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Author Topic: AL-80A Low output 20M and up  (Read 2740 times)
N3DWS
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« on: April 23, 2013, 07:12:39 AM »

I just purchased an immaculate AL-80A from someone off of QTH.com. As it turns out, I had to replace the 3-500Z due to being gassy. With the new tube I am getting full output on 80M and about 850 watts on 40M. The problem is on 20M = 300w, 15M = 250w, and 10M = 175w into a 50 ohm dummy load. I have contacted Ameritron support and they suggested  "Make sure the plate choke is not burned for starters. Also, if you have ALC plugged in, disconnect it entirely. Make sure the tank circuit coil turns are not touching each other as well." Did all of that but all seems to be in order.

Any help with past experiences or direction on how to troubleshoot the issue. I already have over $1000 into the amp that is worth less than that. Thanks for your input in advance.
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AD4U
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« Reply #1 on: April 23, 2013, 08:08:30 AM »

Assuming you verified that all is in order per the recommendations from Ameritron, what you are seeing (decreasing output as the frequency increases) usually indicates a soft tube.  I know your 3-500 is new, but "is it good"?  Unfortunately the only to tell is to replace it with another 3-500 that is known to be good.

With proper drive power - is the plate current around 400 mA on all bands or does it drop off as the frequency increases?

Dick  AD4U
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K8AXW
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« Reply #2 on: April 23, 2013, 08:27:05 AM »

No mention of what the input SWR is or what the grid current is on these bands.......

Is the exciter putting out full power on these bands?
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N3DWS
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« Reply #3 on: April 23, 2013, 08:52:59 AM »

Thanks for the reply. Grid and Plate currents are ~125mA Ig and 3-350mA Ip on all bands, slight variations. The exciter is a TS-940 and I am running about 60 watts output from the exciter for full output from the amp on 80M.

Input SWR is about 1.5:1 at its highest. TS-940 is full 150w output on all bands. I am using a Nye Viking MB-A as tuner but the readings thru the antenna tuner and direct into a 50 ohm dummy are the exact same. As far as the tube, the old tube behaved the same way. Any other info ?  Thanks again.
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AD4U
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« Reply #4 on: April 23, 2013, 09:25:12 AM »

Thanks for the reply. Grid and Plate currents are ~125mA Ig and 3-350mA Ip on all bands, slight variations. The exciter is a TS-940 and I am running about 60 watts output from the exciter for full output from the amp on 80M.

Input SWR is about 1.5:1 at its highest. TS-940 is full 150w output on all bands. I am using a Nye Viking MB-A as tuner but the readings thru the antenna tuner and direct into a 50 ohm dummy are the exact same. As far as the tube, the old tube behaved the same way. Any other info ?  Thanks again.

3KV and 350mA = approx 1000 watts input.  You should be getting 500-600 watts output under these conditions.  If you are only getting 125-200 watts out on 20-10 meters, this is VERY POOR efficiency.  If it were mine I would look for the problem(s) to be somewhere between the tube anode to the SO-239 output connector.  Put another way I would look for the problem to be in or around the tank circuit - tune cap, load cap, coil, bandswitch, relay, and / or associated wiring.

Dick  AD4U

PS:  Make sure the wattmeter you are using is accurate.  Do you have access to a Bird 43 or similar?

« Last Edit: April 23, 2013, 09:27:16 AM by AD4U » Logged
NO2A
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« Reply #5 on: April 23, 2013, 12:00:46 PM »

What is your hv for idle and load? Plate current of 300-350ma is too low to make rated output. You can`t get 850 watts out even on 80m with this.
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K4RVN
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« Reply #6 on: April 23, 2013, 04:40:29 PM »

N3DWS
I have an AL 80A so would you please answer a few questions? Do you have a stand alone watt meter not in the tuner, etc? When you load on 20 meters, can you use 75 watts drive on your 940?
What is the number reading on the load control on 20 meters? Where is the plate control when it dips? What is the idle high voltage reading on the AL 80's meter? You mentioned a dummy load will it take the output of the AL80A for a short time? If you will give this info I will try to help you. I have owned mine for more than 24 years now. Also your tuned input could be off but a few other things need to be checked first. Did the previous owner have any problems with the amp?

