Plate Modulating a DX-40

[W6REF]

Gentlemen,

As a brand-new member of this distinguished group I firstly want to thank all of you for your unselfish assistance to those of us who have re-discovered AM. It's been a while for me...but it's like coming home.

My situation: I have a DX-40 that I want to plate modulate. I have ordered a Hammond 1629SEA transformer that I will drive with a 100-watt PA amp (I know it's overkill but it's what I have). I have disconnected the DX-40's stock speech amplifier and modulator.  The DX-40's 6146 will most likely be loaded up to about 100 ma, which at 650 volts provides about 65 watts input.

I was planning to add a 50K, 25-watt resistor between the 6146 screen grid and the plate side of the modulation transformer, so the screen grid can also be modulated. I now realize that the 1629SEA has a "40% screen tap". How do I configure the circuit to best utilize the screen tap?

As a side note, for Christmas I bought myself an Ameritron AL-811 linear. I'm driving it on AM with my ICOM rice-box (a 751A) with 10 watts max, giving about 100 watts out from the AL-811. Actually I try not to exceed 80 watts out to minimize stress on the 811A's. Reports have been, so far, extremely favorable.

Somehow I think it will be even more fun when I fire up the DX-40 barefoot.

Any thoughts or comments would be welcome.

73,
Bob W6REF

[AB2EZ]

Bob

I have done the exact same thing on a DX-20... and I have also used this method (as have many others) to (externally) modulate a Johnson Ranger and to modulate a number of other transmitters.

With respect to the screen modulation: I suggest the following.... since you are using the exact same reverse-connected output transformer as I am using:

1. Make sure that the r.f. bypassing capacitor that is bypassing the screen to ground is less than or equal to .002 uF (so that you won't bypass higher modulation frequencies to ground)

2. Use an appropriate resistor between the screen and the unmodulated B+ (e.g., the 50,000 ohm resistor you suggested, or perhaps a somewhat lower value) to set the unmodulated screen voltage to the desired value. This resistor will be essentially invisible (i.e., a high impedance) with respect to the modulation (as will be clear from what follows).

3. Connect the full secondary (high impedance side) of the transformer in series between the B+ and the plate r.f. choke. Make sure that the common ("blu") secondary lead is connected to the B+. Make sure that the sum (i.e., in parallel) of all r.f. bypassing capacitors that attach between the modulated B+ line and ground is less than or equal to .002 microFarads... so that you don't roll off the high frequencies on the modulated B+ line. Make sure that the plate DC-blocking capacitor between the plate and the pi-network is less than or equal to .001uF (again, so that you don't roll of the higher audio frequencies on the modulated B+ by providing too low an impedance path to ground for the audio via the pi network and the 50 ohm load associated with the antenna).

4. For DC blocking, you will need two capacitors, each having a value of 1uF, and each having a voltage rating sufficient to handle the B+. Connect one end of one of these capacitors to the 40% tap on the output of the transformer (to block the B+ on the secondary of the transformer). Connect one end of the other capacitor to the screen of the 6146 (to block the DC on the screen). Connect the other ends of these two capacitors to each side of a 10k ohm resistor. Thus the two capacitors and the resistor are in series, (1uF capacitor=>10kohm resistor=>1uF capacitor) running between the screen and the 40% tap on the transformer.

As a result of what you have done, so far, you have fed the 40% tap to the screen through a 10k ohm resistor... while blocking DC at both ends.

Finally, place another 10k ohm resistor between the "screen side" of the existing 10k ohm resistor and ground... i.e., on the side that is closest to the screen. What this does is to form an approximately 2:1 voltage divider, that cuts the modulating voltage applied to the screen to approximately 20% (0.5 x 40%) of the modulating voltage that is applied to the plate. The two DC blocking capacitors keep this voltage divider from disturbing the DC biasing of the screen.

