The AM Forum

Here we go, lets try it again. I was able to convert it to a jpeg here at work.

Here is the mod scheem for the now famous (or imfamous) KB3AHE Scream modded 4x1 transmitter. I built this thing out of my head 8 years ago and never penned down the schematic for it. The actual transmitter is not an "afterburner" type, but a complete transmitter including all of the low level rf and audio stages and fits in a 3' tall Budd cabinet easily.

This was a "necessity is the mutha of invention" project! I wanted  / needed a high powered AM rig, and could not put my hands on a large chunk of mod iron at a price that I was willing to pay back then. (I felt that $1.00 per watt of rating was a little steep) (Maybe I'm just a cheapskate) this rig was built completely out of parts that I had laying around, the only thing I had to buy was the 4x1 socket.

There is no reason why a pair or even a trio of 4-400s couldnt be substituted for the 4-1000. You just need to have lots of dissapation rating avaailable to handle the low plate efficiency at resting carrier conditions.

This is a hand drawn sketch of the modulation scheem only. Any home brewer worth his salt should be able to copy, design and or build the low level stages, so to save space I didnt draw them into the skiz.

Any shove-yank pair of tubes that can produce 15-20w of audio will work fine 6L6s,
6550s, etc. I had a 5 gal bucket of used 2E26s so I used a pair of them. (I didnt want to waste a pair of nice audio tubes)

All of the old-buzznard handbooks say that you should also apply some of the modulating audio voltage to the control grid biass as well to improve modulation linearity, so I designed it into the circuit. I used an ARC-5 mod tranny for this chore because the impedance match was fairly close, it had a tertiary winding, and I had one on the shelf. There is no reason why one couldnt use a small multimatch mod tranny (or a very large interstage transformer) and a resistive divider circuit to do the same chore. You want to apply about 25% of the modulating audio voltage to the control grid biass as well as the screen.

If you plan to build something like this as an "afterburner" type transmitter with something like a ranger for an exciter, you could easily use the ranger for the low level audio stages as well. Use the outpoot from the ranger modulator to shake the screen, and a divider to put a little shake on the control grid as well.

You will need around 25 -30w of rf to properly drive the 4x1 and get the desired audio peaks. Mine uses a 6CL6 buffer into a single 6146 into the grid of the 4x1.
You must be able to get 35-40 mA of grid current on the 4x1 to make the audio peaks.

If you would like to see some pix of the individual decks, look over on Steve's amwindow site. Look in the pictures under my callsign.

Here are the operating parameters as I run mine @ somewhere around 400w carrier outpoot.

Ep = 3450v
Eg2 = 290v+
Eg1 = -150v
Ig1 = 35-40mA
Ig2 = somewhere around 50mA at resting carrier
Ik = somewhere around 400mA (and watch the munky swing with the audio!)

Just a last note, the screen supply should be regulated and adjustable, and well "stiffened" !!

Have fun, Mine has been running like the proverbial "swiss watch" for 8 years now with the only problem being the crapout of a 60+ year old plate transformer.

                                                                 The Slab Bacon

Hello Frank,

Thanks for posting the details on your rig.  Interesting that you mod both control and screen grids.  I have heard it on the air, sounds great! Thanks also for sharing the operating parameters.  Just what I was looking for.

I have looked at the photos of the rig, very nice workmanship.  The only thing I do not see is a VFO, is it in the exciter chassis, or outboard, or do you use a crapstal?

Rick,
      I use a remote VFO sitting over at the operating position. I feel very strongly that shaking the grid biass is one of the things that makes it work so well. All of the old buzzard handbooks strongly recomended it, and it's not that hard to do.

The screen current and the plate current really swing with the audio. Adjustment of the resting screen voltage really makes a huge difference in the carrier outpoot power. Raise up the screen voltage and crank up the audio accordingly and you can dramatically increase your outpoot power. Initial testing into a dummy load before putting it on the air with 450v on the screen saw in excess of 650w of carrier before an atomic yaaaeeeellloo into the mike blew the plate blocking caps to smithereens. (10kv doorknobs) (this was a good kind of crapout) Changing them to 20kv caps and running it a little more conservatively has produced a pretty much "bullet proof" transmitter.

                                                               The Slab Bacon

                                                           

Nice rig Frank!!  I guess I will be saving up some cash for a 4-1000 or a couple of 4-400's.

