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W4VLJ
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« on: August 03, 2013, 12:18:47 PM »

Hi Everyone,

I'm new to AM, spending most of my time as a ham working on/restoring SSB rigs from the 60's and 70s.  I've got a Viking II right now, but getting the itch to build my own transmitter.  Goal is 1KW AM output.  I've purchased some handbooks from the 50s so getting into the research.  Today's question is:  assuming I can find the right size modulation transformer, are there any other tradeoffs in directly modulating the final tubes as opposed to modulating an exciter at lower power levels?  Hopefully not a dumb question. Any advice is greatly appreciated.

73,
Mike
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K1JJ
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« Reply #1 on: August 03, 2013, 12:30:03 PM »

Hi Mike,

Both methods have their advantages and disadvantages.

In the end, many will say that high level plate modulation or low level linear still pull the same amount of power from the AC plug... and can sound the same in quality.

The high-level solid state class E PDM rigs are another thing to consider for audio quality and even better power efficiency.

My opinion is to have both - a plate modulated tube system and a low level linear system. (and even a class E rig) This way you can play with all kinds of technologies and keep it interesting. Boredom is one of the biggest reasons guys leave the hobby. You want to mix up the rigs to keep the interest up.  Homebrew, modified commercial, high level, low level, etc is the way to fun and learning.

Start with the one you have parts for and go from there.  You will find you will attract AMer friends on the air who are running similar gear. Mixing it up will diversify your friends too.

T
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kb3ouk
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« Reply #2 on: August 03, 2013, 12:55:26 PM »

As far as overall efficiency is concerned, a plate modulated transmitter (RF final and modulator) will draw close to the same amount of power as a linear amplifier. (EDIT: I see Tom already covered this, but I didn't see it since I was writing this up for probably the last half hour while he already replied)

 But, the tube required for a plate modulated final doesn't need to have as high of a plate dissipation rating as a tube for a linear amplifier would. I would go with a plate modulated final. If you can find one, the modulation transformer out of an old kilowatt AM broadcast transmitter would be perfect. As far as tubes go, if you want triodes, then the 833A sounds like a good choice, a single one at 3kv will do 1000 watts with some air blowing around it, or you can run a pair at a reduced voltage and get away with natural cooling. The Gates kilowatt rigs used a pair of 833s modulated by a pair. You could also go with a pair of 3-500Zs, a pair at 3kv will do 1200 watts. The nice thing with the 3-500Z is since it is a zero-bias tube, when you run it as a class C amplifier at plate voltages of 3kv or less, you can get away with running grid leak bias only with no need for a protective bias supply. For tetrodes, you could go with go with a single 4-1000A, at 3000 volts it will do over 1300 watts. A pair of 4-400s will do 1000 watts at 2500 volts. There were a few broadcast transmitters that used a pair of those modulated by a pair too. If you went the low level stage driving a linear final route, then you would need a tube or pair of tubes that have a plate dissipation of at least 2000 watts. A pair of 4-1000s running 5kv on the plates will do 1000 watts. A single 3CX3000 will do 1500 watts as a linear with 4kv on the plates.
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flintstone mop
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« Reply #3 on: August 03, 2013, 08:57:12 PM »

Just a small word of caution that we are only allowed 375 of RF power for AM operation. We (AM guys) got stuck a few years ago on this and we have to play the P.E.P. game. So we are all on an even playing field with the SSB folks to have "LEGAL LIMIT" 1500 Watts P.E.P.......It was 'unfair' advantage for AM operators to have 1kw of RF output, allowing us to have 4KW P.E.P. (Peak Envelop Power)
A linear is a nice way to go, if you do not want to go through the expense of heavy iron. Modulation transformer, mod reactor, modulator tubes and circuitry, etc etc.
The linear should be designed for an easy P.E.P. rating of 2500 watts. Plenty of headroom for nice big audio. Only need BIG power supply and a big ceramic tube. OR a couple of BIG glass tubes to get you to this P.E.P. goal.
An amplifier making 1500W P.E.P. will be ok too. But large TX audio will not make it through, without flat topping and distortion. The BIG audio is positive peaks in the waveform. 100% negative is the max for any AM operation...no limit to positive peaks. Seems like 130% is about it, before it is distortion in the receive end.
You have taken on a nice HB project, if you have a junque box.
Fred
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kb3ouk
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« Reply #4 on: August 03, 2013, 10:59:40 PM »

Just a small word of caution that we are only allowed 375 of RF power for AM operation. We (AM guys) got stuck a few years ago on this and we have to play the P.E.P. game. So we are all on an even playing field with the SSB folks to have "LEGAL LIMIT" 1500 Watts P.E.P.......It was 'unfair' advantage for AM operators to have 1kw of RF output, allowing us to have 4KW P.E.P. (Peak Envelop Power)

