Mr. Clean - The new 4CX-350FJ driver/ IPA linear

[K1JJ]

Here's a new homebrew 4CX-350FJ linear amplifer I just put together.   Only 3 milliWatts in -  100 watts out. (About 43 dB gain with a single stage)    It is grid driven with electronically regulated screen and grid power supplies and is neutralized. It appears to be pretty stable on 160 to 40M so far.

The 'J' version tube is one of the cleanest tubes made for linear service. I wanted something I could drive with the 3 mW  output of the FT-1000D driver stage and bring it to 25 to 50 watts to drive the 8877 amplifier to 1KW.

The IMD, according to Eimac, is about -45dB 3rd order.   1400V HV, 300V screen, -14V grid.  100 w pep output.

A cool feature: The screen circuit has a failsafe trip circuit. If screen current exceeds 15ma., then a relay opens the cathode return and grounds the screen. This will prevent many errors, like no HV, overdrive, bad antenna, mistuning, etc.  These J tubes are very fragile with  tiny screen and grid sturctures. During testing it triggered a few times and saved the tube.  I used a simple design: 339 comparator, 2n2222 and a sample off the negative lead of the screen.  I plan to build one for the grid circuit for each of my 8877 amps next.

Neutralization accessible from the front panel for touch up as well as the fan speed and trip reset..   I plan to add band preset markings on the front panel once zeroed in.

I used roller inductors in BOTH the input tuning and plate tank so I could experiment with the highest Q's possible to squeak out optimum IMD figures.  Higher Q, the better.

Notice the homebrew cabinets and color scheme. The baby blue cabinets and white panels with black letters are the theme throughout the shack.  

I once saw an ad for a precision target pistol that read, "Every man needs one highly accurate gun."  Well, I figgered every ham needs one highly clean amplifier....

Actually, this project is one cog in a long quest to build a 1500 watt amplifier chain that exhibits -45dB 3rd order IMD or better. It has been a difficult task so far. However, I just ran some tests and it appears that I have met this goal to the 100 watt level so far.

Pictured later, check out the new 8877 amps. There are three of them now, so I can park them on various bands.  "Mr. Clean"  will be the driver for Fabio, Dr. Love and Dr. Love Jr.

Thanks to Chuck, K1KW for his idea to use the 4CX-350FJ and his help with my many questions. Same for Frank, WA1GFJ.  Thanks again, guys!

T

The all-new Mr. Clean:

[K1JJ]

In the 3rd pic notice the power supply. HV, reg screen, reg grid and fils all on one platform.  It is on casters and has its own slide-off cabinet.  All rigs in the shack are easily accessible for servicing. All cabinets slide right off the top and rigs can be serviced in place.

[K1JJ]

#1 is the new 8877 Fabio using the same color scheme. It will be driven by Mr. Clean's impeccible 50 watts.  
At first glance they all kinda look the same.

Pictured second is Dr. Jr., another 8877.

And of course, Dr. Love.   Another 8877.  (3CX-1500A7)

T

[Todd, KA1KAQ]

What??!?? No Plexiglass??? What happened to the Plexi-King??   

[K1JJ]

What??!?? No Plexiglass??? What happened to the Plexi-King??  

OMG, the Plexi-Police are here already!

It was painful, Todd.    Actually, Fabio, the 8877 pictured in the third post, had a Plexi panel and cabinet until last week.  Due to the high gain and chance of RF feedback in the shack, I've gone to all aluminum... [sigh]


The only remaining holdout is the front panel to the class E rig.  It is a stand-alone unit, so not involved with the other amp chain.

T

Rico Suave, the 24 pills class E -

* Update - I replaced it with a clearer pic:

[W4NEQ]

What method of neutralization did you use that holds for all those bands?

Chris

[K1JJ]

What method of neutralization did you use that holds for all those bands?

Chris

Chris,

I use the same neutralization circuit on all rigs that need it. Take a look at this 813 X 813's rig. I used the same one for this linear.   Just a small stub sample put next to the anode that is fed into the bottom of the input tuning cap:

http://www.amwindow.org/tech/htm/813/813.htm

T

[Todd, KA1KAQ]

OMG, the Plexi-Police are here already!

Hey, be thankful you're not a Collins collector. Johnny still twitches whenever someone mentions R-388 PTOs for the abuse he took - and he was never part of their group!  

The only remaining holdout is the front panel to the class E rig.  It is a stand-alone unit, so not involved with the other amp chain.

Pffft. That doesn't count. Nothing going on inside to see like the tube rigs. A vacuum cleaner is more exciting. Even the ones built into the wall. Being in there with the plexi-adorned transmitters all lit up and breathing fire was a sight to behold. I think Steve got some good shots last time we were there.

