The position of the slug tell you a big story. ?Ultimately you would?want the slug to Bering inside of the coil windings when the SWR dips the lowest. ?That would indicate there¡¯s enough of leeway in the inductance. ?So the positioning of the slug gives you a clue as to what is needed. ?Say for instance the slug is inside the winding area and you can¡¯t null the swr at any position of the slug that tells you probably there¡¯s not enough total inductance to the coil and you would add a turn or two. ?If the slug is totally nowhere near the windings and almost out of the tube then that tells you that you have too much and you would take one or two off. ?You got do a simpler test by adding about 10pf to the existing caps to see what happens if it gets worse go lower . ?At some point trying to change the cap values and the swr doesn¡¯t null then going to the coil may be necessary. ?Since ?you are using different tubes and since the quality control on them is all over it¡¯s not surprising you have this issue. ?As I recal from working on hundreds of these amps, the 80 meter slug is always quite far out. ?Good luck
On Wednesday, April 30, 2025, 10:50 PM, AA7CL via groups.io <aa7cl@...> wrote:
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 ¨C AA7CL
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