New 1200 watt solid state MRF-150 linear - first light

[K1JJ]

Some of you probably remember the 600 watt, four MRF-150's project I was working on last year. That unit has been working FB and online for a year now.

I decided to double the power and go with eight MRF-150's using input and output combiners.  These boards are from CCI and based on a Motorola design. The RF boards are put together as a kit. The heatsinks must be drilled and the associated T/R switching, bypassing, chokes and power supplies are not included.

Notice the copper heat spreaders for both boards, sitting on a slab of aluminum, then a standard finned heat sink.  There are lots of ferrite cores on incoming and outgoing leads as per Frank/GFZ's suggestion. The rig pictured is running about 54 watts output with < a watt of drive. (21db of gain)  It is using a small 13.8V supply for testing - the final supply voltage will be 48VDC and make this bad boy do 1200w out.

I plan to build up some air core LP filters for it. So far it seems stable and working as planned.

Considering the high cost of solid state KW linears these days, a few hundred bux into this project ain't too bad.  It's cool to be able to switch bands instantly and have little to no blower noise.   

Notice a lot of extra holes drilled into the copper spreader. I had a tough time with breaking taps and drill bits. Copper is not easy to work with. The solution was to drill the holes two sizes larger than recommended for taps. EZ going after that.

I have a nice desk-top blue cabinet that the whole thing will be housed in.  Should look purty when done.


Wish me luck as I put 48V on it maybe tonight. That's when the reeeeal men start to sweat....



T

[K1JJ]

Here's a shot of 3.8 mhz at about 54 watts. The waveform is saturated cuz of the sagging 13.8v supply.  The intended 48V brute supply will do 40 amps no problem.

[Opcom]

Pls. direct me to the website of CCI if you would, I have an interest in that technology. I keep hitting Cubic and finding 10KW amps..

[K0ARA]

http://www.communication-concepts.com/

[K1JJ]

Right.

And here's the CCI MRF-150 amplifier board and accessories called the EB-104:

http://www.communication-concepts.com/eb104.htm

T

[WA1GFZ]

cool,
Class A driving Mr. Ugly
Makes me want to work on my 16 FET machine

[WD5JKO]

Tom,

    Wow, you brought back a wave of memories I have when I built a big AMP from a CC kit. I don't recall the EB Kit number, but it was 500 watts with two bipolar transistors running at 50 volts. That thing was linear up to about 800 watts output too!

    I too busted a few taps in the copper. It is tough stuff for sure to work with.

    My application was commercial for a friend that was into AMTOR. This was about 20 years ago using reprogrammed PK-232's, early IBM AT computers, and such.

    Anyway once I got the AMP working, I hooked it up to my Yaesu FT-757. About 5 watts carrier let me run about  200 watts of very clean AM. I had several QSO's on 80M, and the AMP was doing really well, until.....................................ZORCH!!  What happened was I dropped my microphone, and the burst of excitation drove the two MRF-? power transistors so hard that they literally turned orange in front of my eyes until a fuse popped. The AMP was shot, and never rebuilt. Some time later the AMTOR ship to shore transmitter booth burned to the ground.

   Looking back at the CC kit, I made some mistakes. I used a mule of a power supply. It used a Sola constant voltage transformer, so the regulation was decent, but the thing could easily supply 25 or 30 amps load all day, and much more into a fault condition. Having an active current limit circuit (maybe set to 15 or 20 amps) may have saved the AMP. I also neglected to add a means of shutting the thing down should the load VSWR exceed some threshold. Also, if your available drive is way beyond what you need, then add an attenuator. Do not just crank down the drive level on your exciter.

   So Tom, maybe the MRF-150's are more forgiving in your amplifier, but even so consider adding the items that I skipped in my test with a big CC RF Amplifier. I'll never forget them two orange colored MRF-? transistors!!

Regards,
Jim
WD5JKO

[K1JJ]

Very interesting, Jim -

I remember the bi-polar transistor kit you built. 

The MRF-150's will also pop with abuse, that's for sure.  I have in there an resistive attenuator. But I just have a dual 15A circuit breaker for the IC. I shud really build something up that will limit current. Hmmm... maybe that QIX class E  board he uses for overload and swr will work here.  I cringe every time I pop on the 48V supply.

The IMD isn't bad on these either -   about -31db or better 3rd.

There's an Italian ham who built four of these fro 2400w out. He added lots of bells and whistles like you suggested.   Heck with the "PW-1" SS 1KW amp going for over $7K, you can do quite a nice job on one of these for peanuts.

T

[KF1Z]

Wonder what you're using for a combiner?