Why go to all of this trouble? Well, by using the 10k resistors, the effective impedance of the signal modulating the screen is 5k ohms. By using a 5k ohm screen modulating source impedance, you "swamp" out the input impedance of the screen... which varies from around 15 k ohms when no modulation is applied... to a significantly lower value when the screen voltage is modulated upward.... and a significantly higher value when the screen voltage is modulated downward. You get much better linearity by forcing the screen voltage to follow the modulating waveform... and using an effective screen modulating source impedance of 5k ohms accomplishes that.

You can adjust the value of one of the 10k ohm resistors to change the ratio of the voltage divider if you need more or less modulation of the screen voltage to get good linearity on positive and/or negative peaks.

How much modulating power is consumed by the use of the 10k ohm / 10k ohm voltage divider? Well, from the perspective of the transformer, you have a 20k ohm load on the 40% tap... which is equivalent to a 125k ohm load on the full secondary. Since the plate load is around 5k ohms, this extra voltage divider only increases the required audio modulation power from the audio amplifier by around 5%.

Best regards
Stu

P.S. You can do the same thing by running a 5:1 voltage divider between the top of the transformer secondary (i.e., the modulated B+) and ground... instead of running the 2:1 voltage divider between the 40% tap and ground. To achieve the same 5k ohm equivalent source resistance for the audio signal driving the screen, you would have to use a 5k ohm resistor and a 20k ohm resistor to make this voltage divider. [Don't forget the two capacitors.] The advantage of this approach is that you don't need a 40% tap. The disadvantage of this approach is that the audio-frequency load placed on the output of the modulation transformer by the voltage divider is 25k ohms with this alternative vs. 125k ohms with the voltage divider taking audio from the 40% tap.

[W6REF]

Hi Stu,

Thanks so much for your detailed response. I have redrawn my schematic to reflect your suggestions, and have a couple of quick questions....

The 1uf blocking caps...should the voltage rating be high enough to accommodate 2X the B+ value? I saw some .95uf oil-filled caps at a surplus store yesterday with a 2KV rating...or is that overkill?

The 10K resistors...what would be a safe recommended power rating? (I suppose I could do the math, but it's still early for me on a Sunday here on the left coast!)

BTW, the reason I chose 50K for the screen resistor is because lower values pin the plate current meter when tuning off resonance...50K seems to limit the current to the meter max of 150ma.

Thanks,

Bob

[AB2EZ]

Bob

With respect to the blocking capacitors:

The issue relates back to the question of what can happen at low frequencies of modulation where the blocking capacitors represent a significant impedance compared to the 10k ohm resistors.

Down to 30 Hz, the impedance of each 1uF blocking capacitor is less than 5000 ohms... so the audio frequency voltage between the 40% tap and ground will be mostly across the 10,000 ohm series resistors that are connected to the 40% tap (by way of one of the capacitors). The peak audio frequency voltage on the 40% tap should be around 40% of the peak audio frequency voltage being applied to the plate... which, in turn, should be approximately equal to the B+. The DC on the 40% tap will equal the B+. For all of those reasons, I think that a capacitor whose rating is equal to somewhat more than the B+ is about right.

I wouldn't go overboard on the capacitors. You can easily find brand new, non-polarized capacitors, suitable for audio applications, with 400 volt ratings... or electrolytic capacitors with 450 volt ratings. These are physically small and inexpensive. For example, you could place a pair of 4.7 uF 450 volt capacitors in series, with a 330,000 ohm 2 watt balancing resistor across each one; and you will have a 2.35 uF capacitor that can handle roughly 900 volts. Meanwhile, the 330k ohm balancing resistors will also serve as bleeder resistors. [Keep in mind that modern capacitors can hold their charge for a long time. I've been "bitten" by a 1uF capacitor, with a few hundred volts across it, 10 minutes after I removed it from a circuit and placed it on a table... and it definitely got my attention]. The resulting 660,000 ohm DC path around each of these "DC blocking" capacitors will not disturb the biasing of the screen.

