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Author Topic: xmitter: grid modulation V.S. plate modulation.  (Read 2312 times)
KC9KEP
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« on: November 26, 2007, 10:46:44 AM »

Hello Elmers!

Would someone care to briefly elaborate on the pros & cons
of plate modulation V.S. grid modulation?

It seems to me that in my studies of the 40’s thru the
60’s of the ARRL handbooks that plate modulation
is a popular modulation method for phone transmission.

Although plate-modulation seems very simple to
understand, my initial impression is that grid modulation
would be a better modulation method.

After all, the whole beauty of an amplifier is the ability
to control a high current or voltage by the application
of a relatively low power control signal, applied to
a grid (or base).

What sayest thou?

Thank you!

--Tom Nickel KC9KEP
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N2EY
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« Reply #1 on: November 26, 2007, 11:25:22 AM »

Plate modulation was popular because it is efficient. The various forms of grid modulation (including grid, screen, suppressor, cathode, and clamp-tube modulation) were popular because they are simple.

With plate modulation, the RF carrier output power can be as high as 70-75% of the DC input to the modulated stage. But you need a modulator capable of audio power equal to one half the DC input of the modulated stage.

With grid modulation, the RF carrier output power can only be abaout 30-33% of the DC input to the modulated stage. But you only need a modulator capable of a few watts of audio power.

For example, say you want an AM transmitter with about 300 watts carrier output.

With plate modulation, you need about 400 watts input to the modulated stage to get 300 watts of carrier, and about 200 watts of audio to modulate it. The final tubes need to be able to dissipate about 100 watts on their plates.

A typical setup would be a pair of 811As in Class C modulated by a pair of 811As in Class B, and a 1500 volt, 500 mA supply to run all of it.

With grid modulation, you need about 900 watts input to the modulated stage to get 300 watts of carrier, and a couple of watts of audio to modulate it. The final tube(s) need to be able to dissipate about 600 watts on their plates. That calls for much bigger tubes in the final, and a bigger power supply. But you don't need a big modulator.

If you try to run a pair of 811As grid-modulated, you'll only be able to run about 200 watts total input, and get about 65 watts carrier output from them.

Read those old Handbooks in detail. It's all in there, in the chapter on amplitude modulation.

73 de Jim, N2EY

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W5RKL
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« Reply #2 on: November 26, 2007, 12:08:45 PM »


Tom,

What do you mean by "grid modulation" method of
modulating a transmitter?

73's
Mike
W5RKL
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AC5UP
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Posts: 3834




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« Reply #3 on: November 26, 2007, 12:11:09 PM »

Interesting coincidence... I'm currently considering building a Part 15 AM transmitter-ette to tickle some of my BCB boat ankers with some appropriate tunes from http://www.hylit.com

Been scrounging the web for design ideas. Since I have more tube vintage parts in the junkbox than Oz has Munchkins, hollow state is probably the way I'll go.

Check this out: http://groups.msn.com/GospelRadio/cathodemodulationbycunningham.msnw

Dude claims to have whipped cathode modulation into submission and offers * Super Modulation * up to 125% on peaks. I guess I'll have to take that claim on faith...

For the purposes of this thread I think this link offers the better advice: http://www.montagar.com/~patj/cathmod01.htm

Note that cathode modulation was chosen, but not for technical merit or efficiency... The lack of a proper modulation transformer was the deciding factor. Since I'll be working at much lower power levels I'm thinking plate modulation is in my future. If it was good enough for The Orlons back when, it's good enough for me today.

As for screen or grid modulation, Heath did a bunch of that in the 60's and (if I recall) their smaller rigs were not well regarded for audio quality. The DX-100 and Johnson Viking were, and both are plate modulated. Click here

http://bama.edebris.com/manuals/heath/

for a peek at how Heathkit did low level modulation in the DX-35, DX-40 and DX-60. Transformers were expensive parts when bought new. Still are today, but through the miracle of junque...............
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AC5E
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« Reply #4 on: November 26, 2007, 12:28:52 PM »

That question brought back some memories, by no means all good. At the time I thought many of the 30's and 40's modulation schemes were derived from some of the screwy mixing schemes out of early superhet receivers.

Tubes make wonderful mixers, with plenty of both mixing products and distortion in their output. I have heard some cathode modulated transmitters that earned their builders a "pink slip" because they would not pass for even amateur work. I built a couple of screen modulated transmitters that did all right but would never be mistaken for BC quality.

