OSG-50W Important Modification for high SWR between your Radio and Amplifier.

In this video I talk about a modification to reduce the very high SWR the OGS-50W presents your radio. For some radios, this isn't a problem as they will reduce their output power to save the finals from burning out. However, there are some radios that are known to have very fragile final amplifier stages, and this modification will present them with a reasonable (but not perfect) SWR.
This, internal resistive load ensure a reasonable load for the radio, while limiting any negative effects on the performance of the amplifier.
You make this modification at your own risk! Make sure you double check everything before and after you make this modification.

[@rockshelter]

Thank you so much for this, Tom. I will do this mod to my amp when I am able to aquire the right resistors!

[@tomsworkshop5856]

Sir, thank you for pointing this out! Hopefully this will help lots of folks!

[@StephenMaughan-bx7vi]

Hi Tom, I have a single 6w 420ohm resistor in that position so maybe they are trying to fix the issue. Keep up the videos. Now to start work on the usdx.

[@alvarogaitan2529]

Great job Tom you safe my amplifier jejejejej thanks

[@rockshelter]

So I did the mod and it sure made a real difference in the swr presented to the radio. So far I only tested with an usdx and the reports were bad, to say it mildly. I dont think the usdx is anywhere near a finished product, and thus it does not like this amp it seems. I could test the PA with my 817, but the risk and the pain involved in renewing finals on the sweet little Yaesu makes me a bit reluctant to mate these two. Maybe I am overcautious... I dont have deep experience with practical electronics but is there any way this amp could be put to work? Any bands with less unwanted emissions? 10-15-20 meters maybe ? Simple filters?

[@tomsworkshop5856]

Ya, the USDX is a pain. And the 817’s finals are fragile to say the least. As far as making the USDX almost useable, check out my other video. And search Facebook for the USDX group where you can find better firmware and how to upload it to your radio. Mine’s doing OK now, but dang, RF getting back into it via any connection, power, mic, etc.. makes it sound very bad - and adding the amp really compounds the problem. It seems, for me at least, I get the best results on 20 meters. I hope this helps, brother. 73s!

[@PaulReedy]

I ordered the PA50+ online, and they sent me one of these. Granted it was a great price, and now I know why. The one I got does look different than the one you have shown. They have added 3 large resistors to the board, 2 of them are 420 ohm, one is 120 ohm, so I wonder if they have tried to fix the input VSWR issue. I couldn't get a chance to test it as once I applied power, the power light came on for a brief microsecond, then something in the amp shorted out, so I started a return.

[@tomsworkshop5856]

Wow, definitely worth returning if it only had a millisecond life span. But most interesting that they are still working on ‘improvements.’ Hope you end up getting a working amp in the end. 73

[@petervk7kpc180]

Hi, I have recently received one of these with the extra resistors. Measured at 43 Ohm and 12 Ohm. In PI network theoretical attenuation nearly 9 dB and return loss of about 7dB, equivalent to a VSWR of about 2.5:1. Supposedly better than previous. Just waiting for replacement MOSFETs as the smoke escaped when checking bias voltage. Cheers Peter VK7KPC

[@ricardojvf1728]

Tks for the tip, but I think I heard two resistors and I see four resistors, is there another modification ?

[@tomsworkshop5856]

If you take the top off, you’ll see two in the final section. The two I’ve added are just as described in the video.

[@migalito1955]

Hi & thanks Tom, If I understand correctly the maker or designer of the amp did not allow the input side on the amp to appear as a 50 ohm load to the source radio, correct? Because I did not open the case yet I just want to be clear about where I connect the pair of parallel resistors. I saw the proper location on the relay so I am good on that. I believe you said for the other the ground tab of the two BNC connectors, correct? I guess the two connectors share a common ground tab, but as long as the connection is made to what the connectors ground to should be fine, correct? Also, how critical are the 200 ohm in parallel =100 ohm 6 watt resistor assembly values? I'll probably have to order them anyway since 3 watt resistors are not typically in my build box and I am wondering if it was just a workable value you had and perhaps there is an even better selection. Thanks again....