And you are completely wrong. There is no rule that says AM'ers shall not run over 375 watts of carrier. You can legally run 1000 watts if you want, just reduce the modulation down to about 60% positive. If you want to run more modulation, reduce you carrier power. 600 watts at 80% modulation will still do pretty well. You can make up for the apparent lack of audio power with a decent audio chain. Doing a little bit of processing to your audio would help make up for the lower modulation. If you want my honest opinion, you will do better off running less than 100% modulation than running over 100%. Keep in mind that WWV only runs 75% modulation during voice announcements, and I have no problem copying those here even during periods of high band noise. having more carrier will help quiet down the background noise on the receiving end, which will help make up for the lower modulation.
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kb3ouk
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« Reply #5 on: August 03, 2013, 11:50:48 PM »

Ok, I discovered a little bit of a flaw in the program that I wrote that I was using to calculate PEP values for an AM transmission. I did some more accurate math and came up with this. I ran the numbers for a single 833A running 3kv 415mA at 80% efficency (1000 watts output) and modulated at 75% and 60%. 60% modulation on that final gives you about 2020 watts PEP, while 75% would give you about 2330 watts PEP. The same tube running at 2.5kv 335mA 75% efficiency (635 watts out) and modulated to 80% will give you about 1560 watts PEP. The math I used to figure that out was pretty simple. I'll use the 1000 watts carrier final modulated to 60% to demonstrate.

3000 volts / 415 mA = 7230 ohms
3000 volts * .415 mA = 1245 watts input
(60%2 * 1245 watts input) / 2 = 225 watts of audio required for 60% modulation

Then I used Ohm's Law to figure out the voltage and current that 225 watts of audio is developing across a 7230 ohm load. This works out to be 1275 volts at 175 mA. Next add that voltage and current to the plate voltage and current that is provided by the power supply.

3000 volts + 1275 volts = 4275 volts
335 mA + 175 mA =590 mA
4275 volts * 590 mA = 2525 watts * 80% efficiency = 2020 watts PEP
3000 volts * 415 mA = 1245 watts * 80% efficiency = 1000 watts carrier

NOTE: I rounded all of my numbers up to the nearest 5 or 10.
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kb3ouk
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« Reply #6 on: August 04, 2013, 12:33:29 AM »

Disregard all of the above power values, the math is correct, however, I forgot to take in consideration that the audio voltage and audio currents that I was finding were for the effective AC value, not the peak AC value, so in order to get the peak AC value, you divide the found voltage and current by .707, then the PEP comes out accurately. So with the 833A running at 635 watts of carrier, you would only be able to run a maximum of 55% modulation. Running the same tube at 2kv 335 mA 75% efficiency at 75% modulation would give you 500 watts of carrier at 1500 watts PEP.
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« Reply #7 on: August 04, 2013, 01:11:37 AM »

A Doherty amplifier is probably the best type of linear amp for a low-level modulated stage.  Overall efficiency can exceed 60%.

It takes but two tubes, triodes usually.

The circuit came out in the mid-thirties, and was used extensively in broadcast TX for many years.

+ peaks are easily possible, and there is no critical tuning if built for a single band.

73DG
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« Reply #8 on: August 04, 2013, 07:11:30 AM »

Welcome to AM transmitting!!  Cheesy

The answer to your question depends on what you really want in a transmitter.

If you don't mind low efficiency, and you have some big tubes lying around, a linear amplifier if certainly fairly easy to do.  If the transmitter driving the linear is good, the amplified signal should be equally as good.  However, the efficiency of a linear on AM will be around 30%, meaning for your 1kW carrier, you will be running a little over 3kW power input, and creating about 2kW in heat, plus the heat generated by the tube filament(s).  Lots of people do high power AM this way.  It is very popular.  You could build a solid state linear as well, and if you only want 75 and 160 meters, it won't cost you a whole lot of money to do it.

If the wasted power bothers you, in the tube world, modulating the plate is best.  If you're pretty good technically, you could use a pulse width modulator (still a plate modulator) and eliminate the mod transformer all together.  I have done this and it works very well.  The solid state (class E) world affords even better efficiency and generally better audio performance.  There is a lot of prior art out there, and plenty of schematics, pictures and components available if you want to go this route.

Another thing to consider:  If you want "ok" to "pretty good" audio, plate modulation is generally not too hard to pull off if you can find a modulation transformer.  There is nothing wrong with this, and most AMers with whom I talk use transmitters that fall into one of these categories, audio-wise.  

If you want "really good" to "excellent" audio, and you want to use plate modulation with tubes, it can be lot of work.  The audio driver design becomes super-important (and should be direct coupled) and the quality of the mod transformer matters a lot.  High quality mod transformers are hard to find and can be quite costly (not to mention, very heavy!!).  You will probably need a modulation reactor as well.  "Really good" to "excellent" audio is easier to achieve using solid state (I.E. class E) because the nature of the technology naturally lends itself to excellent audio.

