Low output on all bands
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Author:  nn9l [ Mon Sep 18, 2017 2:08 pm ]
Post subject:  Low output on all bands

I finished the build about a year ago, but never really used it until recently. While it looks to be all good, I have used it with the KX3 and with a 5W drive, the best I can get is about 19W out per display (i did not measure the power any other way) into a dipole with 1.0:1 SWR. Similare or worse results on other bands. I did read that the bias current should be set higher now on all the versions (mine is still the original build settings), could that be the cause for my issue?

Author:  jimveatch [ Mon Sep 18, 2017 7:38 pm ]
Post subject:  Re: Low output on all bands

Yes the output power will be lower with 50 mA per MOSFET instead of the recommended 150 mA per MOSFET that we use now. But the difference is a few watts, maybe 5 or 6W not over half the power.

One of the most common build issues is an extra turn on the directional coupler, T3, and if that's the case the power will also read low. Before diving into trouble shooting we should verify with an independent measurement what the actual power is. If you don't have a calibrated wattmeter you can build an RF probe for your DVM and get reasonable readings with just a few parts. Instructions for building and using an RF probe are here:


If you can determine that there truly is low output power, let me know and we can engage the trouble shooting engine!

If you just want to throw in the proverbial towel, I'll be more than happy to check it out for you, we don't charge to get amps up and running even if they are more than a year old. Contact me on WA2EUJ (AT) ARRL (DOT) NET and we can make the necessary arrangements.


Author:  nn9l [ Wed Sep 20, 2017 8:58 pm ]
Post subject:  Re: Low output on all bands

Jim, spot on! I had 11 turns on each side of the T3. I corrected that, and I readjusted the gain as per new instructions. 20M output is 48W with 3W drive. However, and not that I am complaining, but I was showing 70W on 40 and 80 meters with the same drive. My radio shack digital power meter was showing slightly less, around 63W, and all this into a 50Ohm dummy load. Should I be concerned? I'd hate to burn out the finals.

Author:  jimveatch [ Thu Sep 21, 2017 8:21 am ]
Post subject:  Re: Low output on all bands

The Hardrock-50 has the lowest gain on 20M, requiring about 3W for 50W output and the highest gain on 80M which takes 1-2W for 50W output. 6M not withstanding which only get 35-40W output and saturates at 3-4W input.

I use my amp with the KX3 and I set the KX3 to 2.4 watts. The output is a little high on some bands and little low on others but I don't think it's a significant difference. The MOSFETs are rated at 16W each and there's four of them so you should be good for 60W all day and I've seen over 80W on 80M.

Another note is that the HR50's internal meter is reasonable accurate at 50W, within a watt or two 80-10M . If you try to measure above or below this level it becomes less accurate and it's really only a relative indication on 160/6M. It does a good job at what it's designed for; setting the amp to 50W output and as long as the output power is above 20W, the SWR is fairly accurate.

The power sensor on the ATU board is a much more accurate device and provides good measurements of forward and reflected power, SWR, average and PEP. Since it uses the same sensor to measure forward and reflected power, the return loss measurements are extremely accurate and it calculates the SWR based on the return-loss.


Author:  kx9rt [ Sun Mar 25, 2018 7:17 pm ]
Post subject:  Re: Low output on all bands - KX9RT's Amp

I am finding that my amp seems to also be outputting a little on the low side at around 37-42W depending on the band.

40 Meters quickly saturates with about 1.6W of drive and topping out at 42W.

The measured gain is around 14.5-15.5dB with 1.0W of drive.

I have the QSK board installed.

I can't take measurements on 6 meters because the amp keeps reseting. (different forum post)

My measurement technique is using the N5ESE RF probe design, but have also taken independent measurements with a calibrated Communications Analyzer (General Dynamics R2590). I went with the RF probe because the analyzer is limited to a calibration down 5MHz and wanted to learn something new.

Is this power level expected? It seems 10-20% lower than what I am reading in the specs. I did bias the transistors to the nearest 10mA because that is the best I could do with my DMM.


My data is summarized in this graph:
https://docs.google.com/spreadsheets/d/ ... sp=sharing

Author:  jimveatch [ Mon Mar 26, 2018 5:22 am ]
Post subject:  Re: Low output on all bands

The gain seems about right but the output is low. On 80 meters the amp typically saturates between 70 and 80W it's well over 60W 40-10M. With the QSK, 160 and 6M suffer a little. Maybe 40-45W on 160 and 35-40W on 6M.

The typical things to check are:

Terminal voltage: It needs to be 13.8V at the amp during TX If it's dropping, so is your power.
Bias: Should be set to 150 mA per MOSFET.

Looking back through the thread, it seems like the amp was making more power are some point and the output has decreased. If you've been running it consistently above 50W, you might try replacing D1-D4 on the QSK board, they are pretty cheap 1N4007's. If that doesn't help it may be time for a new round of MOSFETs


Author:  kx9rt [ Mon Mar 26, 2018 9:50 pm ]
Post subject:  Re: Low output on all bands

Thanks Jim.

I made such a rookie mistake. The internal voltage on the PCB screw terminals was sagging as low as 12.8V. I have about 1.4V drop in my power supply cabling and need to go through the inline fuses and need to beef up a wire jumper to 12AWG. Can't get around Ohm's Law.

The power quickly increases to over 50W with a short power supply cord and a stiff supply set at 14.2V. I saw over 60W in a couple quick tests and definitely don't want to push it anymore. A "fun" exercise nonetheless.

Now one more gremlin to work out with the 6 meter reset in my other forum post. It has temporarily gone away.


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