The AM Forum

Looking for 40 to 50 watts out without pushing things too much, with clean audio, D104 input, 40 meter xtal control.
A 4D32 screen modulated with a DX60 modulator does not sound or work good, so I need to do something else.
A 6146 with 6l6 modulators is about right, but 6l6's are low voltage tubes and the 6146 likes 700 volts or more.

I have a Viking 2 mod trans, a dx100 mod trans, and a 25 watt mulit match mod transformer.
I have 4D32's, 4x150's, 6146's, 6l6's, 6b4's, el34's, KT90 and KT100's, and other tubes.

Looking for something small, so would like to nix the driver transformer and run AB1 in the modulator.

Grab a Heathkit HP-23 (or most power supplies designed for vintage transceivers) and that will take care of your power supply needs.  Or take a look at the "economy" l power supplies as described by Lew McCoy in some of the 60s/70s era ARRL handbooks which use a full wave bridge rectifier and center tapped transformer to simultaneously provide HV and 1/2 HV for the high and low level stages from a single transformer.

don

That is an idea, a dual voltage power supply, run the 6l6's at 350 to 450 volts and the 6146 at 700+ volts.
That might work well.

I would use those KT-90s even to modulate a 50-60watt output rig.  Build two power supplies 650-700 volts and a 300 volt supply.  6L6s probably won't make enough audio.  807s can be run up at 700 volts but the 6L6s are only good to 450 volts which is more of a PITA to get with most common xfmrs.

The economy supply will also work but you'll need a big 700-800 vac ct xfmr.  A big old TV xfmr might work but most won't yield more than about 650 DC volts after a filter,  and will run cooler if you use a separate filament xfmr to lighten the VA load.  This type supply requires two filters, low voltage and high voltage lines.  I use one in my 6146 x 807s rig but the xfmr ran warm to hot.  It would have run much cooler had I used a separate filament xfmr.  I added a separate 650 VDC supply xfmr especially when I removed the 807s and put in 6550s.

What xfmrs do you have on hand??

Fred

I have a nice 1500 vct transformer, 750-0-750, 150 ma I think.
One 6146 is going to need about 35 watts of audio to drive it, so 50 should do.
I do not need to run things at 70 watts input tho, I could do 50 and get 35 or 40 watts out.
The DX100 mod iron looks like the best bet, the 25 watt Stancore multi match looks a bit light for the job.

I have 6ca7's, 6l5gc's, 1625's, 807's KT90's and KT100's, and some other hifi type tubes.
Its got to give 40 to 50 watts cleanly in AB1 I think, that is not easy to do with a low parts count.
Everything has to fit on a 17x10x3 chassis, power supply, tx, modulator, speech amp.

I've got a single 6l6gc modulated by a single ended kt88.

Sounds great.   Use it on 10 am.   12 whole whopping watts of carrier.   Capable of fully overmodulating that carrier.   Both tubes run on 440 volts resting,  about 400 under full steam.

--Shane
KD6VXI

That chassis is going to be really tight for space.  I built mine 50 watter on a 17x12x3 and things were tight.  Mine has a built in VFO, yours is just going to be crystal control so you might have enough space.

That 1500vct 150ma. xfmr is just enough for the 6146 and not much else.  You will only get about 600-650 VDC after a filter.

If you lowered your power requirements a bit you might be able to build the whole rig on the one chassis with a single xfmr economy supply.  Problem is you need the right xfmr.

Besides the xfmr you mentioned do you have anything else in xfmrs??

Fred

They get big above that. I will check what the current is tonight.
Its icas so I think the transformer will do, I do not have to run things at full bore.
Got a dx100 HV power trans and others, but they get big, plus a big choke is needed.

Ok, maybe 20 to 30 watts out?

Got a diagram?
Can it be scaled up a bit?

Surely do have the schematic.   It's posted right next to all of yours... Lol

I was talking to a friend earlier.   Time to start making them on the pc.   Have a few scribbled on lined paper,  printer paper,  etc....   But need a better method of filing them.

The power supply and mod xformer came from a junked Cb,  a Tram d201.  That much I can say,  off the top of my head.   That's my limiting factor,  I believe.  

