Hi Jim doing the sweep thing gets you in the ball park but you need to be under full power to tune the input. ?You just need a little tweaking for the 572b tubes. ?Ameritron never adjusted those coils for 80 meters, the slugs were always way out of the area of the coil windings. To zero in on the best match always takes ending trial and error. ?I¡¯d say removing ?one turn and trying it is a good start. ?You definitely want the slug in the area of the coil winding for maximum ?adjustment. BTW, a flat swr isn¡¯t necessarily going to be what forwards you maximum output. ?You may very well obtain maximum power at 1.2 to 1. ?It all depends on the radio¡¯s filter network and how close it really is to 50 ohms. ?I always shoot for maximum forward power while adjusting the inputs. If the slug is in the coil winding and the swr isn¡¯t all the way down now you¡¯ll need a cap value change. I usually try around 20pf on the lower frequency bands readjust the coil to see if I¡¯m going the right way, as a last resort try a little less. ?Just be glad you don¡¯t need to do all of them. Lou
On Thursday, May 1, 2025, 11:50 AM, AA7CL via groups.io <aa7cl@...> wrote:
Lou,
Thank you for your suggestion. I was thinking of doing the same. I have several capacitor values lying on the bench to do just that.?
I've read that some have suggested using a VNA to sweep the input by substituting a resistor for the tubes. Have you found this to be effective? I believe the post I read indicated 200 ohms for 572b tubes but I don't know how accurate that is.? Not sure if
you pull the tubes, sub in a single resistor between the cathode and ground and then sweep to see where the SWR is at. Would of course need to operate the relay.?
Jim - AA7CL
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
Lou
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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