need ideas for a low power rig.

[K1JJ]

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

[N2DTS]

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.

[K1JJ]

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

[N2DTS]

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.

[Opcom]

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.

[VE3LYX]

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   

[KD6VXI]

What sort of tube makes a good series modulator?

Sweep tubes.   Gobs of cathode emission.

--Shane
KD6VXI

[steve_qix]

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.

[Steve - K4HX]

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).

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

[VE3LYX]

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

[K1JJ]

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

[N2DTS]

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:

[N2DTS]

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

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   

[Opcom]

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.

[Opcom]

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:

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!

[KA2DZT]

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

[N2DTS]

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!

[N2DTS]

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.

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

[N2DTS]

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.

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

[w8khk]

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!

[VE3LYX]

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  

[N2DTS]

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!

 

[N2DTS]

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...

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!

[w8khk]

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.

[N2DTS]

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).