With respect to the sizing of the 10k ohm resistors in the voltage divider:

The maximum power dissipated by each of the 10k ohm resistors is v**2/10,000, where v is the maximum rms voltage across the resistor. The rms voltage across each resistor in the 2:1 voltage divider will be roughly 20% of the rms plate modulating voltage. If the peak plate modulating voltage is 600 volts, then the rms voltage across each of the resistors will be (for a 100%-modulating sine wave audio signal) .7 x 120 volts ~ 84 volts. Thus the power (at 100% modulation with a sine wave modulating signal applied... which is pretty much the reasonable worst case from the perspective of the heat that the resistor must dissipate) dissipated by each resistor will be less than 0.75 watts. I would suggest using 1 watt resistors or larger.

With respect to the screen biasing resistor:

I won't try to comment on the effect you saw... but I would pick the screen biasing resistor to produce the
recommended screen bias voltage for this application. According the the 6146A specification sheet: for plate-modulated Class C operation, and for a plate voltage of 600 volts, the screen bias voltage should be 150 volts. I would select the screen biasing resistor to produce somewhere around 150 volts of screen bias voltage when the plate current is adjusted (via the loading) to be around 120 mA.

Best regards
Stu

[W6REF]

Hi Stu,

Thanks again for your input. Not only did you answer my questions, but you gave me priceless education along the way. This board is fortunate to have your expertise and generosity.

I will let you know the results when I receive delivery of the Hammond transformer and have the opportunity to implement your suggestions.

Thank you.

73,

Bob - W6REF

[AB2EZ]

Correction on capacitor polarity:

If the capacitors are polarized, note that the point where the two capacitors come together (on either side of a 10k ohm resistor) will be at DC ground potential, because of the 2nd 10k ohm resistor that goes from there to ground.

Therefore, the capacitor that is connected to the 40% tap should have its + side on the 40% tap
The capacitor connected to the screen should have its + side on the screen.

Best regards
Stu

[Steve - WB3HUZ]

Just to offer a different point of view, there is no need to plate modulate the DX-40 if you plan to use it to drive an amplifier. You can easily modify it to straight up screen modulation. It will produce as good or better sounding audio than plate modulation. I've heard numerous DX-60s done up this way.

[N3DRB The Derb]

Ditto on what Steve said. Don't plate modulate it if yer gonna drive a amp with it. Set it up for screen grid instead.

One of the best transmitters to use for this is a Johnson Viking Challenger. they are cheap, already screen grid so you dont have to convert anything, 100 watts out in cw, about 15 to 20 carrier out in AM. With the speech amp modded, you cannot tell it from a plate modded rig. I had one and it was one of the best sounding rigs I ever had. One of the simplest to get right as well. Only drawback is it has sweep tubes so you have to be quick on the tune up. You can get a good one for  under a hundred bucks. 

[W6REF]

Hi Steve,

Thanks for your alternate viewpoint. When you say straight-up screen modulation I assume you are not referring to the controlled-carrier modulation, which is stock on the DX-series. My understanding (and I believe this is a valid generalization) is that plate modulation usually results in cleaner, more robust, 100%-capable modulation than screen methods...but I would really like to know what you have concluded from your own experiments and observations, and how you would personally implement a comparable screen-modulation method.

Actually when I plate-modulate my DX-40 I won't drive the linear....too much drive power. I wouldn't gain much anyway...50 or so watts from the DX-40 versus 100 watts from the linear without exceeding dissipation from the 811A's. However, I was toying with the idea of constructing a 10-watt home-brew plate-modulated rig that would then drive the linear.

But please...as the chorus line of a tune from one of my favorite musicals, "Grease", echoes...."Tell me more, tell me more!...."

Regards,

Bob 

[W6REF]

Hi "The Derb",

I guess your post came on the same time as mine...and thanks to you also for your input. Now the basic question is: If one decides to use a linear with a drive requirement of about 10 watts, is there a general consensus that it doesn't matter whether the driving transmitter is screen modulated or plate modulated?

And the follow-up question would be: If you have two 100-watt transmitters, one transmitter plate-modulated directly, and the other transmitter consisting of a screen-modulated driver and a 100-watt linear.....is it possible that both could have identical quality? In other words, the astute observer could not tell either from listening to his receiver or viewing his oscilloscope which transmitter was which?

Bob

 

[N3DRB The Derb]

I'd have put my Challenger up against any plate modulated rig. You could not tell the difference on the air or on a scope.