--Shane
KD6VXI

Also have a 6l6, screen modulated by a 6bh7 1/2 driven by  traditional carb mic circuit. Other half of 6BH7 is a hartley VFO. I get about 6 to 7 watts out. No schematic, free build.
don

The transformer is rated at 175 ma.
100 ma for the RF stuff, and the modulator would be low duty cycle.
Get a 300 ma one and a 300 ma choke and I might as well build a dx100.
What is a ranger running? Don't tell me they have a huge power transformer in that rig...

No reason why it wouldn't work.  Not sure what the xfmr is but many can handle more current than their ratings.  Most are rated for continuous operation, ham xmtrs are on and off.  You'll still need something for the low voltage and filaments.

I have a similar xfmr here 1500vac ct @250ma but it's a 6in cube.  I tested it with a FWB and pulled over 200ma from it without any problems.

I just took apart a Johnson Challenger (was missing parts) and the power xfmr was a fairly big one for the rig it powered.

How about a Johnson Ranger

The idea is to home brew something to keep me busy and out of trouble.
I did bid on a DX40 though, seems like a well made thing for a heathkit, just low power.

Homebrew is a lot of fun. 50 watts seemed like QRO to me however I did build a 20M rig with a 6146B final screen modulated by a 12ax7 and another 12ax7 as VFO and buffer. I have a 829B 40m rig I am poking away at. Cathode modulated and a PT15 Supressor grid modulated simplified T1154 I have poked away at when things were slow. However some of the most fun I have had is building and using one tubers , pre 1929 rigs. Why they are so much fun I don't really know but they are. Even with 5 watts in I can work the east coast from mid Ontario. I find as long as I can work somewhere I am happy. The simpler circuits really fascinate me as they often work surprisingly well. It is all Lou's fault VE3AWA as he encouraged me to built my first tunable power oscillator and now I am addicted. Before Christmas I built three rigs. Each taking a saturday to build after gathering parts and finish and tune up on the following monday.  Had two of them successfully in the BK rally and one even has phone ability. Old rigs like rangers etc are fun(I have a pair of ARC5s and a DX60B) but homebrew is really a blast. Free builds are for me even more fun (not from a previous plan or schematic ) I admire your wish to homebrew one for yourself and wish you every success. The thrill of the first contact on something you conceived and built and trouble shot is a thrill that is hard to equal.
don

I enjoy the building, and everything I build is a mis/mash of designs and circuits from all over.
Since I have a bunch of high power rigs, medium power rigs, receivers, modulators, vfo/exciters, I wanted to try a self contained 20 to 40 watt xtal controlled AM rig that sounds very good, all in as small a size as I could get.
It could be combined with the prototype home brew receiver as a standalone station.

I find getting 20 to 40 watts out of something on AM takes lots of space they way I do things....

I like the two tube screen modulator setup for the fidelity and small size, but to get any power out I need a big tube or a lot of voltage.
A single 4x150 with 1800 volts on the plate would do 40 watts, but 1800 volts!!!

Plate modulation takes more transformers, more power supplies, screen voltage regulators or driver transformers, lots more tubes.

A ranger was amazing for the amount of stuff they packed into it, but its not as hard if you can pick any part you want or have parts made for you.

Small is hard.

With that low power (50 watts), you *could* build the ultimate tube final transmitter!

The modulator can be DC coupled with no transformers.  Perfect sound.

Use series modulation.  I did so for MANY years back in the day (1971-1972) with a 50 watt transmitter.

You could use a tube modulator, or you could an IGBT or MOSFET.  1600V IGBTs are not expensive at all.

So, run 400V on the final and a total power supply voltage of 1200V which will give you 200% positive peak capability!!

A 4D32 would make a pretty awesome final with an IGBT (or 2 of them - would have to do the math on the power dissipation) in the modulator.

If you want an efficiency improvement, you could go class H with the modulator which would bump the modulator efficiency up to around 70% at carrier.

Audio-wise, you probably can't get better than series modulation.  And it's simple to do, too!

What is the downside of series modulation?
High voltage on the cathode/filament or something?

Hi Brett,

Since you've been playing with screen modulation a lot lately,
why not use one or two 814 bottles.

According to the specs, they would do about 25/30 watts ccs, and 35 icas per tube... (1000v - 1250v)
The screen will take about 2mA, so you could easliy screen mod two tubes with the solid state modulator, or just with a single 6L6 as cathode follower.

