I’m working on a used AL-811 for a friend. The 811 tubes were bad, so I replaced them with some Penta Labs 572b tubes (A whole another adventure). I also replaced the tube sockets, and installed GDTs on the tube sockets and added some bias diodes on the center tap of the transformer. I made sure there were not MOVs in the input board. The issue I’m having is high SWR (over 2:1) on 80 meters and I can’t get full drive from what appears as the exciter is folding back. All the other bands are fine.

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I’ve attempted to adjust the 80-meter input coil with no effect on SWR/PWR in. I’ve checked the input switch for continuity and from the input through the 80-meter input coil with no problems. Switch alignment looks good. I remove and tested C4 (for 80 Meters) a 500pF cap and it tested good. ?I’ve removed the 200-ohm padding resistor just to see what effect it had on SWR and nothing. Obviously, I get more output for the same amount of drive. If I’m at the high end of 75 meter with the tuning slug all the way at the end of the coil form, the SWR just starts to drop, and I see an improvement in drive level. With the padding resistor in circuit, I’m showing about 50 watts for drive and 60mA of grid current. With the amp in bypass the radio will do 120W no SWR.

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I’m wondering about values of C4 and if anyone else has experienced this behavior after swapping to 572b tubes? Don’t know how the amp performed prior as the tube replacement.

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Thanks for any productive comments in advance.

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Jim – AA7CL

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