Frank
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N3DWS
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« Reply #7 on: April 23, 2013, 08:57:55 PM »

Frank,
I will try to answer all of your questions in order. Do you have a stand alone watt meter not in the tuner, etc? YES
When you load on 20 meters, can you use 75 watts drive on your 940? YES but the extra drive doesn't raise the output or the Ip significantly.
What is the number reading on the load control on 20 meters? About 3.75  15M=6.75
Where is the plate control when it dips? Right at the 20 on the plate stencil. At 15 on 15M.
What is the idle high voltage reading on the AL 80's meter? ~3150 v, 2900v under load.
Did the previous owner have any problems with the amp? None reported. Says he had it packed away for 5 years.
It is very clean inside. Had to replace the tube due to gassy after sitting. Thanks for your help in advance.

Eric N3DWS
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K4RVN
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« Reply #8 on: April 24, 2013, 08:21:35 AM »

Eric,
You did well on the questions, the bad news is that everything looks normal except the load should be somewhere around 5 on 20 meters. The manual preset is 5.5 and I just checked my amp loading on a hex beam is 5 for about 750 watts out with 65 watts drive. Tom W8JI who designed the amp writes on his site that the power out should be about 12 times the drive power.
Here is what I would suggest if you feel comfortable doing it safely.

1.Connect a watt meter and dummy load to the amp output. Don't connect the ALC for this.
2.With the amp on standby and the transceiver on CW mode, 100 watts drive, key the transceiver and see if the power out is 100 watts. This will make sure your coax and transceiver output
are OK and the transceiver is not backing down on power under load bypassing the amp.
3. If this checks out OK, I would suggest checking the tuned input for 20 meters only at this time.Here is how I did that on my AL 80A:
Take all the screws out of your amp cover, and slide it back to expose only the tuned input behind your band switch. You will see a top, horizontal group with three openings for a tuning tool. Ameritron has the tools or can tell you what size to buy. I had one about 40 years old but don't know the size.
On the top horizontal group, the openings are as follows left to right. 10/12; 15/17: and 20 meters is on the right side. Set the amp up for 20 meter only adjustment at this time.
There is another three openings on the side of the tuned input and they are as follows from top to bottom in a vertical configuration. I suggest that you write these down for future use.
They are top to bottom 40; 80; 160. These are not identified on my amp so you might want to save the info. Ameritron gave the info to me when I had a similar problem to yours.

Insert the tuning tool until it engages the 20 meter slug so you can turn it. Make sure the amp cover is only exposing the tuned input for safety purposes. The cover should be heavy enough to keep the interlock engaged , if not put a book or other weight on the cover.
Turn on the transceiver and amp. Set the transceiver power on 5 watts drive out in the cw mode. Make sure that the radio and the amp are each on 20 meters and the dummy load is connected. Turn the amp to transmit and key the transceiver with the 5 watts drive. Tune the plate for max out on the watt meter, then tune the load for max out.
Go back and tune the plate again for max out. Advance the load control for about a half line and you are ready.
Turn the tuning tool and watch the watts out, if they increase you are turning the right direction, if not go the other way. Using the tuning tool, peak the watts out. Once the watts out are peaked the tuning is complete. Using only 5 watts drive with the amp in transmit won't hurt it or the dummy load and you should get about 45 to 50 watts out on a watt meter. The watt meter does not have to be accurate as you are just using it to peak the output.

Keep the direction and number of turns in or out in mind as you do this. In case it does not help you could retune the input back to near its original spot.

