AL-80A Adjustment

[K1FBI]

I picked up an AL-80A and during tune-up the Peak R.F. Watts on the amplifier's multimeter is showing 1800+ watts output. With other meters it is showing the correct 1100 watts.

My question is how do/can I calibrate the meter? I see an adjustment pot on the meter board, but don't know if that adjusts just the Peak R.F. Watts or is tied into the Plate Current and Voltage. ( or does not adjust any of those; be nice if it was identified on the board).

[WB2WIK]

Potentiometer R5 (100K) on the meter board adjusts only the RF OUTPUT meter indication. That's not a "real wattmeter" at all, it's really a "relative RF output" meter which should have been calibrated at the factory to read correctly with a perfect 50 Ohm dummy load termination. If the load impedance changes and isn't 50 Ohms, the meter reading will be "off," and could be quite a bit off. And of course the circuit may have drifted over time, since the AL-80A hasn't been manufactured in many years.

Feel free to adjust R5, it won't affect anything else. But don't expect that meter to ever be accurate unless you're using the amp into a 50 Ohm dummy load, and even then it may change a bit from band to band, as it's not really frequency compensated. The detector used is coupled to the RF output via a pair of 10 pF capacitors in series which I would think would make the detector more sensitive on 10m and less sensitive on 80m/160m. It's really just a "relative output" indicator.

[K1FBI]

Thanks for your reply.
I'll dial it in on 40 meters (my main amplified band) to match the Swan meter and use for "relative RF output" only. At least it will be somewhere in the ballpark now.

73 K1FBI

[WB2WIK]

Thanks for your reply.
I'll dial it in on 40 meters (my main amplified band) to match the Swan meter and use for "relative RF output" only. At least it will be somewhere in the ballpark now.

73 K1FBI

Good idea. But don't expect the calibration to hold with changes in the load, or even with changes in the band. The manual's discussion of this being a Wattmeter is kind of silly.

The AL-80B does have a real directional wattmeter in it.

[K4RVN]

Having owned an AL 80A for 24 years now, I would advise you not to load it to 1100 watts. About 900 would be better for a clean signal.
The 1100 is really putting a strain on the old amp I think.

Frank

[K1FBI]

Having owned an AL 80A for 24 years now, I would advise you not to load it to 1100 watts. About 900 would be better for a clean signal.
The 1100 is really putting a strain on the old amp I think.

Frank

This one is 21 years old and still on the original tube. I load it to 1100 watts but back the exciter down so it puts out 600 watts keydown and about 800 peak on ssb. Is this still a potential problem?

[W1QJ]

This one is 21 years old and still on the original tube. I load it to 1100 watts but back the exciter down so it puts out 600 watts keydown and about 800 peak on ssb. Is this still a potential problem?

This type of loading is great for good IMD but loading for almost 2 times the power then backing off almost 50% on the drive is not good for efficiency. If you want the amp to only put out 600 watts you should load the amp up for maximum power output at the 600 watt drive level and then slightly advance the load about half a tick on the dial markings. This is better for efficiency, but if your radio has alot of overshoot then it will splatter pretty badly since the amp is loaded for the lesser drive power. Assuming you had an exciter that only put out 50 watts you would then drive it with the full 50 watts and load for maximum output at that drive level. Many people load up with full power and back off the drive maybe 10 to 20% so that IMD is good, but 50% is too much in my book. I would not do it. Not that it is really bad, but the efficiency lacks.

[K1FBI]

This type of loading is great for good IMD but loading for almost 2 times the power then backing off almost 50% on the drive is not good for efficiency. If you want the amp to only put out 600 watts you should load the amp up for maximum power output at the 600 watt drive level and then slightly advance the load about half a tick on the dial markings. This is better for efficiency, but if your radio has alot of overshoot then it will splatter pretty badly since the amp is loaded for the lesser drive power. Assuming you had an exciter that only put out 50 watts you would then drive it with the full 50 watts and load for maximum output at that drive level. Many people load up with full power and back off the drive maybe 10 to 20% so that IMD is good, but 50% is too much in my book. I would not do it. Not that it is really bad, but the efficiency lacks.

Would backing off 30% be OK or do you still recommend loading at a lower power?
Thanks for your reply,
John

[W1QJ]

As I recall the AL-80B claims the amp can run 1000 W PEP output. I believe the manual tells you how to properly obtain this amount of power. In all honesty I would follow those recommendations. Don't think just because you run the tube "a little less" than what it can handle that it is doing worlds of good. As long as it is run inside the guidlines there is no harm done. You'll probably get as many opinions on this as there are readers who care to respond and then you can get really confused. I say read the manual and do what it tells you. If you have a peak reading wattmeter I would load the amp as specified and adjust the drive level so that the PEP does not go beyond the 1000 watt limit. Simple enough I'd say.

[K1FBI]

As I recall the AL-80B claims the amp can run 1000 W PEP output. I believe the manual tells you how to properly obtain this amount of power. In all honesty I would follow those recommendations. Don't think just because you run the tube "a little less" than what it can handle that it is doing worlds of good. As long as it is run inside the guidlines there is no harm done. You'll probably get as many opinions on this as there are readers who care to respond and then you can get really confused. I say read the manual and do what it tells you. If you have a peak reading wattmeter I would load the amp as specified and adjust the drive level so that the PEP does not go beyond the 1000 watt limit. Simple enough I'd say.

Yes, I was going by the manual for the AL-80A which says to load to maximum power (while staying within other parameters) which should reach approximately 1100 watts with 100 watts drive and then back it down to 1000 watts max or less.