I see it behind the modules....

Is it from CC ?

If so, I thought their combiners were only good to 300 watts.... maybe that was their LPF kits...

Oh yes... I guess they are good to 1kw..... (just went and looked...)

Is that what you're using?

[K1JJ]

Wonder what you're using for a combiner?
I see it behind the modules....
Is it from CC ?
If so, I thought their combiners were only good to 300 watts.... maybe that was their LPF kits...

Their LP filters sucked and did not like more than 300w, so I'm making up some air core LP filters.

Yes, the combiners are from CCI.  They claim the pictured output combiner will handle the two modules. (1200w)  We'll have to see.
The input is the same model, just reversed.

T

[WA1GFZ]

I bought two 8 FET MRI amps that are easy to convert to HF. Each has 4 push pull stages. Very well built medical stuff. Just need to replace about 30 caps and rewind a few transformers to get it down to 160. I have a single pair in a 3 stage strip working 160 to 6 meters for the HPSDR set up.  Tom I think you need to ground the splitters and combiners. Check the mounting holes.
I only paid $175 for each amplifier.
Those CCI filters have a design problem. They start rolling off in the top of each band

[K1JJ]

Testing it more, I'm getting lousy efficiency.   The heatsink is getting hot with about 1500w input and only about 450w out. Maybe the combiner is saturating.

Idling at 3A per module, 15A peak each (30A total) at 50V = 1500w in, 450w out... Sux at  30%.

Both modules are working the same.

I did mount the splitter and combiner on the heatsink to ground.

How do I determine if the combiner is saturating?   As IC increases the power stops going up?  Heck, I was getting more power out with ONE module...


T

[WD5JKO]

How do I determine if the combiner is saturating?   

Tom,

   I suspect that the combiner will quickly overheat if it is saturating. Do you have one of those Laser led assisted IR "point and shoot" thermometers? Can you insert a SWR bridge between one amp output and the combiner input? If, so the VSWR might change quite dramatically as the power is run up. Also having an impedance bridge (MFJ or Autek) might be helpful to see if the combiner is behaving at low power (0 dbm or so) levels.

   Don't rule out a parasitic oscillation that is hogging power and lowering efficiency.

You sure seem to get involved in fun stuff Tom....

Jim
WD5JKO

[w3jn]

The most important question is, whattaya gonna name it, T?

[K1JJ]

Tom,
 I suspect that the combiner will quickly overheat if it is saturating. Do you have one of those Laser led assisted IR "point and shoot" thermometers? Can you insert a SWR bridge between one amp output and the combiner input? If, so the VSWR might change quite dramatically as the power is run up. Also having an impedance bridge (MFJ or Autek) might be helpful to see if the combiner is behaving at low power (0 dbm or so) levels.
Don't rule out a parasitic oscillation that is hogging power and lowering efficiency.
You sure seem to get involved in fun stuff Tom....
Jim
WD5JKO

OK Jim -
The combiner is staying cool throughout the "saturation."   I may separate the boards and test them in place separately to double check they are working. I'll try some of your suggestions. I'll be working on it.

Johnny:  This SS amplifier is named, "Girlyman."  It has no tube BA's and is kinda limp.


BTW, Looks like this K5 is using a combiner that is X2 for the job.  Maybe I need to double the size of mine too: (Actually, he is using FOUR modules and this is a 4-way combiner. So maybe mine is large enuff to support two modules being  half the size. Maybe I have a different problem here.)
http://www.k5er.com/IMG_1691A.JPG

Here's his whole project:

http://images.google.com/imgres?imgurl=http://www.k5er.com/IMG_1697A.JPG&imgrefurl=http://www.k5er.com/ss_amp.htm&usg=__OD6O82dTnZckXaDolwmJaE-okVY=&h=480&w=640&sz=45&hl=en&start=6&sig2=ebO1W-sTyta0bFBkFx-f-Q&um=1&tbnid=2_e7AWa9sZ-21M:&tbnh=103&tbnw=137&prev=/images%3Fq%3Dmrf150%2Bcombiner%26hl%3Den%26sa%3DN%26um%3D1&ei=LP_RSsqDMYGzlAft5bWpCg

He mentions "equalizing resistors" for both the splitter and combiner - what is that about?

T

[W3SLK]

John said:

The most important question is, whattaya gonna name it, T?

Maybe the 'pill-box.' 

[K1JJ]

UPDATE...

Just so no one spends any more time thinking of solutions:

As usual, it appears to be human error.   I forgot to add a jumper that bridges the output transformer for the new module. As suggested, I tested each module individually and found the new one was putting out zero RF. The old one was doing all the work when combined!