Looking at the transfomer in the schematic, I got to thinking,

I've got a TV power transformer here that I think I could put into audio use, Ala Timtrons tech article on the AMwindow site. Should be good for as much audio power that I would want to push through it and windings for screen and feedback too...

Whatcha think about that?

Ya know something, Ed, you might just be onto something! I never even thought about that.
If not ARC-5 mod trannies and small multimatch mod trannies still show up pretty cheaply at hamfests. There are also some other mod trannies out there with tertiary windings.

My basic skiz shows the concept, a little creativity may come up with other stuff that will work equally well.

                                                                the Slab Bacon

I am leaning toward a trio of 4-400A tubes.  Rather than transformer coupling, I believe I will use a stiff series voltage regulator, and pass the audio through it; that will allow easy adjustment of the DC screen voltage as well as modulation level.  Independent regulators for the screen and control grid will make for ease of adjustment.  I will use a couple of 20DKT5 doorknobs like I did in my 4-400 linear - thanks for the caution on the 10KV jobs.  I recall cutting corners and using a TV doorknob in the past, it could handle the voltage but not the current.  Live and learn!

I really like your layout, with the final on one chassis and all the biass and exciter stuff on the other chassis.  Keeps the weight and size managable.  Thanks for all the info, should be fun to build this one!

Good idea on the 3 4-400s - if you can get them more easily.  The 4-1000 is a wonderful tube - why, I even own one :-), but they're getting scarce.

The only other suggestion which you will definitely not regret is to include a negative peak limiter!  Use a 2 or 1 diode limiter.  With a tube, you could probably just use one diode, tied off to a "keep alive" supply which will preven the screen from falling below some voltage which gives you say 90 or 95% negative modulation.

Regards,

Steve

I already have several 4-400s, two NIB.  Also have the sockets, and fil xfmr.  Seen many 1000s, but never had one. 

I was reading the ORR  handbook 1970 edition, and in that book they indicate to fully screen modulate, it is necessary to drive the screen somewhat negative with respect to the cathode (filament).  The negative limiter could be referenced to the negative potential that equates to 100% negative swing, I presume.

I was also considering some negative closed-loop feedback, in the form of a modulation detector at the linear output, fed back to the audio voltage amplifier differential stage.  Not sure whether this would be a good idea if I also used negative peak limiting.  Just looking to get as linear as possible.  All up in the air, lots of testing to do with the dummy load and scope.  Fun in the winter when we need the heat, not like now in HotLanta!

I am also thinking of trying the screen mod route with a quad of 4CX250Bs, only because I have them.  Could make a small desktop rig, with separate power supply.  It could do double duty as a linear or screen mod AM rig. 

Also consider a hi-fi output tranny turned around backwards. This will allow you to use a solid state amp as the modulator, assuming your sense of electronic aesthetics allows you to mix tubes and solid state.  ;) 

The schematic at the link below shows how. The secondary connections would be in the screen supply instead of the plate supply (B+). A smaller value reactor would likely be required too. If you use a multi-match tranny, one of the other windings could be used to modulate the grid, as in Frank's lash up.


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

My aesthetic sense doesn't limit me at all  ;D  I would mix ss with hollow.  It's just that I don't have lots of drawers with FETs in them.   :-\   

I'm a use what you've got kinda guy. 

Good way to go - use watcha got! NU2B's circuit might  be a good starting point for you.

http://amfone.net/Amforum/index.php?topic=6051.msg44706#msg44706

Rich,
       When I built it I designed it so that every rf stage is completely sealed (shielded) from the next. Even the final grid coil is in the exciter chassis. This made it extremely stable without any neutralization. Between the shielding and the heavy swamping of the 4x1 grid from the -150v bias, no matter what I have tried I cannot get it to oscilate or "take off". Even with the transmitter keyed  if I drop the input from the VFO, both the driver and final plate current drop to 0! It just doesnt get any better than that.

If you all would like some more pix, I have some saved at home showing the bottom of the Exciter / Driver chassis, and some close ups of the stuff inside of the shielded rf section. Let me know and I will find them and post them.

FWIW the 4-1000 is just a great bottle. They are very forgiving, undemanding, and easy to work with. They are also still cheap and plentiful used on hamfester tables. The socket is the harder thing to find.

