The AM Forum

ok, so i've been floating around a few ideas in my mind for some kind of transmitter that would be an improvement over what i have now, something that was purpose built (by me) for AM and that could actually handle more that 3 minutes of continuous transmission. i've been looking at solid state designs, but really didn't like many of them, so i went back to looking at tube designs. i've been looking at the 4CX300A, a neat little ceramic tetrode, capable of 160 watts output at 1000v 200ma. does anyone have any schematics or anything that use this tube as a class c amp on 160 and 75 meters. and does anyone know of any other tubes with similar specs for power, like at least 150 watts output at 1000 volts? 4X150A comes close, at 140 watts out with 1000v 200ma. 4X250B comes out the same as the 4CX300A, 160 out with 200 watts (1000v 200ma) in. i'd prefer to have a single tube, but i could deal with two tubes of any type, as long as the input power for the PA stage doesn't exceed 200 watts, i recently got a 100 watt audio amp that i want to use in a modified heising setup to modulate the amp, so the input can't be over 200 watts if i want to use that amp. an 813 running 1250v 150ma would also work, gives me 140 watts out. looking at triodes, an 811A at 1250v 140ma puts me at 135w and isn't over my 200w input limit. or using more than one tube, a pair of 6146B's gives me something like 124w at 600v 140 ma, or 3 807's at 600v 100ma gives me about 126w, but that's getting a little low in output. going a little higher in voltage, a 4-65A will give me 145w out at 1500v 120ma. any other ideas, remeber, the total input power for the tube or group of tubes has to be 200 watts or less.

The 4CX300 uses that funky and scarce socket. The 4CX150/250/350 family can use a cheap Russian ceramic socket. All need pressurized chassis and blowers.

A 4D32 can run 120W out at only 600V, use 2 and let them loaf. They also use a cheap and common socket. You can also use a pair of 5894's with all sections in parallel at the same voltages, same socket. Makes PS and RF components cheap and small.

The 814 is a sleeper and dirt cheap even on fleabay, especially the VT-154 version, socket the same as 807. Push the HV a bit if you want 160W out, it doesnt bother it or loaf a pair at a lower voltage.

Carl
 

OTOH build up a 4-400 socket and heater supply and you can run 4-125, 4-250 or 4-400 but it takes a bit more plate voltage. Modulator can be a pair of 813s

I'm kind of partial to the 4X150A. For some reason, that tube worked better and put out more power than any 4CX250B I tried in my old 144 MHz amplifier.
http://www.w0btu.com/files/misc/W0BTU_2_meter_linear/

Using two vertically-stacked 8-element Quagis, I worked a lot of DX between 144.00 and 144.25 with that amp from my old Toledo, Ohio QTH.

ok, i forgot about 4D32's, and as far as 4CX300 sockets, i've found plenty of them on ebay, some of them even with the tubes, usually for around $150 though (but that is tube and socket, when just the tube can be had for something in the area of $50). i'd like to run a heising setup like this: http://amwindow.org/tech/htm/ssmod.htm (http://amwindow.org/tech/htm/ssmod.htm)
http://amwindow.org/tech/htm/obmod.htm (http://amwindow.org/tech/htm/obmod.htm)
would that work at the power level i want (around 150 watts)? that's the reasoning behind the 200 watt limit, my amp i have is good for 100 watts, and in order for that setup to work, the audio power has to be at least one half the plate input, whci gives me a maximum of 200 watts input to work with. 814 sounds like a good tube. 5894 looks good too, but running 2 tubes with all 4 plates tied together puts it over 200 watts in, however, using one with both plates together at 450v gives about 135w in, which would let me get over 100% positive peaks modulating it the way i want to. and as far as the 4X150A, i'm leaning toward that, but i want to know if anyone has used one on 160 or 75 meters before, it does fit in my criteria though.

At that power level, and given that your major interest is 160 and 80, I would stick with simple common tubes.  A trio of 6146, 807, or 1625 tubes would be fine for 200 watts input class C.  Sockets are cheap, no cooling air is needed, and a 600-700 volt power supply is likely to be a bit cheaper to build.  The Valiant/Valiant II uses three 6146 tubes in parallel for its 200 watt AM input rating and the tubes live a very long time in those transmitters.

I have the original Hallicrafters HT-33 that uses a pair of 4CX300 tubes and they are nice tubes but I wouldn't design anything around a tube with expensive sockets and a somewhat fragile grid structure.