Some say the 814 is worthless tube with a flimsy anode, but I think it is a great tube for low power purposses.
They shine nice... that is a plus. :)

Oddly enough,  I'm doing just that,  Steve.

Have a pdm rig in the works.   Runs at 170 kHz,  and onto isolator coupled to the pdm pass mosfets.   This allowed any mosfet to be used as the switches,  at any voltage,  with the pwm running at normal voltages.

Had hoards made up in Asia.   Waiting on them to show up in the mail now.

Figure as designed,  should be good for 75 Watt carrier,  and 400 pep.  Want more,  parallel more mosfets on the output.   

The pwm is working,  just waiting on higher voltage fets and a transformer to test with.   Worked at 180 volts thus far...

--Shane
KD6VXI

Not interested at all in the solid state stuff.
That is a much different junk box that I do not have.
And I think I would much rather have a new flex into a big amp, it does so much more then a solid state AM transmitter if I was to go that way.

I am very much more interested in the method, not the end result.
I have piles of parts to use up, and do not want to add piles of solid state type stuff.
I have lots of tubes that want to do something, they are tired of just hanging around...

I have series cathode modulator (homebrew) on my ARC5s. For me the downside is the HV used by the moduating tube. Simple enough to cure by adding the lost voltage across it to the power supply design except I dont have a bunch of transformers or a lot of gold to spend on it. Currently I net around 500 volts on the pair of 1625s and another 200 or so across the modulating tube. That isnt all bad as many ARC5ers only like 500 to 550v on their finals. In the same vein I made a single tube series cathode modulator for one of my TNT rigs, a 45 tube deal modulated by a 01A. Recently realizing my Bare Essentials 50l6 CW transmitter was also cathode keyed I tried the 01A modulator in the key jack of the 50L6 rig. I had to supply its own heater voltage but all HV is stolen from the rig being modulated. Much to my surprise it worked very well. The 1940 radio hand book (not ARRL) has a lot of info on this type of modulation plus several builds using it in some form or other. I found it very intriguing and know I have just scratched the surface. No mod transformer was the most interesting part. Some designs use a choke. I have tried it with and without. I have used 6J5s and 6l6s and everything in between so far. There are trade offs. Do you use a big enough tube to handle all the current or multiple small ones for less total resistance? Here I like carbon mic circuits because I like that dry crackly audio however not everyone does but that is simply a matter of building a speech amp to match your mic choice and has little if anything to to with the type of modulation since the same parameters exist for all types. Series modulation can also be applied to the plate with not a lot of design differences. That circuit is also in the 1940 handbook. I have also used the simple grid modulator as ARRL (1956 ARRL handbook) insists on calling it which is a variation of this idea and it works well too. It also requires no B voltage as it also steals from the rig when plugged in. Of course built in would be a no brainer. For me I find this very interesting. Simple uncomplicated designs the actually work quite well. In fact the only plate modulated rig I have in this shack out of around 15 transmitters is my benton harbor lunch box , the TWO er.
don

You can build a tube series modulated rig just as easily as a solid state one.

I think you would like the result!  Absolute high fidelity and DC coupled.  Doesn't get much better  :D

The plate efficiency of the modulator is low, but the overall efficiency is probably not much lower than a transformer coupled modulator when you consider everything.

When I built my first series modulated rig back in '71, I was absolutely shocked by how good it sounded.  It was totally unbelievable as compared to anything I had ever used before.

Another possibility:

After building my quad 4D32s PDM tube rig, I have  new respect for the 4D32.  Four will put out  400 - 500 watts of AM carrier.

A classic rig would be a pair in class C plate modulated by a pair triode-connected.

Power supplies would be minimal.

1)You would need a ~ 600-800V plate supply. 
2) A ~75V protective bias supply for the finals.
3) A common filament transformer for the finals and modulator.

The final's screen voltage could be a dropping resistor off the plate.  Use a grid leak resistor in series with the protective bias.  Use a zenor or regular diodes in the cathode lead of the modulator for stiff bias. The modulator screens and grids are tied together and driven. No external bias needed. That's it.

Add negative feedback to the modulator and a good mod xfmr and you have a classic rig.  Drive the modulators with either a MOSFET audio driver or a backwards connected audio transformer.

If desired, blow some air directly down on the tubes and you're golden.

