circuit should go below the
>chassis, into the area of the the tube. Every VHF amp I have built
>(and HF amp also) using the 8877 has been totally stable. It's a
>great tube if built properly, it's actually difficult to make one
>oscillate without doing it intentionally!
Totally agree.
>The grid is the dominate element, ground it well and problems
>disappear. At 50 MHz and above, I'd use a flapper cap rather than a
>conventional cap to keep Q and losses down.
A flapper cap almost always presents a mechanical construction problem
unless an above average workshop is available. On 6M it is not necessary
since ordinary air variables work fine when modified properly. The rotor
bearing surface available on a Johnson/Cardwell 154 series cap keeps
heating losses to a minimum. I replied to Bill earlier so msgs are
probably overlapping, but I offered suggestions on modifiying.
73...Carl KM1H
>
>73, Tom W8JI
>
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>
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