Honestly though, I was amazed there wasn't a fireworks show before now with the plexi panels due to the close proximities and strap factor.

R.I.P. Plexi-King. Seems fitting that he leaves us along with another stalwart of transparency: Dear Abby.

[KA0HCP]

No wonder you have been so scarce, but obviously putting the time to good use!   

[W7TFO]

It is good to see matching meters on projects.

Meter hodge-podge drives me nuts.  Not that I'm not almost there anyway.

73DG

[W3GMS]

Very nice Tom.  It will be good to see the final numbers when you get to that point.  Mr. Clean is a great name for the beast!

Joe, GMS

[K1JJ]

Very nice Tom.  It will be good to see the final numbers when you get to that point.  Mr. Clean is a great name for the beast!

Joe, GMS

Thanks, Joe -

The work could be better, but I have just so much patience for details and then move on to the next project.  I use recycled scrap yard aluminum and the parts I have lying around here. They are built strong and work well, but I have to stand in awe when looking at the professional work that some of the other guys do here...

I just received a 2-tone kit board from ElectroCraft, so shud be able to run some accurate IMD tests soon. But it's easy to tell the splatter level just by talking into a dummyload and tuning with a receiver. This new amp is at least 15 db cleaner than my 200 watt FT-1000D, so it's doing the job.  After hearing how sharp the skirts are on a signal that is -45dB 3rd, I became hooked.  The difference is tremendous.   If a station is transmitting 5kc AM audio, after the 5kc point the signal just drops into the noise... no buckshot, no artifacts.  Same on ssb with those 3kc channelized QSO's. It's a worthy goal.

Hope to get on with the boys this weekend with the 'J'  driving the 8877 and get some on-air acid tests.


DG, yes meters are important to match. Those old round 3" relics are of beautiful quality. The plasdick junk of today cannot compare.  A hole punch or even a drill/ hole saw from ACE Hardware is all that's needed.

T

[W3GMS]

Well, I am sure when you get the two tone test run done you will be happy with the results.  Very impressive on the AM sideband performance!  I can think of some stations on 75M that could really use your gem!!!  

The constructions looks fine.  Sometimes its better to prove the concept and move on to other projects.  We are all cut from different cloths on that.    

You will have the best fidelity narrowest signal on the band!  

Joe, GMS

[K1JJ]

For the techies following the thread....

Even though the FJ amplifier works well at 43dB of stage gain, I decided to reduce the gain and add a one watt lab amplifier (ZHL-3A, 24 dB gain) between the 3 mW FT-1000D driver and the 4CX-350FJ.   43dB is axing a lot of one stage and can be prone to instabilities.

I added a 3db pad before -  and a 6dB pad after this one watt amplifier.  This adds isolation and a level of protection.  Then I added a 4.7K swamping resistor from grid to ground of the 4CX-350FJ stage reducing its gain down to about 30 dB.  So the amplifier should become even more stable at a lower gain - good when changing bands and tuning up on odd frequencies.  I saw no effect on IMD, being that the lab amp is super clean.  Just a way to make the overall system more bulletproof.

The FT-1000D's  3 mW driver works a little easier now too.   The neutralization seemed to tweak in sightly better too, which is the sign of a more stable stage.  I now see a perfect screen current peak, plate current dip and power output peak coincidence. It was close before, but right on now.  

So, we're basically axing a tube rated for 350 watts  to put out 25 to 50 watts PEP in service. What an EZ life!

T

[W1RKW]

Love the very Navy like blue cabinetry.

Tom, can you post a picture of the grid drive input circuitry?

[K1JJ]

Love the very Navy like blue cabinetry.

Tom, can you post a picture of the grid drive input circuitry?

Tnx, Bob!

I really like that blue color. Kinda like the old IBM computer cabinets I think.

The amp is all put together and in the operating position, so next time I have it apart I'll snap a shot of the bottom.

I just finished up the little lab amp and the two pads and put them in service too.  This is the pre-IPA between the 3 mW  1000D driver and the 4CX-350FJ input.

Notice the 24 vdc 2.4A Sola supply. This regulated supply is only about $10 on eBay, brand new. What a deal.  I also picked up a + - 12V and 5V Sola to power my HPSDR modules.  The wimpy supplies that were stock are now replaced with some man-size Solas - cheap.

This lab amp runs about 12 watts in class A to put out 1 CLEAN watt.  I wasn't happy with the heat sink temp so added a small 12 vdc muffin fan running on 6V.

Shown below:

T

[W1AEX]

Mr. Clean is real pretty Tom. He can walk the walk, but can he talk the talk?