T

Tom,
That is what I do with my 3x4D32 rig.
It runs three 4D32's modulated by 811's, everything runs at 1300 volts and it runs well at 300 watts carrier out.

300 watts is not a low power rig for me though.

I need to look into series modulation (cathode mod?).
I never tried that.

Sounds like you've already arrived, Brett ...  ;)

Yes, a series modulated rig can make beautiful audio - flawless.  I've built a few over the years. They certainly sweep well, but a larger series modulated rig sure throws off a lot of heat. It's almost ridiculous.  It's quite a contrast to see the final in class C and the modulator in class A.  (It's all relative, of course)

Except for my little 5 watt 6AQ5 series modulated rig, I eventually recycled every one into something more efficient. 

A smaller series modulated rig is the ticket.  The audio quality vs: the heat is probably a worthwhile trade off.

T

What sort of tube makes a good series modulator?

Looking at what I have, and with the good results I have had with it, I was thinking maybe the dx60/qix modulator into two 6146's might work well and be small, 25 watts carrier? but I never did series modulation...

One thing I like about the dx60/qix modulator is the small parts count, only two 9 pin tubes mic to screen.
It also takes care of screen protection.
I thought an 811 would be a good modulator, but it likely needs power to drive the grid.

How about trying an inexpensive Octal based high current sweep tube like a 6CD6?
The G2 volts requirement of 150-200 makes it more complicated too, but that is DC and low current so a supply could be floated, and it's not a signal to be faithfully reproduced and coupled as with the grid of a zero bias triode.

6CD6 will take more current, 200mA average, than a 6550 and stand well over its rated 'DC' plate voltage of 700V. Has a 5KV peak plate voltage rating, not too likely to arc at 1000V. The specs usually say 20W CCS but it will take a little more. There are similar tubes. Look for which takes the most dissipation, that not current will be the issue.

If all you want is to make 25W carrier from a 6146, the voltage can be low.
RCA TT-4, 175MC data:
Eb 320V
Eg2 180V from 13K resistor
Eg1 -51V from grid leak of 27K or cathode bias of 330 Ohms
Peak RF grid volts 64V
Ib 140mA
drive 3W
Po 25W

So, there you go, a 700-1000V supply, paralleled 6CD6's in series with 6146.

Maybe 3 or 4 6CD6's in parallel due to 60W resting dissipation @ carrier.

Dissipation is what you really have to deal with. The series mod has to be able to dump some heat, have a low drop at 200+mA, and and stand a high voltage at low current.

I don't know how you can get away with just one tube for a series mod unless it is a pretty big one and is capable of a low voltage drop. OK the 4CX250 but that s another problem due to air. That's why I suggest looking at the 6CD6, they cost about $8 NOS so 3-4 is no cost and they look cool.

I have only one real experience with a series modulator. When I played with series modulation I used a modified variable voltage DC power supply as the modulator of an 807 making only 10W. IIRC the supply used two 6L6's, and it was pretty small. Peak plate volts on the 807 was about 300V. Schematic is around somewhere.

6v6 was commonly used up to 4 in parallel which will handle a pretty stout rig. There is one in the 1940 book similar using 4 6l6s in parallel that will handle rigs of 500 to 1000 watts. The same can be built with any number of tubes with in reason so 1 6l6 will handle a 250 watt rig easily and a 6v6 will handle lighter applications. The actual modulator section is quite simple however most would use a speech amp in front of it. I have tried it with triodes as well. In my modulator for my ARC5s  I am currently using a 6l6G and driving it with a 6sl7 speech amp when using a dynamic mic. Like a lot of rigs the speech amp could use a bit more or an amplified microphone(as can most of my rigs including my DX60B) but that said is a practical set up and does work well. When using the carbon mic I use a modulator with a 6sl7 or similar dual triode parallel connected which would be not unlike a pair of 6j5s.I built a few different ones just to see. Some just clipped together.  Essentially the plate of the modulator tube (6l6 or 6v6 ) is connected directly to the cathode of the final in the transmitter. The cathode of the modulator is connected to ground and biasing is not that different from what you would use with those tubes anyway. Idle current can be altered by changing the resistance in the circuit. I personally use a small lamp there as they come in various resistance values and are easy to change. (IE #40 #47 #53 etc) As mentioned by others it will surprise you as modulation is full sounding. It can be installed in any rig with cathode keying in minute, just plug into key jack. Only supply needed is heater as the tube is in the cathode return circuit and gets all its B voltage automatically. If you would like a picture of a couple of representative circuits from the old handbook email me direct and I will do that for you. I am good in QRZ.com
don
Wooden deal was one a cobbled together as I wasnt sure but it works wonderfully. Transformer is for the carbon mic circuit and is a universal audio output . otherwise it is an 01A tube , a .1 paper cap and a 5K grid resistor. 6 vlts to heater K circuit also supplies carbon mic bias. This with a 45 tube will modulate the arc5s nicely but I hate using my collector tubes regularly. Too hard to replace.
Bottom is the dual triode carbon mic modulator and I have used it quite a bit with pleasure. It should really have 6 volts mic bias but this was the holder I had and it does work.
Top is the so called simple grid modulator from the 1956 arrl book and it uses the same method. (in series with cathode return of transmitter finals. It works well also. It handles a modern mic rather then a carbon   

