I have a 811H and it does not do well in a contest situation. It does NOT
like heat or contest stress at all. Especially with 811 tubes. Replacing
them with 572b's is a good choice. Albiet more expensive. But they last a
lot longer.
Check for a crack in the coupling capacitor from the plate choke to the B+.
Check for over heated, or just plane poor connections on the PCB where the
coil connects to. Consider adding a fan to the left side blowing into the
cabinet.
Hope that helps a bit.
Dan/W4NTI
----- Original Message -----
From: <ku8e at bellsouth.net>
To: <amps at contesting.com>
Cc: <secc at contesting.com>
Sent: Monday, December 12, 2005 9:08 AM
Subject: [SECC] AL-811H Amplifier Problem
>
> John, K4BAI, has been nice enough to lend me one of his extra amplifiers -
> an Ameritron AL-811H.
> I have had it since CQWW CW weekend. Unfortunately, In just a few weeks I
> have done something
> wrong an It pretty much died on me this weekend during the ARRL Ten Meter
> contest.
>
> I noticed thoughout the weekend that the output power I was getting out -
> about 500 watts - slowly started to decrease to the point that on Saturday
> night it was only putting out around 200 watts. This amp is rated at 800
> watts PEP, which I would think would be around 600 watts output max.
> That's close to what I was getting out during CQWW CW with around 75 watts
> of drive.
>
> When tuned the grid current has now dropped to less than 80 mA. The plate
> current is around 450mA. On Sunday morning I opened up the cover. I didn't
> see anything noticably wrong - no burn marks or blown components. I
> started to remove the four plate anode clip connections from the tubes to
> pull them out to look at them. On three of the four 811A tubes the anode
> cap of the tube just broke off the tube. the inside of the cap was very
> corroded. These tubes were probably on their final stages of use. I'm
> not sure how old they are but when I talked to John he said he thought
> they were the original tubes. I have ordered four new tubes (Taylor
> Select) from RF Parts. Fortunately for me these are pretty cheap tubes.
> Less than $20 each.
>
> One thing I noticed when I was using this amp is the SWR was pretty high
> between the radio and amp. I used the antenna tuner in my radio to tune
> out this SWR (around 3:1) I'm not sure if John did the same thing when he
> used this with his FT100MP ?? Anyway, There an adjustable rear panel slug
> tuned input circuit adjustment for each band. There is no mention in the
> instruction manual
> on how to do this adjustment or what type of tool you use ?? Anyone else
> have any ideas on what might of happened or anything else to look for when
> I insert the new tubes ??
>
>
> Jeff, KU8E
>
>
> http://www.ameritron.com/man/pdf/AL-811H.pdf
>
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>
>
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