After adding the new jumper, the second module is behaving FB.  Now gonna combine the two and see what happens.

T

[WD5JKO]

Tom,

   Good deal on finding one amp loafing. If this had been a push pull triode final, it might have been obvious since one tube might have a dark filament.

    You asked about those resistors on the splitter and combiner. I found my old Motorola RF Device Data books (vol I and II), and looked up AN749. Then I found it on line:

http://www.ir3ip.net/ik3umt/proj/AN749.pdf

   Look for Figure 10, you need 25 ohms, and on the output it will need to be rated for 1/2 the power of one module. This way if one AMP fails totally, the I/O impedances stay in line as the output falls 6 db; i.e. the resistor gets HOT. When both AMPS are running at the same power, and in phase, the output resistor is COLD.

Fun Stuff!

Jim
WD5JKO

[K1JJ]

Jim,

Thanks for the info. I will add those mods later.


Well, still baffled here.

I'm still pulling in about 1500 w and getting out only about 400w when running both modules combined.

I can connect each module up separately (without the combiner) and with 15-20A get out 500-600w.  BOTH modules do this.  But when I hook them into the combiner, I get only about 400w output. Both modules are pulling down about 15-20A, which is now 30-40A total.  The power supply stays stiff at 48V.

I put an swr meter in line with each module between its output and the combiner and see about a 2.5:1 swr for each combiner input.   That doesn't seem too bad.  I don't see any signs of instability. The combiner is staying stone cold.   I notice the output transformers from each module are luke warm after a carrier of 30 seconds, but that's seems about normal.   The dummy load after the combiner is a Bird and measured at 1:1 swr.

I'm using the same combiner/splitter board as the input, so it appears to be constructed and wired right.

Right now, a single board is putting out more than two combined....

T

[K1JJ]

Another update:

I put it all back together as a combined unit.  I found on 40M it will do 750w out.  On 14mhz it does 700w.  One 160M it will do 850w out.   BUT on 75M (where I've been testing it) it's limited to only 450 w.   Sound like maybe a parasitic or some other strange thing going on there.  At least I'm happy with the results, even though it's not 1200w yet.

We'll keep looking at the 75M problem.

T

[KF1Z]

Can you put RF into both modules at the same time through the splitter, but with the outputs seperate, each into a 50 ohm load?

That might tell you something......
Is the problem the splitter, OR the combiner...

[WA1GFZ]

Sounds like there is a problem in the combiner. It should be flat. Do you have all three coax shields terminated to the ground on the combiner? zero degree combiners should be very efficient. The Italian guy had a mis wire in his combiner from CCI if I remember. AN749 has an article on the 2:1 combiner. Check the combiner output transformer. The input should be a tap on the shield at about turn 3 up from ground. The output is the center conductor of the coax. the ground end of the shield and center conductor both go to ground. the high end of the shield floats. The transmission line transformer should have a 1 to 1.4 turns ratio step up. The combiner is 2- 50 ohm inputs producing a single 25 ohm output that needs to be stepped up to 50 ohms. Sounds like there is no step up. PM me some digital pictures , also compare it to the input side. Don't push it until you fix it or you could take out FETs.

[K1JJ]

I pulled out both splitter/combiner boards and checked them carefully. They appear to be wired right and OK.  I then swapped them around.  The same results.  75M is low output while the other bands are acceptable.

The power seems to go higher as the freq gets lower as you'd expect, with the exception of 75M.

I continue to test and try things, but running outa ideas.


I measured the power at the input of each amp and see I'm nearing 10 w, max drive.  This gives about 700w out.  The gain isn't there or the efficiency is low.

T

[WA1GFZ]

http://www.duessler.de/MosFet-Title.htm
Tom,
Maybe the single bead on each output coax is doing something weird on 75 try adding another. This guy wound coax around a torroid. Too bad you  don't have some small teflon coax. Thee ERB 8 holer has a lagre 2 hole bead with a turn through each hole. BTW the PP 2 holer has solid output 160 through 6 with a slight dip in gain on 15 meters. I think Jay had similar results with his board.This guy has some interesting mode but I like my grounding / power mod better. Carefully check each picture. I remember it was good reading last time I went through them. What do you get on each board stand alone for output? Turn your bias down to an amp until you sort this out. this will help if something is taking off by going more toward AB.  You could be getting feedback between amps?? Layout is a bit off. maybe you need to rotate 1 module.

[KF1Z]

Is it at all possible for the capacitor and combination of the 2 input transformer's inductance causing a nice resonant circuit, and shunting some of the 75 meter energy to ground?