Anyway, there is not a "right" answer.  The right answer is completely personal, and it depends on what you want, how much you know, and what you're willing to put in to get it  Cool

Sounds like the start of an adventure to me!

Regards, Steve
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« Reply #9 on: August 04, 2013, 12:04:48 PM »

By now you have probably figured out the answer depends on your priorities.

Some have been looking at high level plate modulated rigs for years and know many of the design rules.

That's something many have to try.

Low level AM followed by a linear amplifier is the most flexible as far as multibanding.

Other schemes sport low light bill usage for the power out.

Then there's the boatanchor experience.

So you really have to ask yourself what the station will be expected to do performance wise.

If you have a junker ssb xcvr that works it only takes a few steps to make a FB AM transmitter.

No matter which way you go there are experienced people available to help here.

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W4VLJ
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« Reply #10 on: August 05, 2013, 08:13:32 AM »

Thanks everyone for the great info.  I'll stew on this for a while and will probably have a follow up later.  Thanks again - Mike
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W3GMS
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« Reply #11 on: August 05, 2013, 08:38:22 AM »

Hi Everyone,

I'm new to AM, spending most of my time as a ham working on/restoring SSB rigs from the 60's and 70s.  I've got a Viking II right now, but getting the itch to build my own transmitter.  Goal is 1KW AM output.  I've purchased some handbooks from the 50s so getting into the research.  Today's question is:  assuming I can find the right size modulation transformer, are there any other tradeoffs in directly modulating the final tubes as opposed to modulating an exciter at lower power levels?  Hopefully not a dumb question. Any advice is greatly appreciated.

73,
Mike
W4VLJ

Hi  Mike,
Great on your goal and welcome to the world of AM home brewing.

You may keep your eye out for a broadcast rig.  In some cases they can be had for free.  If you choose not to keep the BC style of construction, its a great source of parts to build your first big rig for AM.  You will have most of the parts including the mod iron and plate iron and tubes. 

My preference would be to build a high level class C rig plate modulated rig but as pointed out there are many good ways to go.

Have fun with your project!

Joe, W3GMS   
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W4VLJ
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« Reply #12 on: August 11, 2013, 02:53:24 PM »

Thanks again for all the info.  I would like to request help on this net with my project, so I am starting a new thread entitled AMFONE transmitter project.  There are no Elmers I have found in this area with knowledge of building their own transmitters, so I thought it would be fun to keep you all updated on my progress.

Thanks,

Mike
W4VLJ
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W3GMS
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« Reply #13 on: August 11, 2013, 04:45:13 PM »

Thanks again for all the info.  I would like to request help on this net with my project, so I am starting a new thread entitled AMFONE transmitter project.  There are no Elmers I have found in this area with knowledge of building their own transmitters, so I thought it would be fun to keep you all updated on my progress.

Thanks,

Mike
W4VLJ

Hi Mike,
That sounds like a fun project.  Kind of like "Blog Cabin" on the DIY channel! 

Be prepared for many different options and often times they will conflict since many design choices can be made.  So in the end, your going to have to digest the material and make the decision on what way you want to go with your new rig.  There are many ways to accomplish what you want to do.  Folks generally have a strong opinion on what they feel is the best way.  Listen to them all and then you can make the decision for your own rig. 

As others have said your tube lineup is well proven so in the end you should have a great rig.

The most important part is to have fun and I am sure you will learn a lot in the process. 

Joe, W3GMS   
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« Reply #14 on: August 12, 2013, 12:08:18 AM »

Just a small word of caution that we are only allowed 375 of RF power for AM operation. We (AM guys) got stuck a few years ago on this and we have to play the P.E.P. game. So we are all on an even playing field with the SSB folks to have "LEGAL LIMIT" 1500 Watts P.E.P.......It was 'unfair' advantage for AM operators to have 1kw of RF output, allowing us to have 4KW P.E.P. (Peak Envelop Power)

And you are completely wrong. There is no rule that says AM'ers shall not run over 375 watts of carrier. You can legally run 1000 watts if you want, just reduce the modulation down to about 60% positive. If you want to run more modulation, reduce you carrier power. 600 watts at 80% modulation will still do pretty well. You can make up for the apparent lack of audio power with a decent audio chain. Doing a little bit of processing to your audio would help make up for the lower modulation. If you want my honest opinion, you will do better off running less than 100% modulation than running over 100%. Keep in mind that WWV only runs 75% modulation during voice announcements, and I have no problem copying those here even during periods of high band noise. having more carrier will help quiet down the background noise on the receiving end, which will help make up for the lower modulation.

'tis true. I run a 500W carrier and less than 100% positive modulation and stay within 1500W peak power. It's just the way the TX works most efficiently with the new rules, it was designed to run with the old 1KW input rules. No one complains, sounds great. never fear large carriers.
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