The nice thing about it was put it on cw and you got your 100 watts power for tuning the amp. all you then had to do was set the mode switch to am and bang you had your 10 to 15 watt carrier. didn't have to retune anything, TX or amp. I ran mine into a SB 200 for some time. if your power supply can handle more current, drop in 2 572B's for the 811A's in your amp. You'll have 5X the plate dissipation you had and be able to run a little less than double the carrier power you had -

****IF**** your amp's power supply can handle the increased current. I dont know that.

I will say as a blanket statement that controlled carrier transmitters sound like crap on AM without lotsa mods, and there's precious little to be gained over a straight screen grid setup. IMHO every dx 40 or 60 or similar TX should be changed.

[AB2EZ]

I agree with Steve and The Derb.

For a tetrode or a pentode: the screen voltage is what controls the plate current.

Likewise: the plate current at the fundamental r.f. frequency*** x the r.f. voltage (at the fundamental r.f. frequency) across the output tank circuit (not the plate voltage) is what defines the r.f. output power at the fundamental r.f. frequency.

The r.f. voltage across the tank circuit at the fundamental r.f. frequency (which is the plate current at the fundamental r.f. frequency x the impedance looking into the tank circuit at the fundamental r.f. frequency) is what we want to faithfully follow the modulating audio signal.

For a tetrode or a pentode, the r.f. voltage across the tank circuit at the fundamental r.f. frequency is insensitive to the plate voltage... unless the plate voltage drops below the screen voltage, or unless the peak r.f. voltage of the tank circuit tries to exceed the plate voltage.

*** In  Class C operation, the plate current at the fundamental r.f. frequency is 2x the average plate current. That is why the r.f. output load impedance that gives the maximum output power for a Class C stage is [plate voltage / plate current ] / 2

The principal reasons to modulate the plate voltage (while at the same time modulating the screen voltage, of course)... i.e. the principal reasons to use plate modulation instead of screen modulation are:

1. It reduces the average plate dissipation of the tube by keeping the total plate to ground voltage proportional to the peak r.f. voltage being developed across the output tank circuit... thereby reducing the excess, non-rf voltage from the plate to cathode.

[As an aside, if you think about this, modern Class H audio amplifiers are using a similar technique to improve their efficiencies (vs. Class A): lowering the voltage on the drains of the FETs during portions of the audio cycle when a larger drain voltage is not needed]

2. By increasing the plate voltage during the positive half of the modulation cycle, you can run the tube to a much higher r.f. output level... while not exceeding the allowed plate dissipation of the tube.

As Steve and The Derb point out... in cases where you a running the tube at much lower output power than the tube is designed to handle (i.e., no problem with plate dissipation), the principal reasons for modulating the plate voltage [1) and 2) above] are not relevant.

The above is also the reason that I suggest that... even when using plate modulation... we design the circuit  to modulate the screen from a relatively low impedance source... so as to force the screen voltage to faithfully follow the modulation.

Best regards
Stu

[W6REF]

Hi Guys,

So from what I'm reading, I can take my DX-40 and either plate modulate it at full power (50-60 watts), or screen modulate it at greatly reduced power (10 watts) and drive my linear up to 100 watts or so..and end up with essentially the same quality modulated signal in either case. This assumes proper implementation of modulation mods in both cases.

Now my question is: If I wanted to modify my DX-40 so the 6146 is running at greatly reduced power (10 watts) can I simply reduce the plate and screen voltages proportionally to allow the 6146 to loaf along...or are there other considerations?

My next question is....how would Heising modulation (used in the Globe Scouts) compare to the above two scenarios?
My second next question is...have anyone tried the "Modified Heising" modulation scheme that uses a hi-fi amp driving an output transformer driving a 30-50 henry inductor/modulation reactor? I'm talking about the WB3HUZ circuit available on the web.

Comment to The Derb: thanks for the tip on the Viking Challenger. I had heard the stock audio isn't great but improves dramatically with some simple mods. Also, I too had thought about replacing my 811A's with 572B's. The AL-811 uses three 811A's, and reports I've read on the web indicate no problems so far with this tube replacement. I think I will call the Ameritron tech department, though.