Sweep tubes.   Gobs of cathode emission.

--Shane
KD6VXI

Just to be clear: cathode modulation is not the same as high level series modulation (even if the modulation is applied to the negative side of the RF amplifier - AKA, the cathode side).

Cathode modulation is really grid modulation with some portion applied to the cathode-plate differential.

True series modulation of the negative side of the RF amplifier involves varying EVERYTHING with respect to the PLATE.  So, the grid-cathode voltage does NOT change under modulation - E.G., the DC grid circuit floats along with the cathode.  This is NOT difficult to do.

With class A series modulation, you are not pulling an inordinate amount of current.  If you're RF amplifier runs 400V @ 150mA and your total power supply is 1200V, the max current seen by the modulator is 450mA, on the highest positive peak the system will do (in this case, 200% positive).

Choosing the modulator tube:  Using the 400V @ 150mA example with the 1200V power supply, the modulator will be dissipating 800V @ 150mA at carrier.  That's 120W of power.  To be more conservative, I would suggest 150W of plate dissipation in the modulator so you're not pushing the tube(s) to the max.  Digression:  This is where class H would be good.  The carrier dissipation would drop to something like 15 or 20 watts with class H, but that's another discussion.

So, whatever will give you 150 watts of dissipation would work.  A triode is easier to deal with as a modulator, and will probably work better because of the greater plate swing available.  A couple of 811As might work (or 3 of them, which would be better).  There are probably other tubes as well.

Exactly. Frank Jones helped to popularize this method of modulation with articles in Radio magazine in the 1930s. His approach was to pick the amount of cathode (effectively the same as plate) modulation versus grid modulation to maintain a reasonable final amp efficiency while easing the amount of modulator power required (as compared to plate modulation).

I am confused. If all the potential is going through the modulator tube, 100% and if it doesnt go though there it is not getting to the cathode IE ALL B-as it is in my set ups ? Modulation is not APPLIED anywhere. It is the gateway for all B- for the finals. If it doesn't go through the modulator tube to the cathode which is in series with it there is no cathode connection and nothing flows. I must be missing something here. The modulator tube in my set ups is THE ONLY connection the final has to B- or ground. What did I miss?
don

Don,

Take a look at this schematic showing a true series modulated rig. As Steve said, the grid is tied to the cathode and follows the high level series modulator voltage. So there is no grid modulation.

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

Whereas, a cathode modulated stage would have the grid voltage independent of the modulator causing grid fluctuation. (A combination of cathode and grid modulation)

T

So it takes 3 or 4 811a's to modulate a 6146?
No thanks!

Maybe this will work, a pair of 6146's with the dx60/qix modulator and a minimal power supply:

Nice stuff.
How does one 6l6 modulate a 250 watt rig?
I don't get it.

stylized pictures tell a story.

The cathode modulated stage is variable efficiency, not as efficient as a constant efficiency class C stage modulated only by its supply voltage.

The trade of is you can't use one 6L6 or two 6V6's unless you want 5-10 Watts.. Got to have some more watts and volts in the modulator to do more.

OK, but wasn't QRP asked for? We just suggested QRP-sized series modulation schemes that are simple and economical.

The choice of efficiency, outside of using a switching modulator, is whether it is in the RF stage requiring a little modulator tube or in the modulator requiring a big modulator tube.