For a single tube, a 811A should work and you could always plug in a 572B (and increase the plate voltage) if you want to increase power in the future.  

ok, so does anyone have some good schematics to start from that would work for any of those tubes, and how to hook my audio amp to use as the modulator. what kind of transformer do i need for that, it would have to be lo-Z on the amp side, but hi-Z on the side of the mod reactor.what impedancedoes the hi-Z side have to be though, or doesn't it matter too much? here's how i think i'm gonna hook it up.
http://www.hamelectronics.com/k1deu/pages/ham/transmitters/am/pages/using_audio_reactor.htm (http://www.hamelectronics.com/k1deu/pages/ham/transmitters/am/pages/using_audio_reactor.htm)
but mine will be with a regular soild state audio amp driving a transformer. i have a 1625 in a box in the shed, so i get another and i'll be good to go. i have a schematic for a single 811A modulated by a pair, so i could start there for some ideas if i go that route, just need to take a look at it and see what i need to change to get what i want. a trio of 6146B's at 475v gives me 178w in, 126w out, a pair at 600v gives about the same. if i go that route, i'd take more tubes at lower voltage, that way they will last longer.

Do you already have a modulation reactor?  They aren't so easy to find! 

I noticed in an earlier post you were concerned about going over 200 watts by using 2 tubes (i.e. 5894) with all 4 plates in parallel.  Keep in mind those ratings you see are maximum so just because it can run more than 200 watts input doesn't mean you must (or should).  Many of the tube combinations mentioned have CCS (continuous commercial service) ratings well below 200 watts.

Have you considered buying a parts transmitter (with good modulation transformer and plate transformers) and using that to supply the key parts for your first purpose built transmitter using a conventional tube modulator and modulation transformer?  I believe this would provide a simpler path than trying to set up a heising modulated transmitter as your first major AM construction project. 

i've been think about that too, i think if i try anything it will be something like a trio of 6146s modded by a pair of 807s or something. or i could use the 811 schematic i already have, running 1250v will give me 135w out. i think that will be simpler. just need to find the iron (hv and mod) that will work for that. here's the schematic i was looking at that used 811's. only thing different is it was designed for 75/40m, i want 160/75.
http://schmidling.com/811_am.png (http://schmidling.com/811_am.png)

there was a johnson viking II on qth 100 bucks !

i jus reread what was on amwindow about modified heising, it says there that you can use a power supply filter choke, or a string of them series or paralleled up, as long as the total inductance is 30 to 50 henries. doesn't say i need a reactor. so now would that way sound a little more reasonable, since power supply filter chokes should be fairly easy to find.

That will work Shelby, BUT as you add more in series, the more the series resistance will reduce the B+ to the final. Inductors work like resistors in the formula.
Unless you build a big 2700 volt supply and make the tubes sweat a little.

This sounds like the way to go. Noone on the other end is going to be able to tell the difference between 100 and 140w, it's less than 3db so it wont even show up on their guess meter.

A Vike II for a C-note will end up saving you a lot of time and money. Besides they are a workhorse, pretty hard to kill, and use all easy to get (and cheap) tubes.

Just my $.02...................

fred,
if i can find a single choke that big, that would be perfect. but, if i put a few in parallel, would that increases the amount of current they can pass? i can find some with high enough inductance, i just can't find high enough current, i'd need something like 400ma capability.

what is the best/right amount of inductance to use in this circuit, amwindow's article says 30 to 50 henries, somewhere else i think had 20 to 50H.

and i'm looking at just building an amp, driving it with my icom, then using my audio amp to modulate it. i think i might use either an 811 or two, a 4D32, or some 6146s or 807s.

I wouldn't parallel chokes unless they are identical in this situation (or at least be prepared to make some calculations followed by experimentation).

I really think trying to build a heising modulated transmitter in order to use your existing solid state amp is going to be more complex and expensive than building (or buying) a more conventional rig.

If you do the solid state thing - go modified Heising. A backwards audio transformer of size, an oil cap and a big choke. This is a very forgiving setup with 50W class solid state amps like an old Bogen PA.

To test the theory, use a big 6.3V filament transformer like a 15 Amp job (backwards) and a 20 Hy choke and a 4 uF cap and run 400 Hz into it and see how it modulates a pair of 6146's or an 814. You can add diode negative cycle loading and can even wrap some feedback around if you can find a spot on the amp that you can inject negative feedback into.

Mike WU2D

that's exactly what i wanted to do. with the transfomer, you would feed the amp into the secondary, then have one side of the primary hooked to the cap, and the cap is hooked to the tube side of the choke. now for the opposite side of the transformer primary, what does it get hooked to? i've saw multiple setups: hardwired to ground, cap to ground, resistor paralleled to the primary then grounded, and resistor paralleled to the primary then connected to the power supply side of the choke. which is best? i also saw somewhere to float the choke and not ground it, because any voltage breakdown would take place between the windings and the core or case, so by floating it the only stress on the insulation comes from the audio voltage at the output of the modulator.

Putting two chokes in parallel reduces the total inductance the same way as two resistors in parallel.

You need to select a tube that will run on higher voltage and less plate current to get the desired power output.  You can then find some common PS chokes and put them in series to get more inductance for the Heising set-up.