[@tomsworkshop5856]

Yes sir, you've got it right. However, there's a different version of this amp being sold now, and it is identifiable by the blue power light instead of red. I recently bought the second version, but have not yet been able to do the modification as they went through-hole for the relay. I'm hoping to make a follow up video on this newer version as to how the modification can be done, because, well, it still doesn't present a good 50ohm load to the radio. Hope that helps!! & 73

[@migalito1955]

@@tomsworkshop5856 Thanks Tom. Yep, I have the blue light version. I will wait for your video and I did order the resistors.

[@tomsworkshop5856]

@@migalito1955 Sounds good, hopefully this will be a simple (ish) mod, I do know I need to pull the board out, and that means the temp switch and unbolting the FETs. So, might take a bit of effort, But I'm hoping to find some "top side" connection point after I go through it. We shall see.

[@petervk7kpc180]

@@migalito1955 Hi, Blue light version has an attenuator already fitted Cheers Peter VK7KPC

[@MaciejSzewczyk-x6w]

Funny thing, I got mine yesterday. I did some quick test on dummy load and quess what - mine produces 90-95W from circa 6W input. Any ideas? Regards

[@tomsworkshop5856]

Two things come to mind. Drive power should be 2 to 5 watts, and if you're using an mcHF/RS-918 type radio, (Or similar experimental radio,) harmonics. Often times the 3rd harmonic.

[@MaciejSzewczyk-x6w]

@@tomsworkshop5856 Hi Tom, thank You for your answer. Yes, the drive power exceeded 5W - in fact it was around 6-7W. But would it make such a difference? Today I tried handheld Albrecht 2,5W and USDR reduced to similar power. The amplifier gave 40W on output , and a few qso-s were made with good reports. Harmonics - yes, but would it not affect SWR (and it was exactly as expected)? Anyway - best regards - Maciej SP5ZE

[@tomsworkshop5856]

@@MaciejSzewczyk-x6w They often do effect SWR, but in some cases, especially with non-resonant antennas that are being made to transmit with a balun or unun, such as the popular 9:1 random wire. The SWR, even with out a tuner can often be under 2:1 on all HF bands, thus, the harmonic(s) might not affect the SWR as you would expect. Now, if it is a mono band antenna, I'd be surprised. But, not everything works out to simple answers. Also, I often run my amp in excess of 75 wats on 80 meters. It gets warm, the fans run a lot, but it seems happy doing that. ;) I just wouldn't recommend doing it, if you don't have replacement IRF530s on hand.... All the best and 73's VE7NGK

[@MaciejSzewczyk-x6w]

@@tomsworkshop5856 Thanks, 73!

[@richardsenining7410]

Hi sir. How are you can i ask about two meter linear what is best tube type or mosfet thanks 73

[@tomsworkshop5856]

That's a tough question to answer because everyone uses VHF a little differently. If you're just looking to boost your handy talkie a bit, then Btech (Model # AMP-V25) makes a nice little amplifier that also has audio (in and out) to turn your handy into more of a mobile / base setup. If you're looking to do VHF SSB, AM, or Digital then I'd be suggesting that you look at the offering from RM Italy. Hope that helps. 73! VE7NGK

[@richardsenining7410]

Thank you sir... But i need to purchase linear vhf 144 to 148 if have can you help me about that sir and im willing to buy thaks 73

[@tomsworkshop5856]

@@richardsenining7410 Sorry, I don't have any VHF amps.

[@plotdoc]

Appreciate the video! However, I set up several jigs with two samples of this amplifier, and loaded it with both tuned antenna and dummy loads, and measured the VSWR on the exciter (a (tr)usdx) on all bands, with the OGS either powered on or off. Worst case was on 80 meters, and was approximately 1.5. Best measurement was on 11 meters (no surprise). I could not repeat your observation. Can you display the actual test setup you used? Also, I would be very uncomfortable using this amplifier without some filtering. It is sending large spikes at the second harmonic, on all frequencies measured, making it unacceptable as an out of the box amplifier, even for 11 meter operation. Wonder if the manufacturer has something to do with the problem: different stores in China have wildly different pieces with the same name. Not doubting your findings! Just want to know what accounts for your findings. Again, thanks for all you do.