So there is always inefficiency somewhere unless high tech methods are used like PDM/PWM. Then yes one sweep tube would be ok, maybe, I'm not the expert on PDM! Nothing's free, so the extra complexity required for PDM. - but I see that as experimental fun, if I were going to do it.

OK, go for the traditional modulation, no one can complain!

Why not just build a single 6146 modulated by 6L6s.  Plate voltage around 450 volts for both tubes.  Use a dropping resistor for the 6L6s screen voltage, maybe add a few VR tubes.  At 450 volts you should get maybe 25-30 watts output.  The 6L6s will run AB1 so you can use a phase inverter.

That xfmr you have in the picture is a Triad or one from a piece of HP gear.

Fred

Well, say I am looking for 50 watts carrier, with 100% modulation and clean audio, what sort of setup could be run?
It seems a lot easier to build a big rig then a small (in size) one!

That is an idea.
Can make a dual voltage supply and run higher voltage on the 6146 as well, that would not take more space.
Its a triad transformer, r118a.

That is an idea.
Can make a dual voltage supply and run higher voltage on the 6146 as well, that would not take more space.
Its a triad transformer, r118a.

A 6L6 push pull plate modulator is an interesting idea, but then it would be the same as so many other smaller rigs, and the weight and size go up as you add tubes and transformers. 

You have made so much progress evaluating various tetrodes for screen modulation.  The 4x series modulates cleanly, but if I recall correctly, you were not impressed with the 4D32 or 813 screen modulation characteristics. 

A 4x150 would work in a smaller rig, but for screen modulation you still need the noisy fan.  I was thinking that it might be interesting to evaluate the performance of one or two Eimac 4-65A tubes with either a transistor or tube based screen modulator.  They are about the same size as the 4D32, they use the same socket, 6 volts on the filament, and can take the high voltage necessary to produce the positive peaks.  They would not need a fan, and they might make a more compact rig, eliminating all the excess weight and bulk of a plate modulated configuration.  The only additional weight and size of a higher-voltage power supply is more capacitors and equalizing resistors. 

If this seems interesting to you, there are NOS 4-65As in my stash,  but it would be a week or more before I could get them on the way, because I am travelling now.  Look at the specs, maybe they would need higher voltage than you wish to use, but it sure would be interesting to see if they modulate linearly.  If they do, it would surely make a more compact rig with lots of headroom!

Take a look at the schematic? Whose? That is not mine. I have built a half dozen of these. No the grid IS tied to the cathode lug and ALL is controlled by the tube underneath when I use it on the one tuber. I suspect it is also so on the ARC5 although RF comes in from the OSC on a cap. I will check that out this afternoon. One was already converted to cathode keying when I got it so in truth I didn't look. I know if I built it the grid resistor would be tied to the cathode lug cause i always do that. The mod tube is I believe only path to DC ground and B- for the finals and certainly by far the most significant.  I think some are confused. There is Parallel Cathode modulation  which I have also done with my ARC5s. In that case I used the Modulator with the 2 D batteries and fastened a transformer to the front . I used a Universal TV horizantal transformer since I knew it would stand the voltage and it worked. I worked Dave and the group in Rochestor area with it. It was a bit light and needed more push from its speech circuit it seemed. Advantage with it is you don't have to deal with the voltage drop across the modulating tube so it had a good strong carrier as the 1625s were running full tilt (and a bit ;>)). For instance in series cathode modulation if your modulating tube drops your B voltage (remember now it is in series with the final.) then you lose that amount of voltage across your final so ideally you would plan for that. And you try to avoid tubes that steal too much by taking internal resitance etc into account instead of buy the biggest tallest tube you can find but use all that is required and no more .How can a 6l6 control that much? I offered the circuit. Not one taker.  It is straight out of the 1940 Handbook pg196. That circuit uses four 6l6s to series modulate a pair of 860s and the article says it will handle finals from 500 to 1000 watts. This is not what I said. It is what the book said. It was from these articles that I began experimenting with it. I postulated if four 6l6s can handle 1000 watts then most likely one can handle a 250 watt amplifier. Didn't seem unreasonable to me. I know a 6v6 by itself has no trouble with my pair of 1625s on the ARC5 transmitters. However in sorting through all my 7AC base tubes I found one 6l6 that sounded better to me hence its use. A 6sl7 can also handle them nicely and does with the carbon mic modulator as does a 45 in the wooden one. Across the pond I found some hams using a single high voltage NpN transistor to modulate Arc 5s in the same manner. I like tubes so I went down this path. I am beginning to suspect I am about the only one playing with this and I can see it isn't going to happen here so I think we should just let it die and I will continue to use it and enjoy it when I run my Arc 5s occcasionally early mornings before sunrise. However it does work, is quite simple to do but if you haven't actually tried it it may be hard to understand.
don  