[W2NBC]

Close..

[K1JJ]

Close..

You bery vely funny man, Jeff!

Yes, there are striking similarities, I must admit.  Unfortunately, National went out of biz about the time they adopted that slick color scheme...  


Well, I put together the Electrocraft 2-tone tester board tonight. It appears to produce a very clean two-tone, good enuf for reasonable IMD testing.  The lab method would be to inject two separate RF carriers, mixed and spaced 1200 hz apart for the purest test.  But this 2-tone method will use the same station transmitter with audio tones, so will simulate the total enviroment as is.  

Due to station artifact pickup, I was only able to see down to about -45db 3rd with the HPSDR, and that's what the FJ is producing.  I then drove the 8877 linear with it and I still see about -45db 3rd at 500 watts output.  When I go above about 750 watts output, the 8877 starts to draw normal GG grid current and the IMD starts to go south. Still it's better than most ricebox IMD figures even then.

It was interesting to see how drive, loading, screen current and many other factors caused the 3rd to drop but the 5th and 7th orders to increase. And vice-versa.  They go up and down and change positions, just like squeezing a balloon. There are definately sweet spots to tuning an amplifer and if we do our homework and premark these settings, they may help us to tweak in an overall better IMD result.

All in all, so far this is the cleanest lashup I have had to date and will continue working to see if there are any more combinations of current, voltage and other parameters that improve things.   The days of being stuck at -31 db 3rd because of my marginal FT-1000D final amplifer are over...

Hope to get on tmw with the 8877 and get some daytime-quiet-band bandwidth reports. Caw mawn.


BTW, Frank/ GFZ says he's making some good progress cleaning up his SS amplifier. His IMD is getting good at low power levels so far.  It's amazing how much an amplifier cleans up when run 1/2 to 1/4 power. Running full strap and drawing grid current is like letting the wolves out.

T

[W1RKW]

Tom,
No need to pull it apart for a picture on my account.  I meant schematic of grid input circuitry.  Brain fart on my end for not using the correct wording.   Is the input circuitry also similar to the 813 setup?  I have enough stuff to build a linear but wasn't sure what I wanted to do on the input side of things.

Nice metal work, BTW!

[W3RSW]

Go Tom !
Your a true experimenter.
You've got me thinking. I think I'll give a low IMD/IP3 intercept amplifier chain a try.
Right now the best, low IMD exciter I have is the QS1E (mating to the top of the QS1R.)  The QS1E has only 2mw output though.
http://qs1r.wikispaces.com/QS1E+Exciter

I've been experimentally running it into an old TR7 brick (preamp and 150pep amp on same board w/sink) with manually switched TR7 output filters I got off epay and now see the error of my ways.  

So to maximize the "1E's" very clean signal and get up to a super clean 1/4 watt or so I think I'll build Clifton Labs quad GALi 74+ board,  http://www.cliftonlaboratories.com/z10046a_high_gain_amplifier.htm
You'll note that this board's specs. are pretty nice. It is intended as a super clean receiver pre-amp but can also be used to get the QS1E's output up to real world levels.

BTW, Phil, N8VB, designed the QS1E at that low output so it could be used as a lab generator for, say, designing filters by simply feeding the E's output through the filters and right back into the QS1R's input to see the passband, attenuation, etc.

So back to the Clifton board:

Both second and third order output intercepts depend upon selected bias resistor options. OIP2 is typically +87 to +93 dBm; OIP3 is typically +42 dBm to +45 dBm. OIP2 also depends upon the care taken by the builder during assembly.

Well that gets me to 1/4 watt for feeding into decent Class A tube amp circuits, or maybe another clean solid state circuit before going directly to a clean high power final (8874, 3Cx800 or 8877); the 8873/4/5 series alone at low feed levels should easily duplicate your 350.  I have a couple of 8875's and a spare Johnson socket.

You said ElectroCraft for the two tone gen.; did you mean Elecraft?  I need to look up the spurious specs on the two tone generator.

[K1JJ]

Hi Rick,

Glad to see you are excited enuff to build one up too!

Sounds like you have a nice clean starting point at a few mW.  I talked to Jack Smith from Clifton at length about using several of his preamps for this job. The bottom line is he recommended going with the Mini-Labs ZHL-3A. He ran IMD tests on his own at higher power levels (50 - 100 mW) and found they were good, (-45 area) but the MiniLab was about 10-12 dB better.   It takes a real honest guy to rec another competitor's product.  So I bought the ZHL-3A. We both measured it and it's about 58 to 63 dB 3rd IMD. That is stellar performance and complements the low level you or I use.

