[SECC] AL-811H Amplifier Problem

[Dan/W4NTI]

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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