                                                                          the Slab Bacon

Francois,
I first met your xmitter at Bowie and had the pleasure of operating it (well, the PTT anyway.)  I've got a pix of Chuck, FJJ in front of it stashed somewhere.  What a great rig.  Not only is it in a 3 ft. rack but it's really rugged and has lighted up some fests.

I know it made it to Howard Co.  That's the one where a SSB'er and his wife were walking by the corral when I overheard, " why doesn't your stuff sound as good as that?"   ... and that from the receiver.  She'd've been blown away if she'd heard your rig.  - 2E26's no less.   At that time all you'd tell us is that it was not plate modulated but that the schematic was available in just about every handbook around and that, "By G., you were not going to draw it up."  Yeah, that's what you said.  ;D

A true "classic of the waves."

And your too modest, your pix and the rig in ER and all; can't remember the issue but I'll dig it up.

Yes, I would be interested in seeing detailed pics of the exciter.  Good to hear more details on the design.  I used a similar approach in 1968 with my 4-400 linear - lots of grid swamping, and no neutralization.  It was also very stable.

I will keep an eye out for the 4-1000A at festers in the future.  I looked on eBay today and saw new ceramic sockets available (foreign manufacture) for under $40, they look pretty good quality in the photos, so there appear to be no great obstacles in this approach.

Frank,
What value did you use for the swamping resistor value. The 4CX3000A is the 4-1000As big brother.

Frank,
         I swamped it down by not using a grid leak resistor and slamming it with around
-150v of grid biass. It is so stable that you cant make it take off. I have intentionally tried to make it oscilate by shutting down the VFO and fiddling with the tuning and I cant make it take off nor do i ever see "that little something extra" on the scope. I also have the 6146 driver biassed way down deep in class C. It has always been unfinicky and a pleasure to operate. I have even left it in the hands of unfamiliar operators when we did all of the special event stations with it, and none of them managed to blow it up either!

I'd be uncomfy using nothing but a bias supply. since I tend to leave rigs on when I'm not around, i'd be worried about a supersized version of the drb-100 'incident'. But thats because I'm paranoid. In the HB rig, I used a combination of grid leak and fixed. but I could make my rig take off. Me and Huz burned a hole in a 812 one day over at his place. damn tube still worked.  :P

Well Frank, I appreciate your praise of the 4-1000.  It is truly a class bottle.  You convinced me to go first class, I found a bottle today, NIB GE, not Eimac.  It is on the way, and I ordered a socket as well.

I assume the single 4-1000 will be easier to build than a trio of 4-400s, especially with regard to stabilization without neutralization.  I will let you know how the project comes along.   Thanks!

Well, here are some more pix of the rig. Keep in mind that the pix of me standing next to it, Thanks to my wife I am quite a bit grayer and balder now!! ;D ;D

more pix

the pair of 807Ws are not modders, they are a paralleled pair of cathode followers for the regulated screen supply

and at last a few more

thats a JW Miller plate choke. Some good looking metalwork on that bad boy.

nothing beats fine looking new homebrew..... same as white lightning!

Hey, thanks for posting this Bacon!

I've got the battered (yet complete save for a vacuum variable) carcass of a 4-1000 amp, and a supposedly good tube that was cheap off the 'bay.  I'll be attempting to duplicate once I live someplace where I wont win the "Worked all Toasters" cert!

I might, however, try something like this with an 813 though!

Thanks and 73,
Jason kf6pqt

That's a fine looking rig!

Thanks for posting it!

Nice work!!  Thanks for the photos.

It is obvious that LOTS of planning went into the layout!  Cool bandswitching in the exciter.  Thanks for posting all the detailed pics.

I used up quite a few Sharpies worth of ink and a whole bottle of alcohol wiping it back off and redrawing it before I drilled the first hole. It sometimes gets pretty interesting trying to figger out exactly where everything wants to live.

I built it back in a time where my personal life was a little less complicated. It went from a thought / concept in my head to on the air in about 2 1/2 months.

                                                                The Slab Bacon

Great FB looking rig!

Sure wish I could do the type of metal-work.  I'm quite happy with my pair of screen-modulated 4-400's.  I use a UTC hi-fi output transformer wired backwards, with the high Z side wired in series with the adjustable screen voltage.  I have plenty of audio power, supplied from a 20 W SIP solid state amplifier. It will cut the tubes off on negative peaks.  Yes, a little assymetrical without feedback, but after hearing it in the mod-monitor and good-off-the-air reports, I have left the global feedback out of the circuit.