The 6l6 cathode modulator circuit is in the 15th edition of the Bill Orr handbook.
(the red one, page 240)
It says the circuit does not work well on tetrodes, but it works well on Triodes.
What gets me is they run the speech amp circuits off the cathode voltage drop!

 

That is true, when I add a mod transformer and two sockets for the 6l6's things get very crowded, plus I would need to add a choke in the power supply good for the modulator most likely.

A 4-65 would likely make a good tube to screen modulate with the solid state circuit, but the tube circuit would likely have problems with that.
I think all the 4- series tubes work ok with the solid state screen modulator (a low impedance modulator).

A single 4x150 would be great, low drive needed, screen modulates very well, they do not need a lot of air unless pushed hard, and 40 watts would not be hard, but they want higher voltages on the plate to get nice peak power output.
I could likely do 1400 or 1500 volts with a full wave bridge setup and get 40 or 50 watts out with the tube screen modulator.
The 4x150 takes very little drive power as well.
I sure seem to be getting hooked on the 4x150's...

I have a feeling that the 4-65 would probably work better with higher plate voltages than the 4x150 series of tubes.  The solid state screen modulator would probably be just as compact as a tube modulator, and the heat sink could be mounted on the back of the cabnet instead of inside the chassis. 

I think the progress you have made on screen modulation points the right direction for clean modulation, lots of headroom, and a rather light weight design.  Back when the power limit was 1KW plate power input, screen modulation made little sense, but on 40M, as you have stated, lots of power is not really needed.)  As I mentioned earlier, once the 250TH plate modulated rig is completed (a couple weeks away) I plan to convert my Dentron Clipperton L from 572Bs to a quad of screen modulated 4X150Ds, modulated by your solid state circuit.  I found a site on ebay that sells four of those HV transistors for just under $30.00, free shipping, and they delivered quickly.  So now I just need time to put it together.  I had other plans for the NOS 4-65s, still sealed in the Eimac boxes, but I am sure curious how they would do with screen modulation.  I will keep them available in case you want to go in that direction.

Its not my circuit, its John's, W2IMX.
And you should be able to get the transistors for $2.00 or $3.00 each I think, the last time I got 8 for $20.00.

The tube circuit is very small, two 9 pin tubes and some diodes, and it includes a mic input.
The solid state modulator takes heat sinks for the resistors and the three devices, plus some large high voltage caps.
I used oil filled caps in the high power (high voltage).

On the small chassis you pictured , where do you plan to get 1500 volts?  It won't be from that Triad xfmr you pictured.  The R-118A  xfmr is 750 volts CT. at 175ma.

I have built 1400 volt supplies using two xfmrs like the R-118A.  Put a FWB around each xfmr.  Connect the diode rectifier output (of the first xfmr) directly to the negative terminal of the second xfmr FWB and the two voltages will add together.  Second xfmr has to be able to withstand the higher voltage on its secondary.  You can do this with any two xfmrs and get whatever voltage levels you need.  You can use any combination of FWB or FWct rectifiers.

Fred

Full wave bridge, should give 1500+ volts cap input, the regulation is not important because screen mod is steady average.
The cap is a problem though, I need a 2000 volt cap and dont like series electrolytic caps that much, it would take 4 in series.
The tube modulator will do a nice job on a single 4x150, 100 watts out if you push it some, 50 watts is easy, you just need some voltage on the tube. I do not think they do much at 800 volts, but I should test that again.
I have about 15 old 4cx250b's, good used ones plus some new ones.

They are small,
They take almost no drive,
They do not need much air at lower power levels,
They screen modulate very well and easy,
I have a crap load of them.

Cap input, no load, 750 volts x 1.4, about 1100 volts peak.  Add any load and the voltage drops from that.  Plus it will put a heavy VA load on that xfmr.

