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Author Topic: AL-80b low grid current/Power output  (Read 30425 times)
NC5T
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Posts: 44




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« on: November 23, 2014, 06:33:42 AM »

Good Morning,

   The amp is an AL-80b that I bought used several months ago.  It is wired for 220 and HV is 3100 and drops to 2800 under load.  I drive the amp with a Ten Tec Omni VI+ with no ALC line attached and I have it set for around 120mA.  The following is the results I get with 70W drive into a dummy load:

Band   Drive   lg   lp   Output   Plate   Load
10M   70w   50   250   450w   9.5   6.5
15M   70w   42   300   300w   9   5.5
20M   70w   50   275   500w   8.5   0.25
40M   70w   50   300   600w   7.5   0.5
80M   70w   45   325   500w   3.5   2
160M   70w   45   320   500w   6.5   1.75

  My concerns are the low grid current and the low setting on the load cap, not to mention low power output.  I have done a lot of reading and research and so far I have checked and the load capacitor is fully meshed at 0 and I checked the diode at D117 and it is fine.  Just not sure where to go from here.

Thanks in advance for any assistance.

73,

Bob
NC5T
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K3VAT
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« Reply #1 on: November 23, 2014, 06:49:34 AM »

...<snip>...

  My concerns are the low grid current and the low setting on the load cap, not to mention low power output.   ... 73, Bob  NC5T

For low power out: Are your measurements taken from the dials on the AL80B or do you have an external unit capable of calibrated measurement (Bird, ArraySolutions, etc.)?
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NC5T
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Posts: 44




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« Reply #2 on: November 23, 2014, 06:59:36 AM »

Power out was taken from the amp, but I verified the amp's meter with a friend's bird meter when I got it.  I also use a radio shack power/SWR meter that gives the pretty close to the same results.

Thanks,

Bob
NC5T
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KH2G
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« Reply #3 on: November 23, 2014, 01:25:26 PM »

Lower than expected Ig is normally a function of loading being too light. You can usually load up in a couple of spots but the one with the proper grid current is the proper point.
Regards,
Dick KH2G
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W8JX
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« Reply #4 on: November 23, 2014, 01:33:43 PM »


   The amp is an AL-80b that I bought used several months ago.  It is wired for 220 and HV is 3100 and drops to 2800 under load.  I drive the amp with a Ten Tec Omni VI+ with no ALC line attached and I have it set for around 120mA.  The following is the results I get with 70W drive into a dummy load:


The key word here is used. Likely well used. The voltage drop is about normal. Have you checked the input SWR to amp?
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Ham since 1969....  Old School 20wpm REAL Extra Class..
WA7PRC
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« Reply #5 on: November 23, 2014, 01:56:13 PM »

Many people who use amplifiers for SSB or AM tune for maximum CW output, and then switch to SSB or AM modes. When running SSB or AM, the last step is to advance the LOAD control until the best linearity is achieved and set the drive level such that no flat-topping shows on your monitor oscilloscope. NO meter will indicate that.

For best intermodulation distortion, HV dynamic stability is a good thing. However, no power supply has absolute (0%) voltage regulation. At some point, we just accept what it is. For our purposes, 10% is generally considered adequate. Your HV drops from 3100 to 2800. The regulation is
In your case, ((3100 - 2800) / 3100) x 100 = 9.7%

You're on the low side but it's not awful. If you want to try to improve it, start with the AC line voltage at no load and full load. If you're running it on 120VAC that bounces up/down a lot, there's most of your problem.

vy 73,
Bryan WA7PRC
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W1BR
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« Reply #6 on: November 23, 2014, 03:23:56 PM »

You're 40 meter readings are showing 600 watts out for 840 watts DC plate (.3 x 2800).  That is not too bad.

I agree with the others about loading the amp heavier, that will improve IMD and raise the grid current.

Other than that, I'd verify that your load is still 50 ohms, or very close to it, and again borrow a good know accurate Bird or similar wattmeter to reconfirm that your meters are accurate from 160 through 10 meters.

Pet
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N3DT
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« Reply #7 on: November 23, 2014, 03:49:31 PM »

Just for reference, my AL-80B on with LOAD at max output, 115VAC in, no ALC.

28.5 MHz
drive 70W FM mode
Ep=2700
Ig=140
Ip=475
PO=600W(amp) 712W(LP-100A in tune mode)
Plate=9.3
Load=6.3
Antenna SWR 1.18:1 on the LP-100A tune mode
Notice this is over max plate dissipation (1282-712=570)

7.2 MHz
drive 70W FM mode
Ep=2700
Ig=130
Ip=430
PO=750W(amp) 830W (LP-100A)
Plate 7.0
Load=1.75
Ant SWR 1.19:1 on LP-100A
Plate dissipation 1161-830=331W much more efficient, 80 and 20 about the same, everything 17M and above exceeds plate dissipation with 750-800W out.

