Redesigning the Dual 188XR Test Pallet

Here I will attempt to chronicle the tear down and rebuild of my primary test pallet, which is a dual BLF188XR pallet.

This was one of my early designs and I have since done many more and this one needs updating. Plus I want to try some more new things on this pallet.

I usually try all new experiments on this pallet, then if they are successful, I incorporate them on my dual 1k80h pallet. But now, this old pallet is starting to become too different from the 80h pallet and I need to bring them back closer together.

Updates and changes:

integrate the attenuator onto the input PCB (like the 80h pallet below)
break out the 4 LDMOS Gates and add 1Ω gate stopper resistors to each (shows you hold old this design is now)
populate the output TLTs with my latest TLT designs
incorporate room on the back end for a balun option, so I can test a few ideas
remove all unnecessary PCB copper to reduce board capacitance
widen the clearance widths where the low impedance coax connections are made
incorporate a small signal relay on the front end to switch the bias voltage
possibly add SMA connectors for RFin and RFout
whatever else I’m forgetting

Here’s the dual 1k80h pallet. This pops out of the case with two touches of the soldering iron, 2 spade lug disconnects and one small connector, and two screws. I swap the dual 188XR in for experimentation.

Here’s the dual 188XR pallet, looking pretty old school, and now stripped of any components that might be reusable.

Here’s my method for removing the PCBs from the LDMOS parts. I use a Dremel with a cutting wheel to cut the PCBs away so the only things left are these tiny pieces below each tab. This makes it a LOT easier to unsolder them.

Here we are stripped all the way down. I carefully checked for solder bridges and shorts under the tabs and used solder wick to clean up where needed. Both devices check out perfect with the ohm meter.

Plugging away on mechanicals…

Always amazing to me that these can end up perfectly aligned with the holes underneath…

All the hardcore mechanicals are finished.

Comments

18 responses to “Redesigning the Dual 188XR Test Pallet”

Jesse L Alley, Jr (Jay)

May 21, 2018

I built a dual blf 188 amp about two years ago following you and kf8od designs. It really works well, but I am getting the itch again to up grade to one of the newer mosfets. Really enjoy all your articles. I am an old man now(84) but still enjoy building and tinkering!! Keep the info coming!! Jay N4HD

Reply
1. N4GA – Rob
  
  May 21, 2018
  
  Jay, thanks for the comment. I hope I’m still building when I’m 84!!
  
  The 188 is still a great device. I just completed a newer design using the 188 and it will easily go way beyond legal limit so I’m not sure why I would pay a lot more money for 1k80h devices at this point. I have another 2x done with the 1k80h devices and it’s nice and even more powerful and efficient, but if you’re staying in the legal limit power zone, what’s the point of spending more..
  
  Anyway nice to hear from you!!
  
  Reply
2. W5TTS
  
  January 31, 2021
  
  Its good to see you still tinkering. Its like crafting for radio nerds 🙂
  I built mine in 2017 and still going strong. I am about to build a second one and donate this to a good friend.
  
  Reply
  1. N4GA – Rob
    
    January 31, 2021
    
    Yeah, still at it! Up late tonight in fact designing a PCB!
    
    Its a sickness… 🙂
    
    73!
    
    Reply
Thomas Mize

December 28, 2018

Hello Rob, Thomas here. This might be a slightly dumb question lol but on the original heat spreader the whole that you counter sunk look black in the picture. Is this just a trick of the camera or is that oxidation? Kind of looks like something has been put there.

You have been a huge blessing for me. I am a young 35 year old ham that has been working with a really good friend that has designed a pallet using a old school style binocular transformer for the output. With testing it has worked very well with a single and dual design. We kind of have no choice but to use 1000v DM19 Silver Dip Mica for the Compendating Capacitor for the output transformer. I would love to use Metal Mica but you can’t find any 1000v any where.

I would love to show you what I have to see what your thoughts are. I have took pictures of the last build I did for my self which was a single 188.

Here is my question, I thought you may be the best person to turn to for this. As you know, many hams among st the years have always favored huge amplifiers capable of performing at many times over the legal limit just so they can run them at a quarter or less capacity to do legal limit. For example a Henry 10K or a Ham building with a 3cx3000a7.

Well my best friend is one of these guys KE4MLH lol. Once I introduced this technology to him, his mind started pondering. Then one day he asked me, “Thomas, I can achieve Legal Limit out of 2 of these devices at around half throttle correct?” I replied, “Around that, yes”. Then he let the word roll off his tonge. “Thomas, how about 4 of these devices doing legal limit at only a quarter throttle?” Of course I replied, well there is absoltuly no need for that but, I do understand Ham Radio is all about Preference or it would take a lot of fun out of the hobby. Then I replied with something of the sort of, well if you have the money to waste on this crazy project then I’ll do whatever I can do to make it happen for you lol.

Rob, I was thinking in my mind that I haven’t seen an attempt at a 4xBLF188xr Amplifier. It would be a monster and about he only plus to it would be 3 things.

1. To do it for the sake of doing it since I’ve never seen it done before. I guess the thrill of the build.

2. You could run it at less then 10 percent throttle using higher attenuators to do around 500w or so and you wouldn’t even need fans lol.

3. It would be fun when you have friends over to show it off on the dummy load of it doing over 4kw plus but of course you would need a massive dummy load and to bypass the output filter board.

The thing is, that he can’t run any more then legal limit but if he were to show off to some friend or on a youtube video on his dummy load I have to make the output coupler to handle at least over 4kw for a few seconds.