The easiest tube to use is a 813 running with 1000-1200 volts and about 150-180ma plate and screen current.  You can find 200ma chokes with some higher inductance.  The older Thordarsons usually were 12Hy with at least 5000 volt RMS test voltage.  Even two 12-15hy chokes will work.

Fred

i like the idea of the 813, and running at 1000v, that definitely isn't stressing it out too much. and the old 1953 handbook i have has a couple of single 813 transmitters in it that i could get some ideas from.

An 813 running at 1000 volts will outlive all of us.  BTW, I've had my 1953 handbook since about 1953 and I still refer to it.  The book was given to me by a ham.

You can HB a rig.  It's a lot of work, but you'll learn a lot about building and xmtrs along the way.
Stay on the project and any questions you have, just let us know.  There are plenty of folks on this forum with the answers.

Good luck
Fred

i completely forgot about this schematic: http://amwindow.org/tech/htm/813/813.htm (http://amwindow.org/tech/htm/813/813.htm)
now, is there someone out there that could pick the pieces out of here i need? i'm just gonna be running a single 813, and i have the modulator section already worked out in my mind, so all i need would be how the input and output would be hooked to a single tube.

does the 4 uf cap have to be oil, or does electrolytic work? oil might be easier to find, especially one that handles HV.

2-4uf OIL cap, at least 2-3 times your plate voltage.

The schematic is for a pair of 813s.  You can use most of what K1JJ (Tom) has in the rig.  Grid circuit would be about the same.  You can use a larger grid resistor and not use the grid bias supply.  You don't need the screen supply.  You can use a screen dropping resistor from the B+ end of the large plate choke (RFC).  The relays in the filament CT are for turning the rig. on for transmit and into standby on receive.  You don't have to do it that way.  You can step-start your HV supply on and off instead.

You don't need 1/4" copper tubing for the tank coil.  You can wind a coil with 14-12ga copper wire.

If you don't use the grid bias supply, you should add a clamp tube (6Y6) on the screen resistor.

There are other things that can be done differently from what K1JJ did with his rig.  The 5 turn input link coil on the grid coil is usually put on the cold end of the grid coil instead of the grid end.  Not sure why JJ did it that way.  You don't need the swapping resistor on the grid.

Some of this will give you a start in your thinking and planing.

Fred

you can build up a cap by series connecting a string of 10 or so  450v electrolitics from mouser or some such.  Start with BIG caps.  10 100uF 450v in series will give you one 4500v 10uf cap when put together, Run at a lower voltage and they will be very reliable.

Even at a couple of bucks a piece, 10 or so are a lot cheaper than most oil filled HV caps I've seen around.

Hit rewind.

The issue was that the OP has a limited power audio amp that he wants to use for modulation? Did I get that right??

These days, if ur going to use a solid state amp with a reversed transformer for modulation, no reason to limit urself due to amp power. There are a ton of ultra powerful solid state amps out there at rather low prices. So no reason to limit oneself due to a specific amp that is on hand when ur going to do a homebrew transmitter that will take a lot of time and effort and possibly $$ to build. Imo.

                        _-_-bear

ok how about this don't know this help or not but I can't beleave your passing up the viker 2 but here is a little amp only do bout 125 watt AM for a 813 i like 2 give you more if you need it always build bigger push pull amps are better then just one tube but that's all up to you!

http://www.raibeam.com/zl1axb/813_amp.html


                                                                                                                                 

i have a 100w amp, so i just didnt want anyone to sugegest a tube or combination of tubes that put the plate input over 200w, which is the power level i'm looking at, between 120 and 180 watts. thats why i think i'll go with the single 813.

but why you want 20 to 80 watts more there is no gain there! now your telline me your radio is putting out 100 watts so why you going for 180 watts! now i want to know you building an rf amp or a audio amp push pull 811A or want more 572B's more 813's!

i have a 100w AUDIO amp, that i want to use to modulate a 120 to 180w RF amp, that i want to build using a single 813.

He's wanting to roll his own and experiment, which is quite understandable.

Build the RF deck and the P/S on separate chassis. Plan the RF deck out so that later on, you could add a second 813, and also overbuild the P/S from the start so that it can deliver the necessary current for 2 tubes. Makes it easier to upgrade later on.

Whatever you choose to do, take your time and do it right, learn, and have fun.  


73, Phil

You can run a 4-1000A at only 200 watts input, IF you want.  It is fine (within reason) to use a tube with more capability-just not one with less.  For example, the 813 you are considering is rated up to 200 watts input for grid modulated service and twice as much for traditional class C with high level modulation.  So in traditional terms it is a "400 watt tube".

If you have (or someone offers you a good deal) tubes that are capable of more input you are better off with more emission capacity (extended service at your required power level) and more plate dissipation (bad things do happen and that reserve capacity is a positive).

put 1500 volts on per 813 will give same as a single put a variable power supply on it if you want more you'll have it but that is up to you if you don't you will be sorry!