This is a 2004 model new to me this year, I'm assuming the original tube but I don't know. I'd say you have a worn out tube, but I'm not the one with experience on that. And yes, is the SWR into the amp good, mine is very good according to the meter on the TS-2000 which is quite optimistic, but it shows 1-2 marks swr and no foldback. I get very good reports with this amp and TS2000.
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WA7PRC
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« Reply #8 on: November 23, 2014, 04:25:38 PM »

I agree with the others about loading the amp heavier, that will improve IMD and raise the grid current.
Heavier loading = LESS IGRID.

If you want more IGRID, drive it harder. Again, ONLY an oscilloscope is going to indicate when your drive is too high and/or your loading is too low.
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W1BR
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« Reply #9 on: November 23, 2014, 04:32:10 PM »

Oppps. brain fart, I stand corrected! Typing too fast and not thinking.
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NC5T
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Posts: 44




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« Reply #10 on: November 23, 2014, 05:26:24 PM »

Thanks for all the responses!  I have tried to answer each question below.

W8JX:  The SWR raises a little on the radio, from 1:1 to just under 2:1 at the worst, 80M, and it varies by band a bit but not much.  I checked it with the antennas and got pretty much the same result.

WA7PRC: When I tune the amp, I use the tune feature on the Omni 6+, and then tune for max output with first the plate and then the load caps.  The amp is the only thing on a fresh 220 line so there is very little drop in AC to the amp.

The higher I turn the load cap, the less grid current, and power out I get.  I am not sure if this is what was meant by loading it heavier.

I have always gotten good reports on the audio, but I did go ahead hook up the scope and it didn't see any issues, even with full drive, so I think I am good with the way the ALC is set.

Thanks for the help on this.  I wonder if I am worrying about nothing, or is this an indicator of some issues with the amp or tube?

73,

Bob

NC5T



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W8JX
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« Reply #11 on: November 23, 2014, 05:39:02 PM »

W8JX:  The SWR raises a little on the radio, from 1:1 to just under 2:1 at the worst, 80M, and it varies by band a bit but not much.  I checked it with the antennas and got pretty much the same result.

As SWR increases, drive will decrease and rig folds back power. So rigs are pretty sensitive to SWR. Input on amp is tunable.
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N3DT
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« Reply #12 on: November 23, 2014, 05:48:58 PM »

Did you see the numbers I have on my amp? I'd say you're not getting what you should out of it. 70W in should put 800 or so watts out especially on 80/40/20M and maybe 700-750 on 10M. Unless you've got your ALC engaged. Looks to me like your Ig is real low too, actually all your I numbers are low as far as I'm concerned. Are you tuning the PLATE for max output and also the LOAD? You should only increase the LOAD after you've found the max PO with the LOAD control and then only maybe 10W decrease or so. It's about finding the sweet spot, not tuning to numbers. Heck, I can get 1KW out of mine with maybe 80W in, but it's over rated at that and it's pushing .160 Ig and .500 Ip.
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NC5T
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« Reply #13 on: November 23, 2014, 06:03:18 PM »

Thanks again for the responses.  I thought about the power folding back as well, but the Omni doesn't.  I have checked it all the way to over 3:1 and it still puts out what you set it at.

N3DT:  Yes I saw your numbers and I have seen others posting their numbers on here, which is what led me to testing and trying to figure out what is going on with mine.  As for tuning, that is exactly how I tune mine.  I set the plate and load at the presets, key it with the tune control on the Omni, set the plate for max output, set the load for max output, check the plate again, and then raise the load until the power dips a little. 

Thanks,

Bob
NC5T
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N3DT
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« Reply #14 on: November 23, 2014, 06:23:11 PM »

Well, I have no problem getting 150ma grid current if I drive it hard enough, but I don't, I like to keep the Ig around 130 and the Ip around 425 (70-75W in). It always gives me close to 800W out, even if I'm pushing it a bit on 17M and up when the Ip goes up more. I think the PO meter on the amp reads low at least on mine comparing it to the LP-100A which I trust a lot more. I was surprised at the low Ig and Ip numbers you posted, I have to reduce my drive to maybe 50W or less to get those numbers and I'm sure it's not meant to run like that, at least what I've read in the Ameritron AL-80B manual. The input SWR on my amp is real good except on 30M which I don't use the amp anyhow. I've even got good SWR in on 17/15 and 12/10. The output SWR on the amp shouldn't matter much unless it goes above 4:1 or so, but of course lower is better.

I've also got the grid protection circuit and the QSK board in it if that matters, but I doubt it.
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