[@tomsworkshop5856]

Good day George. So, I've stopped using my uSDX with this amp as it creates far too many spurs and harmonics. The class E final setups in those radios are not up to being connected to an amplifier at all. I've exhausted many hours trying to clean up the emissions. With that said, I use this amp with my RS-918, that has had Wayne NB6M's modifications done to its filters. The OGS-50W is a dumb amplifier, and has no input loading, thus, unless they've changed something in the last year, I'd wonder how you got a good match at all. (And I'm not doubting you and the fine manufactures change things everyday!) The tests I did used the SWR meter in the RS-918, confirmed with a direct connection to a dummy load (Showing 1:1ish) and my Random wire without tuner (2:1) and then tuned 1.06:1 Using my ATU-100, and the OGS powered off in between the radio and tuner, and repeated on all bands. Once the the amp is powered on and trips into transmit, the reported SWR on the antenna tuner stays low, 1.15:1 (Some change expected with the increased power, and resistance of coax, balun, etc.) However, the radio's indicated SWR (5:1) was so high that radio was only pushing 2.5 watts into the amp, while set to 5 watts, in many, but not all of the bands. There is good news: The harmonics detected from the output of the antenna are 40 to 50db lower than the fundamental, compared to 9db lower than the uSDX substituted for the RS-918. Test set up was TinySA, using supplied antenna polarized in the same direction as the random wire and 40 feet from center of that transmitting antenna, with the TinySA set to measure harmonics. (Notably the 3rd harmonic was the worst in all cases, with both radios, likely because the amplifier favors frequencies in the 10 to 15MHz range.) So, you are correct. If your radio is dirty, or has high DC out, such as the uSDX families of radios. (This is evidenced by the uSDX radios only generating about 30 watts out of the amplifier, but traditionally built commercial radios without the DC component will do 55 watts with the same 3.5 watts in.) Don't use this amp without a filter system BEFORE the amplifier, unless you've cleaned up your radio's emissions to all harmonics being -50db or better. A filter setup after the amplifier is a good idea, but if you haven't cleaned up what it is amplifying, you will be converting all of your output to heat in the filter network. It should be noted that it will work on 10 meters, or as you say 11, but it is super inefficient. I hope that helps, and all the best to you!

[@plotdoc]

@@tomsworkshop5856 Appreciate the thoughtful reply. I will set up the OSG-50 with an RS-918 (one of my favorite portables!) and see what I measure. My measurements were with the SWR meter and transmatch at the base of the antenna. The OSG-50 was left inline, both powered and unpowered, and worked well, with many spurs noted, which likely reflect the Class E operation, as you noted, of the (tr)usdx. I use an LPF-7, as well some other stiff filters on most of my Chinese equipment, as the second harmonics are often troubling. I am a product of the early sixties amateur scene, and recall "pink slips" for inappropriate emissions. All said, this amplifier is a work in progress, and needs a bit of work to be a good ham radio product. V73 George KF5WFA

[@tomsworkshop5856]

@@plotdoc Hey, Thanks for the kind words! I do agree, the amplifier is a work in progress. BUT, I can say I've now logged over 100 hours of TX time on it. So, at $1 an hour investment, seems to be rather good. lol All the best George!! 73's de VE7NGK

[@plotdoc]

@@tomsworkshop5856FB, Tom. I set up a test rig, using a QRP SWR/Wattmeter between the exciter and the amplifier, and another SWR/Wattmeter between the amplifier and the load, which, for purposes of the test, was a Cantenna KW load (finally, a use for it!). With the exciter active, and the OGS non-powered, the forward power was set to 4 Watt, and was seen on both power meters. SWR stayed at 1.1 or better, on all bands, from 160 to 10 meter. Then I energized the OGS with 13.8V, and repeated all the studies. You are correct, Tom. The reflected power to the exciter was several Watts, making the VSWR nearly 6! The lowest value, 4, was obtained on 15 Meters. I will certainly try your fix, but excellent pickup on this one, OM! 73 George de KF5WFA

[@tomsworkshop5856]

@@plotdoc I honestly can't take credit for finding this. But, once it was brought to my attention, fixing it made a world of difference. It's possible to use two 100 ohm resistors to make things happy, but the amp suffers. (I tried) So, the end result is 2:1, and the amp doing its job. Kind of a compromise, but a good way to not hurt your radio. (The amp's finals are easy enough to replace....) Have a good one friend, hope to talk to you on the air. I love 20 meters on Saturdays. ;)