For that, you pretty much needed to do it the way the big boys did it. Plate modulation. And that's the way I would do it if I were doing it again.

73  Pete Allen  AC5E
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K6AER
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« Reply #5 on: November 26, 2007, 12:32:31 PM »

I have a slightly different take on modulation.  

Plate modulation was done for the audio power combined with the plate modulation to produce very nice 100% modulation. It was expansive to produce a 200 watt audio modulator for a 200 watt carrier, 400 watt PEP modulated transmitter.

Grid modulation was cheep to produce but your carrier had to be fixed at about 25-30 percent of maximum carrier for linear modulation.  Although you could get close to 100% modulation is was never as good as plate modulation. Also there is phase modulation where two exciters are modulated 90 degrees out of phase from each other to produce a combined 100% modulated signal. Most very high power AM station transmitters used this method.

Modulation can only go to 100%. This is where the signal is shut off in the valleys between the peaks.  At above this point you have shut off distortion.
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AC5UP
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« Reply #6 on: November 26, 2007, 12:55:08 PM »

You want a ton of modulation? Check this out: http://hawkins.pair.com/wlw.shtml

About mid page on the right you'll see a pic of one of the twin modulation transformers WLW used when they were running 50 Gallons Plus 10 dB on 700 kHz. Yeah... Half a megawatt.

Rumor has it that at night in downtown Cincinnati whenever they played something with a strong bass line the streetlights would pulsate slightly in time with the beat. Note the outdoor heatsink for the PA cleverly disguised as a pond...

That's what I call serious modulatin'!
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KA5IPF
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« Reply #7 on: November 26, 2007, 06:25:28 PM »

Careful, you might make SOME of the Kalifornia Kilowatts jealous, notice I said some.

Very interesting site.

Clif
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W8JI
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« Reply #8 on: November 26, 2007, 07:22:02 PM »

Here's how to think about AM. Let's assume 100% sinewave modulation.

The envelope voltage doubles in amplitude on positive peaks, and goes to zero on negative peaks of audio. The envelope power of the modulation crest has to be 4x carrier because the envelope voltage is double. Remember P=E^2/R

Now that envelope power comes from something. It either comes from reserve in the PA tube, or power supplied by an external power source like a  modulator.  

Now what the PA has to do is follow the voltage envelope of the audio with a square law power function without any distortion.

In a purely grid modulated stage, even a linear amplifier with modulated input signal, the plate supply voltage cannot change. Peak current will double on positive peaks and go to zero on negative peaks, but anode voltage cannot change. This means to get four times the carrier envelope power on positive peaks efficiency has to change. It has to double. This means the PA has to be tuned so the carrier efficiency is HALF the peak envelope efficiency. So if we have a PA that can do 70% efficiency on peaks, it can only have 35% or less on carrier. The PA gets really hot because most of the applied power is just making heat.

It is totally untrue that the grid modulated PA is less linear or has more distortion. That's false.  It's also untrue the AM "sounds better" from a plate modulated stage. That's false. As a matter of fact it is much easier to get low distortion from grid modulation of a tetrode or pentode than it is anode modulation of the same stage!! Some of the cleanest transmitters have a diode modulator followed by linear amps, or are simply grid modulated.

Now here's another fact. It is impossible to 100% plate modulate a tetrode or anything but a very hard switched triode without applying some grid modulation.

The distortion is terrible if there isn't some efficiency (or grid) modulation of the stage when it uses a tetrode or pentode. Very low distortion requires a correctly designed compensation circuit to make the output envelope follow the audio in a linear manner.

Of course anyone can do something wrong and build a crummy grid modulated stage, just like they can build a poor plate modulated stage, but the bottom line is except for the wasted power as heat it is much easier to build a good frequency response very low distortion grid modulated stage than it is a plate modulated stage.

The only easy purely plate modulated stage would be a very low mu grid driven triode or something equal that switches very hard on and off with the carrier. Anything else requires modulating more than one point, or some form of distortion compensation.

73 Tom
 
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W8JI
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« Reply #9 on: November 26, 2007, 07:29:53 PM »

By the way, the modulator power to 100% modulate a fully plate modlated stage is HALF the carrier power for a sine wave.

In other words a 50 watt average power output modulator will 100% modulate a 100 watt input power plate modulated transmitter.

Flaws in the design can increase the power requirement, and a combination of grid and plate can require less power. But it is about 50% of the plate input power for plate modulation.

73 Tom
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