Once again, thanks guys. I feel like I'm going back to college with Tubes & Circuits 101.

Bob

[WA5VQM]

I have a DX-40 an was planning to have it my starting point for my foray back into AM.

I'll poke around here and look for the screen modulation mods. Sounds like the way to go.

73, Mark

[W6REF]

Welcome, Mark!

I will still plate modulate my DX-40 because I've ordered the modulation transformer (about $95 from Radio Daze). But if your situation is similar to mine (I have a small linear amp) I would certainly experiment with screen modulation, based on the replies on this thread. I plan to do just that with another rig in the future.

As you've probably read the on posts here....Stu, Steve & The Derb have been extremely knowledgeable & helpful.

Good luck!

Bob

[AB2EZ]

Bob

1. To modulate the DX-40 using screen modulation:

A. Leave the B+ at its normal value

b. Without any modulation applied to the screen, adjust the grid drive and the loading to get full output (i.e., around 40 watts)... just as you would for CW operation.

B. Without changing any loading or tuning settings, reduce the plate current to 1/2 of the plate current you had at full power output (i.e., around 50 mA) by reducing the grid drive to the 6146.

By doing the above, the r.f. voltage across the output tank circuit will be 50% of the B+ when no modulation is applied (i.e., "at carrier"). This allows you to modulate close to 100% upward and downward.

Since, at carrier, the r.f. voltage (and also the r.f. current at the fundamental frequency) across the output tank circuit will be 50% of the value it has at 100% positive swings of the modulation, you will have 25% of the output. I.e., the output power will be ~10 watts at carrier.

Note: the above looks very much like the way you load up a linear amplifier... except, you are running the tube in Class C (or maybe Class B after you lower the grid drive); and you are modulating the screen instead of modulating the grid drive.

2. If you are using the single ended Hammond output transformer, which can easily handle the average plate current, you will get no benefit from using a modified Heising configuration. I've used both a modified Heising configuration and a non-Heising configuration to externally modulate my Ranger with the Hammond transformer that you plan to use. It makes no difference in the performance at frequencies higher than 30 Hz.

I have also compared the use of modified Heising and non Heising in conjunction with the Ranger's existing modulation transformer. The modulation (the output of my off-air monitor) looks more linear, when modulating with a sine wave at frequencies below 100 Hz, when I use the Heising configuration. However, when people listen to my signal on the air... they can't tell the difference.

[See also: my thread on this board regarding the use of a low-cost toroidal ferrite-core filament transformer as a modulation transformer (instead of using the Hammond transformer). Since the toroidal ferrite-core filament transformer cannot handle any unbalanced DC, I had to use a modified Heising configuration in that case]

Stu



 

[Steve - WB3HUZ]

If properly implemented, screen modulation will sound just as good as plate modulation. In fact, it could actually sound better, since it's pretty easy to screen modulate without a transformer which is, generally, the largest source of distortion and frequency response limitation.

Check out these DX-60 mods. They should be applicable (with some minor changes) to the DX-40.

http://www.amwindow.org/tech/htm/dx60aud.htm

http://www.amwindow.org/tech/htm/wc3kmods.htm

http://www.amwindow.org/tech/htm/dx60k4tax/dx60k4tax.htm


For more info on properly modulating the screen of the RF final during plate modulation, check this link.

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


Using an external audio amp, a hi-fi output transformer and a mod reactor works very well and is easy to implement. I've done it with a Viking II and an Eico 720 (single 6146 RF final). You could easily do this and still do the screen modulation mods later. With some simple switching, you could run both. That would make for some interesting tests. Try it out, don't tell anyone, and see if anyone notices the difference.

[W6REF]

Stu and Steve,

Stu - I'll try that on my DX-40 when I get home tonight. I recall that normal grid drive is around 3ma. I have a watt-meter at the output of the DX-40 which should show the results.