This is how I did my AL80A and it works. There are other ways to do it I'm sure but this will work to check the tuned input adjustment for 20 meters and is what I suggest you try.
Let us know how you make out and be careful always aware of where your hands are around high voltage. If this does the trick for you, Check the other bands that are in the horizontal top group for max out like you did the 20 meter. BTW, I use 65-70 watts most of the time on my Ten Tec Orion to drive my amp.
Good luck

Frank
 

« Last Edit: April 24, 2013, 08:24:32 AM by K4RVN » Logged
NO2A
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« Reply #9 on: April 24, 2013, 08:50:20 AM »

I`m curious if the amp ever arced with the old tube. Since this affects all three higher bands,I`m thinking something in the tank area too,like a bad plate choke.
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WB2WIK
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« Reply #10 on: April 24, 2013, 09:45:27 AM »

Tuning is a reiterative process, and assuming the amp's components are all intact, it "sounds" to me like you're simply underloading it.

I've had an AL-80B for over 12 years and for it to run full power, the Ip is always well over 400mA; it can get up to 600mA, and for intermittent service, that's fine.  I keep Ig down to about 150mA max using the ALC.

But the first peak you hit while tuning is never the "final" peak; after the first peak in output is achieved (using both controls), increase LOAD more (clockwise) and repeak PLATE.  Then increase LOAD more, again, and repeak PLATE, etc.

The "final" adjustment should achieve 150mA Ig and at least 500mA Ip, and when both controls are re-peaked for MAX, turn the LOAD control slightly more clockwise to reduce Ig about 10%, and then leave it that way.

Regardless of 3-500Z tube ratings, I've checked linearity and IM3, 5, 7 and 9 with the amp loaded to almost 600mA Ip (takes about 75-85 drive power, usually) and Ig = 150mA, and it's still linear (delta input vs. delta output remains the same as at lower power levels, with about 10-11 dB uniform gain; it starts to flatten out beginning at about 800W carrier power output, and is in compression at 1kW output) and worst case IM3 is about -33 dB.  IM5, 7 and 9 are lower, in the -39 to -45 dB range.

Might be something else, but first guess is you're just not loading it heavily enough.
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K0CWO
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« Reply #11 on: April 24, 2013, 09:49:14 AM »

As Frank and Steve point out, it may indeed just be a matter of tuning.  When I first acquired an amplifier I did so in order to overcome lightning noise on 160, 75/80, and 40 meters.  It took me a while to get comfortable tuning the amplifier on those bands and after learning more about amplifiers, plate voltage, plate current, grid current, plate and load controls, linearity and efficiency I started to actually understand what I was doing and what was important with respect to being kind to the tube(s).

It wasn't until later that I found out that it can be a bit more tricky to tune an amplifier on 20 through 10 meters.  On the amplifiers I've owned the 160 and 75/80 meter plate and load controls are "wide as a barn door".  Most folks have a relatively easy time tuning an amplifier on those bands because the point of "satisfactory" power output is easy to find with the wide and slow behavior of the plate and load controls.

As one starts tuning an amplifier on the higher frequencies the plate and load controls behave a lot more actively and are much narrower to where the "preset" numbers in an instruction manual are basically useless.

As such I don't use preset numbers in manuals anymore.  For grounded grid triode amplifiers I start of with low drive and the plate and load control capacitors at half mesh (12 o'clock position on most amps).  With low exciter power and into a dummy load I first adjust the plate control for max power out with a quick follow up on the load control again looking for maximum power output.  A few more quick back and forth adjustments between the plate and load controls will reveal where those controls need to be for maximum output.  From there I go up in steps, increasing exciter drive until I achieve maximum output within the grid current, plate current, exciter drive, and output limitations of the amp.

This method more than numbers in a book have helped me understand how to tune an amplifier.

I hope not to insult anyone’s intelligence nor do I know if this method of tuning will help N3DWS’s problem with his AL-80A.  I’m just pointing out some things I’ve learned in order to get my AL-80A to a state of resonance and linearity.

