[SECC] AL-811H Amplifier Problem

[ku8e@bellsouth.net]

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