On Fri, 5 Dec 1997 23:47:24 -0500 "Walter Kornienko" <zak@goes.com>
writes:
>Hey all. Ran a bit of the 160 contest tonight and noticed that after
>about 45 minutes of heavy use power output started falling off on
>my homebrew 3 x 4-500 amp. The input SWR isn't changing but
>the output tuning seems to be effected. After 5 minutes cool off
>its back up to full legal output. Could the plate blocking caps, a
>pair of TV door knob types, be the culprit? Any ideas?
>
> 73 de Walt Kornienko - K2WK - zak@goes.com
Ouch, after the YEARS of comments in QST, etc about the unsuitability of
TV doorknobs at QRO I am suprised anyone still takes the chance.
Replace with a pair of 1000pf 5KV or larger real RF doorknobs and the RF
heating drift will be minimized at that point. At over 2KW even four
500pf in parallel would be a good idea.
Other culprits are any fixed caps used in the Tune and Load circuits.
Replace any that are more than just a bit warm to the touch after a
minute or so key down. In many instances doubling or tripling up smaller
values works fine since the lower values often have a much lower TC
rating.
If you have the room, look for the hockey puck style caps that show up at
low prices, usually stripped from RF generators and the like. BUT
AGAIN...make sure they are RF rated and not just DC types.
Finally, is your plate choke up to the task?
GL Carl KM1H
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