Steve - I remember seeing those DX-60 mods. As I recall they were mods to the control-carrier circuit and the speech amp...but they're worth a second look. Thanks for the links. I actually have an Eico 720 I recently bought on E-Bay, but haven't had a chance to fire it up (hopefully not literally) yet. Any sources for the mod reactor?

Thanks

Bob

[WA5VQM]

Welcome, Mark!

I will still plate modulate my DX-40 because I've ordered the modulation transformer (about $95 from Radio Daze). But if your situation is similar to mine (I have a small linear amp) I would certainly experiment with screen modulation, based on the replies on this thread. I plan to do just that with another rig in the future.

As you've probably read the on posts here....Stu, Steve & The Derb have been extremely knowledgeable & helpful.

Good luck!

Bob

Thanks Bob! I wondered about plate modulating the DX-40 also. I can see why now that the project is kicked-off you'd want to finish it. Experimentation is the name of the game.

You're right about the resources here!

73, Mark

[AB2EZ]

Bob and Mark

See my new post about separately plate modulating and screen modulating my DX-20.

In order to screen modulate a DX-40, you will probably need to build a separate, negative grid bias supply for the grid of the 6146... in order to adjust the plate current to half of its full-output-power level. I tried simply reducing the grid drive on my DX-20... but since the grid of the DX-20's output tube is "self-biased", that doesn't work. You will need an adjustable negative voltage supply... approximately  -50 volts => -30 volts... capable of delivering a few mA of current. Because the current is so low, this should be a simple supply to implement.

Stu

[kc2ifr]

Steve,
Not sure where your coming from, if screen modulation is so great, why the hell dont we see screen modulated broad cast transmitters. I know.......I know......there have been a few shortwave transmitters that use screen modulation BUT it was a matter of cost.......................not quality.
Bill

[AB2EZ]

Bill

Plate modulation is more efficient. For a given plate dissipation that a tube can tolerate... you can get much more 100% modulated r.f. power out of the tube if you modulate the plate voltage along with the screen voltage. On the other hand, you will need to provide the plate modulator.

Stu

[kc2ifr]

Stu,
Yup.......that is actually what my point is.........thanks
Bill

[The Slab Bacon]

Time to stir the pot a little    FWIW, I have been running a high powered screen modulated transmitter for years. It is a screen modulated 4-1000a final, loafing along at
"near legal limit" power levels and making fairly good audio to boot. It was my own wacky design when I built it years ago. I wanted a big power transmittah, but couldnt find a big enough chunk of mod iron at a price that I wanted to pay. So necessity wuz the mutha of invention here.

It is all home brew, and my own design. I modulate the screen grid, and the control grid bias as well. the control grid bias is given about 25% the audio modulating voltage, and in phase with the screen. I use no grid leak bias, so the class c control grid biass HAS to follow the audio waveform as well. I am using transformer coupled modulation to both grids. It has no trouble making 100% positive peaks.

Keep in mind that "alternative" methods of modulation were scorned and put down years ago when plate efficiency really mattered. When, in the old days you were limited to 1kw Dc input to the final, who would want to build a screen mod rig at 33% when you could build a plate mod and get upwards near 70%.  330w out vs 700w out .................not a lot of thought process there. Howeva............................ since the invention of the Bird Thruline wattmeter and the rules now being changed to 1500w PEP out poot, it really doesnt matter anymore how one gets here. Plate mod, scream mod, cathode mod, or God forbid; linear AM, it is now a level playing field!! It dont make no matter how you do it as long as you do it!!     Just my $.02 worth

                                                                             The Slab Bacon

[Steve - WB3HUZ]

Steve - I remember seeing those DX-60 mods. As I recall they were mods to the control-carrier circuit and the speech amp...but they're worth a second look. Thanks for the links. I actually have an Eico 720 I recently bought on E-Bay, but haven't had a chance to fire it up (hopefully not literally) yet. Any sources for the mod reactor?

Bob:

The mods all more or less change the controlled-carrier screen modulation to straight-up or fixed carrier level screen modulation.

You can use power supply chokes for your mod reactor. If you need more inductance, string several in series. Also check AES. Rick, W3RSW just bought one from them.