I am going to attempt to use W6pql’s output combinner/ coupler and to just some how make it to where it can handle twice the amount of power.

What are your thoughts about this? Also can I send you some pictures of the board to see what you think?

Thanks in advance rob, sorry for any miss spelling.

Reply
1. N4GA – Rob
  
  January 13, 2019
  
  Thomas – thanks for the post!
  
  Well, you and I know for Amateur use, there’s really no need for a 4x amp. But that said, if you search “LDMOS” on eBay you will see a 4x PCB from Israel.
  
  I would probably make two 2x amps and combine them. There are very high power combiners on eBay as well that could handle that task. You will see them under them same “LDMOS” search.
  
  73s!
  
  Reply
2. Brandon
  
  December 9, 2020
  
  Hey I know this is old. But I thought I’d chime in….I don’t play with the higher voltage toys yet but the time is near. Lol anyways. From my time in the 12 volt world…I had struggled to find higher kv devices at times. And just a thought. You could get a couple of 500 volt capacitors and run them in parallel. Right ? Communications concepts has all kinds of metal clads. Also I know so many people crap on the smds as compensating capacitors but in my experience if you get a quality part….they will perform every bit as well as metal clads. And they are seemingly held to a higher standard as far as tolerance.
  
  Reply
3. Timothy S. Kraus
  
  May 31, 2022
  
  Mr. Mize,
  
  I’m Tim Kraus in Georgia.
  
  I built a dual BLF-188. I copied it from e-Bay.
  
  I can get no more than 1,200 watts from it. Each gate biased for 150mA, 600 mA in all.
  
  You speak of the compensating capacitor on the output transformer.
  
  Mine is 1,500pF, 500V. Maybe I ought up the voltage to 1,000V.
  Thanks for listening.
  
  Kraus/KC4ZGP
  
  Reply
  1. N4GA – Rob
    
    June 1, 2022
    
    Kraus,
    
    That cap won’t matter much, that’s just for stray inductance cancellation and only minorly affects distortion.
    
    Your bias is too low. Those devices need 1A each or more.
    
    Try a total bias of 3A, which would be 1.5A per device.
    
    If that’s not better you can try a total of 4A, 2A each.
    
    If that’s no good, you have some other issues.
    
    73!
    
    N4GA
    
    Reply
    1. Timothy S. Kraus
      
      June 2, 2022
      
      Rob and Group,
      
      I upped bias to one ampere per BLF-188.
      
      My output transformer is a 16:1 impedance ratio of FT-87-61 sixteen cores in two stacks of seven cores each.
      
      The one turn low side is coaxial braid wrapped with copper foil tape to give it form.
      
      I thread 14AWG stranded four times.
      
      My d.c. input 50 volt chokes are FT-104-43, 15 windings, 14AWG stranded. One on the positive lead and one on the negative lead.
      
      I’m doing something wrong. I will get it however.
      
      Kraus/KC4ZGP
      
      Reply
      1. Timothy Kraus
        
        August 7, 2022
        
        Ladies, gentlemen and anyone else who cares.
        
        I think I found the problem. My #61 transformer gets rather warm at 14MHz, 1,200 watts but at 1.8MHz I get about 2,200watts and the transformer runs quite cool.
        
        Looks like #67 is called for. Eh?
        
        Kraus/KC4ZGP
      2. N4GA – Rob
        
        December 10, 2022
        
        That’s unexpected because I can hardly ever get 61 material to warm up at all.
        
        Did you ever test the 67?
Damir Eterovic

June 4, 2021

Hi guys
Nice to read your expirience with PA 2xBLF188XR.I built one and can get more than 2kw.But on some bands (80m and 15m) higher SWR occured before LPF. So on this bands I run no more than 1kw output.I supose probem with harmonics before LPF trigering my protection for High SWR. My design is “eb104.ru” .Still have present this trouble.Any idea how to reduce harmonics.
Best Regards
73 9A5BDE
damir.eterovic1@gmail.com

Reply
1. N4GA – Rob
  
  June 6, 2021
  
  1kW and above, you have 3rd harmonic only down 10-12dB.
  
  Maybe try a diplexer LPF, or one with a better design.
  
  Reply
  1. 9A5BDE op.Damir
    
    June 9, 2021
    
    Hi Rob
    Thank You very much for above reply.I have used LPF for 2KW from “eb104.ru” russian source.When passing signal only from radio ,120W,SWR is perfect at all HF bands.So LPF looking ok.Probably at high power change caracteristic.Palets are ballanced and not noted overheating at 100 Ohm resistor or ferites.I want to buy LPF diplexer for 2kw but can not find for more than 1kw power.Do you known some source,please for info.Thank You for all nice posting about LDMOS PA .
    Best Regards
    9A5BDE op.Damir
    
    Reply
    1. N4GA – Rob
      
      June 10, 2021
      
      Thanks Damir.
      
      I do not follow whats out there for LPFs. If you want a 2kW diplexer, prob best to make it yourself just for the band(s) you operate on.
      
      73
      
      Reply
Bob, N4PSK

April 15, 2022

I have always dreamed of a HF wideband directional circulators that dumps all the reflected & rejected harmonics into a load. We use them on VHF & UHF equipment but imagine one for HF.

“Shoot for the moon. Even if you miss, you’ll land among the stars.” –Les Brown

Reply
1. N4GA – Rob
  
  June 1, 2022
  
  I’ve got dreams but none of them include electronics.
  
  🙂
  
  73!
  N4GA
  
  Reply