The Clifton will work FB, but we never know what we will be doing in the future, so might as well get the best.
I was able to add some pads (3dB and 6db) to make the ZHL happy.

IMD figures are generally hard to come by. They depend on a lot of factors but the biggest is how hard are we driving the stage.  For example, for extreme cleanliness, I never want to drive the 8877 harder than about 750 max watts output.  This is when it begins to draw grid current. Sure, we can get 2KW out of it and it will probably be about -30db 3rd, but maybe there's a way to do 1.5 KW with a bigger tube running 1/2 its max power.

Or do like some guys do... scream at full power in the pile up, and then back it way off when the contact is made...

Yes, I mean the Elecraft kit two-tone board.  It went together in 45 minutes and worked the first time. They say it's good to -55db harmonic distrotion. It apears as clean as I need for measurments.  It was cool trying different idle bias settings with the 8877. Running the two-tone, I could clearly see the best idle. The IMD dropped like clockwork whenever the idle was near 150 ma at 3500V. That's about 525 watts of diss, which is roughly 1/3 of plate diss. That's a good rule of thumb and works in this case.   Funny that if I idle it harder, the IMD creeps up slightly, opposite of what we normally think.

Keep us posted as your plans evolve for the amplifier chain. As a word of encouragement... last night as I tweaked the settings of the overall chain and saw the interactions, I felt like I was looking at an SDR pan adapter (actually I was) and finally seeing the real picture for the first time.  I tuned my other receiver that uses a 6" antenna stub. The two-tones were at 60 over while the 3rd and 5th sounded like weak whistles that disappeared with the higher orders up the band. This was at 500 w out.  When I increased output to 1KW, these tones popped up out of the noise as usual when seeing grid current.  The AM signal using voice disappears after 5kc with gentle wisps. No tearing garbage.  It's well worth the extra effort, and it IS a lot of effort.  


Bob: I do have a working schematic with iron burns - and will take a pic for you once redrawn..  The input circuit is the same as the 813 rig, minus the grid leak resistor.   I also used a tranzorb on the screen for protection. See G3SEK's tetrode board and suggested circuit precautions for more info.  I designed up my own screen trip circuit and regulated supply, so avoided the cost fo that board.


T

[K1JJ]

BTW, Rick, speaking of choosing the right tube....  Check out the typical operating data on page two for the 4CX-350FJ.   1400V, etc =  -45dB 3rd.  I've proven to myself that the tube will do about 43dB of stage gain for a single stage.  Use Hi-Q circuits for both in and out.  

In contrast, I think the power grid triode triode family will do 15dB of stage gain?  So, even though a tetrode is more of a hassle, we might need two or three triodes to do the same job for total gain.  

http://www.g8wrb.org/data/Eimac/4CX350FJ.pdf

The alternative is a solid state amp for 10 watts IPA, but I've had trouble getting the usual MRF-150's to do any better than -32db 3rd or so. The CCI kit boards were no better.   The Yaesu FT-5000 DXXX thing advertises class A at 75 watts, super clean, but I can't seem to duplicate this with regular ss amps.  Frank/GFZ is working on the problem as we speak and feels he is making some progress. All about cores and windings.

Let me know what tube chain you settle on for a design there.

T

[W3RSW]

Tom,
Ok on the ZHL-3a. At 24 db gain that would get me to 500mw clean output.
Well I was hoping for 20 db gain from a xxxA7 triode giving me 47dbm or 50 watts at such low drive levels but your 15 db gain from a 8874 etc is probably more realistic.  Perhaps one clean class A stage in between, say an 8874 feeding a pair of 8877's (very expensive, and redundant, given excellent tetrode specs as you say.)

I like the idea of very clean amps but there is a point of no return in having huge power supplies to achieve it in the last two or three stages.  Vis, starting with the Mini circuits amp, for 1 or 2 watts out you have 24 volts at .6 amps or 14 watt supply and a comparatively huge heat sink.  Full high power voltages, cabinets, etc. for a 50 to 100 watt output amp also is very eye-opening.  There is a middle ground particularly at hf where even atmospheric noise will swamp IM products, even at 35 db down, etc. Yes distortion products are cumulative with the addition of stages, so I will be seeking something a lot better than the usual fried transistor and positive grid voltage class of amp but maybe not as perfect as the art allows.

Also of note to you, audio is injected to the QS1e via USB, either from a USB mike or normal mike through USB digitization, another area of distortion concern to me. There are software two tone generators right from sound cards so I need to research the distortion products from these sources too. 

[W3RSW]

Concerning the Yaesu 5k, wonder if a lot of their success comes from super regulated bias too, i.e. keeping it in tightly same point class A.  That seems to be a recently, better explored design hot topic.