You need a bigger xfmr or a second xfmr.

Do you have any other similar xfmr like the R-118A??

I have some others, but they get bigger.
The trans is 750-0-750, 175 ma,  2100 volts no load cap input, 1350 choke input.
A single 4x150 would pull about 150 mills, 202 watts in, 50 to 60 out, 150 watts plate dissipation out of 250.
The high voltage is a pain though...

My catalogs show the R-118A as 750v CT.  To me that means 375v-0-375v.

I better check that out then!

Yes, that trans is 750 volts center tapped.
I tried tests with one 4x150 with the tube modulator, at 700 volts you can get about 10 or 15 watts out.
They really want 1500 volts and up for screen modulation, so that is out.
I have transformers that will do it, but things tend to get large with high voltage.

Voltage doubler.

--Shane
KD6VXI

If you have another xfmr like or anything close you can get a 1400 volt supply from two.  I described how in an earlier post.  Two smaller xfmrs can fit nicely on a small chassis than one large xfmr.  A FWB on a 750v ct xfmr can give you the low voltage supply (300V) off the CT and the the high voltage from the diode bridge is then connected to a second 750v ct xfmr that also has a FWB rectifier.  You connect the HV output from the first FWB diodes directly to the negative terminal of the second xfmr FWB.  You end up with a 300+ supply for the low level stages and a 1400 supply for the PA stage.

You can put a PTT relay on the HV line between the two FWB rectifiers and maybe a second relay contacts can be used to PTT the low voltage supply.

Keeping it simple, you can use a RC filter on the low voltage supply and a LC filter on the HV supply.

I've made a number of these supplies and they work perfect.  The only thing is the second xfmr has to be able to withstand the high voltage on its secondary.  Most common xfmrs is this class should be able to handle the high voltage.  On some I've improved some of insulation on the wires coming through the metal end bells.  Also added some additional insulation on the winding core and inside of the end bells.

Fred

Oddly enough I have some smaller transformers that would give 2000 volts out in full wave center tap, I think one is 120ma or its 220 ma, the first number is rubbed off.

I don't think I want to go that way tho, 1500+ volts has space problems, I do not like strings of electrolytic caps at high voltage, plus that and oil filled caps good for the voltage are huge.

I can go 2 ways, a pair of 6146's screen modulated at 25 watts output, or a single one modulated by a pair of 6l6's at 40 watts output.

 

You should be able to get more than 10W. By way of encouragement, the plate modulated transmitters T-217 and T-282 use a pair of class C 4CX150's at 850VDC to make 100W carrier at 225-430Mhz.

G1=-115V,
G2=180V to 300V maximum,
Eb=860V.

Modulation B+ is also 860V and is by a pair of 811 and the other a pair of 4-65 for modulators. The mod transformer is about 8KCT to 3K (pair of 4X150's).

Manuals are here.
http://www.bunkerofdoom.com/lit/mil_uhf_01/index.html

My HB 6146 x 807s used one 730v ct  265ma TV xfmr with an economy supply.  Worked FB, xfmr ran warm to hot after being on for hours.  The xmtr grew larger over time and I now have a second big 730v ct 300ma xfmr TV xfmr for the 650V DC supply and the original supply just handles the 300V DC.  The second big TV xfmr is also used to bring up the 813 x 811s supply from 1200V DC to either 1500V DC or 1800V DC depending on which output from the FWB rectifiers I feed into the CT of the HV plate xfmr.

Very complicated xmtr with a lot of relays and switching.

For your small scale project I like either of the two you mentioned.  You should be able to get by with the one R-118A xfmr,  current rating is a little light but for short duty cycle it may not get too hot.

Fred

Not in screen modulated service if you want clean audio.
Plate modulation is fine at any voltage, but with screen modulation, you only get 100% modulation with 700 volts on the plate at up to about 10 watts, above that, positive modulation drops off a lot.
At 1300 volts and up (more is better) it works great and you can get huge peaks at higher voltages.

Yes, I could plate modulate a 4x150 at 500 to 700 volts on the plate, say 600 volts at 200 ma for 120 watts in and 90 watts out would work fine, but with a pair of 6l6 modulators its pointless and needs air on the tube.

I could likely run it at 600 volts and 100ma and it would be ok and need hardly any air.
Not much advantage over a 6146 though.