Tuning the AL-80A on 160 meters is a much easier and lazier affair than on 10 meters with respect to plate and load control movement.  This can make it more difficult to get maximum power out simply because the plate and load control are in the wrong place.

I know because I’ve done it.

73, BJ
k0cwo
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N3DWS
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« Reply #12 on: April 25, 2013, 11:01:06 AM »

All who have given input, thank you very much for the input you have given and for taking the time to write it all down. Makes me proud to be an Amateur Radio Operator with people like you that have responded.

Last night I took my time trying to tune the upper bands as suggested, and I feel like a Doofus. This being my first "Real HF" amp I was timid about tuning and loading the amp properly. At the lower drive power the load would be... for example ~4. After dipping the plate and the adding more drive, the load would move up to 5.5 for a peak. As a result, I thought I was tuning improperly. I didn't know that much of a jump in the load was normal. I was following the directions in the manual but wasn't sure if what I was seeing was normal. After spending $$$ for the new tube I didn't want to blow it out from improper tuning. With that being said, I now have full output on all bands except 10M. For reference the load is about 4.75 on 20M, 7.25 on 15M.

10M is a wild exercise in knowing what is correct. While tuning 10M I was getting ~600w with 10w of drive power (not right) in my mind, so I stopped trying 10M for fear (again) of causing damage to anything. Frank, if I may ask, where is your load for 10M Huh I'm not even sure this amp has a 10M mode... LOL. I think it does, or I would have nothing at all on the output.

Once again thanks to all who helped. God bless you.

Eric N3DWS
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N3QE
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« Reply #13 on: April 25, 2013, 11:33:59 AM »

10M is a wild exercise in knowing what is correct. While tuning 10M I was getting ~600w with 10w of drive power (not right) in my mind

Wattmeters/SWR Meters, especially the "ham" not-especially-calibrated kind that might be found in a rig or an amp, tend to be most off at the highest or lowest band, and can be way way off when the load isn't a flat 50 ohms.

Me, I tend to trust most, a calibrated oscilloscope and correctly compensated probe, measuring RF voltage at the 50 ohm dummy load itself.

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K4RVN
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« Reply #14 on: April 25, 2013, 12:49:04 PM »

Eric,
10 meters, my load is 8 plus 2 lines. The plate is at the end of the white beyond the number 15. The preset is 8 in the manual. I do have a 10 meter wire on my hex beam, but my tuned input is peaked for 12 meters a favorite band. The same coil in the tuned input is used for 10 and 12 meters. Likewise I peaked 17 meters which shares a coil with 15 meters. I like the warc bands.
I can only get about 750 watts out on 10 meters which I don't often use. I get about 900 on 15 meters on some frequencies., less on others further from resonance on my antenna.
I would suggest that you make several copies of the Amateur tuning log from your manual and record your amp settings. This will assist you in speedy tuning for switching bands. I also would
suggest that you copy the presets and always start from there on your load and plate settings. I usually advance the load about a half line after tuning for the max power out. W8JI suggested this I think.
Also I find that all my current readings are way below the max so I don't tune by them, I go  by the watt meter. Should I see a high grid current while tuning, I go to stand by if I cannot immediately drop it with the load and plate tuning. I have a clip board on the desk with the presets and my particular tuning settings  even after over 24 years with the AL 80A.
BTW, WIK, Steve, nailed your problem so congrats. there Steve, you haven't lost your touch. That was the one thing I called your attention to that was not normal when you answered my questions, but I was not convinced that was it like Steve. Also your old 3-500 is most likely OK, as I just put my 17 year old Amperex back in after  storage for over 4 years.
Good luck  Eric,and I'll be listening for a loud signal from you. My advice is don't run the old amp at max out. You won't see any improvement of 1K over 850/900 max. is my experience.

Frank
« Last Edit: April 25, 2013, 12:52:18 PM by K4RVN » Logged
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