The AM Forum

Season greetings
It's been a long time since I posted here. Many projects and work related things among others.
After finishing a 4 month long ART-13 restoration and power supply build project, I am now setting my sights on a year long project to build a large AM transmitter. My plan is to implement a fully self contained design
in a standard rack package. My criteria is simple. It must kick ass and contain easily replaceable (not rare) tube types.
A pair modulating a pair platform comes to mind but then there are other designs like a single tube modulating a pair or vice versa. I'm not an engineer but I can build anything and make it work, so all of this depends on what makes logical sense. An old buzzard build for the present day. After lusting for the Collins 30K-5 that I cannot afford, I will build something that allows me to be creative and have fun. I seek to read comments from experienced builders with suggestions that are doable and can direct me to circuit designs already drawn.
I would also add that frequency agile DDS injection is very much a priority.
I look forward to your comments and I thank you all in advance.
73 de Billy N6YW

Pair of 813s modulated by a pair?
http://amwindow.org/tech/htm/813/813.htm

All the high power work is taken care of, all you would have to design would be the speech amp and audio driver sections and the RF driver section (for the RF driver, I think a single 807 or 6146 would be plenty, maybe even a little too much, perhaps a 2E26 would be better).

Having said "it must kick ass",  ...

Pair of 4-400s modulated by another pair.

Chris

 

Pair of 3cx3000's modding a single 4cx15000???  ;D

Find the best mod iron you can scratch up and go from there.  I have a version of a pair of 813's mod. By 572b's but am using only old BC610 iron.  If I were starting from scratch with fairly modern tubes, I'd go with the 4-400's as mentioned.  Get the mod tranny, mod reactor and plate iron from a junque RCA BTA or similar.

The 813 design seems like it's a good way to go.
The transformer & tube costs are reasonable and more important, available.
Still, looking at different ideas before I go off collecting the major components.
Trying to keep the cost of the project from getting too crazy but I figure about
$2000 dollars spread out over a year isn't a bad figure.
Is there a cost factor versus performance factor where PP modulation is much better
than single ended modulation? Curious.

A single 4-400 is much better than a pair of 813's especially if its to be bandswitched, cost will be the same or less. A small fan is all that is needed to cool the filament pins and up around the glass with some holes, no expensive socket or chimney. Use the 813's as triode modulators or beam tetrodes with NFB. It can also all be run with one PS if wanted.

Id use a 6146 heavily swamped to broadband and eliminate having to retune it or the 4-400 input.
The VFO can drive a broadband SS output to drive the 6146.

Use a pair of those 6 band tuned input boards available on the Internet and customize for the impedances needed. ARCO trimmers, DIP relays, and toroids can easily handle the RF and makes for more efficient layout and minimize panel clutter. Also eliminates the need for neutralization.

Carl

I took an RCA BTA-1R1 and 're-packaged' it into a standard rack.  RCAs can be had reasonably priced and provide all the major iron. Only things to add are the PA tank components and grid circuitry assuming you want something frequency agile.  I used a Ranger as an exciter.

Took me a bit over 2 years.

Thanks for the replies.
I would like to suspend this discussion in light of the shootings in Conn. this morning.
Thanks,
Billy

How much power in carrier are you looking for?  All of those are great suggestions.

C

I built this:
http://amwindow.org/tech/htm/813/813.htm

It's a solid design and has big BAs!

I drive the modulator with an LM1875 IC as an audio amplifier.  See circuit in the attachment.  Replace the speaker in the schematic with a reverse coupled audio transformer. Audio transformer is a Hammond 1620.  Rail supply is about 40 or 50Vdc if I remember correctly. There's plenty of headroom to tickle the grids of the 813s.  You will need to build a split power supply to power the amp.  The only drawback with this lashup is the audio input is unbalanced. It could potentially pose an RF issue however, I do not have that problem.  I didn't think about this when building.  The amp is well shielded and the audio input is converted from balanced to unbalanced right at the input. The unbalanced to balanced line converter is an MCM 555-8485 line converter.  All is enclosed in the transmitter. I drive the audio amp with a mixer with balanced outputs. 

The trick is to get it all into a tabletop rack of say 36" tall maximum.

Even though I am considering this build, I just got word today about an RCA transmitter that may become mine! This is a real heavy metal box, around a 1000 lbs and there's some pertinent history behind it. This is also a large transmitter, so I might consider downsizing my build project to a more manageable size, like a Globe King 500. At any rate, I am going to build one FO SHO!

805 tubes make good modulators also. A pair would mod a 500w input transmitter and a quad for 1Kw.

A plus for 805 is that old US made ones are fairly easy to come by, but also there are new ones made in China for the audio world.

You just can't beat a pair of 4-1000's modulated by a pair.  That and a good antenna and you will be the channel master for sure.
Joe, W3GMS   

or 3-500Z's by same. They are $150 each from China, easy to use, and will be around for a long time due to all the amps that used them.

If that is not kick-assy enough then go to 800W+ non-glow ceramics, or do something technical like a 3CX3000 modded by a PWM 3CX3000.

Hi - you mentioned a single tube modulator a couple of times.  This is certainly the best _SOUNDING_ option (assuming it is designed and built correctly), but uses unusual circuitry, as compared to a transformer coupled modulator.

I have built a number of single tube modulators that use pulse width modulation - fantastic sound, DC coupled, no mod transformer, very efficient (90 percent for the modulator is typical).

Any other type of single tube modulator (analog-series-modulator) would be far too inefficient to consider, at least at the power level you're looking at.

Sounds like an interesting project for sure.

Regards,

Steve

These are some nice comments guys!
I have been contacted off thread to talk about a couple of things!  ;D
Indeed I have some alternatives to digest here, but what a great way to
delve into the subject. So much to learn, thank you.

By the way, the transmitter I am looking at getting that I mentioned earlier is an RCA BTA-250K, in good shape.
Needs some cosmetic stuff done but it works and has been on the air in the last decade. This will be a great
schooling for me and give me some much needed insight on the "Bigger" AM picture. I have committed to the purchase for now, the rest is logistics and timing. Wish me luck but in the mean time 450TL's being modulated with 450TH's is very much on my mind too... or was it the other way around? I need sleep.
Goodnight, and 73.
N6YW

Why not a Cathode modulated 4-1000.

Good power output, no modulation iron required.

Screen modulated 4-1000 ala Slab Bacons rig good choice.

hi Steve ... I have not been able to find any schematics for a single tube pulse width modulator ... maybe this is a gross conceptual error on my part .... consider me visually impaired ... a leg up would be appreciated ...73 ... John

Hi, here is an article about tube pulse width modulators.  The article presents a very good fundamental look at PWM, how it works and presents complete circuits.  However, if you actually want to build a PWM transmitter, I can help with newer circuitry using more modern (and better) components.

http://www.classeradio.com/pdm_article.html (http://www.classeradio.com/pdm_article.html)

Regards,

Steve

How well would a sweep tube (or several in parallel) work as a PWM modulator? The high peak current and voltage capabilities have me wondering.

Okay guys, let's slow down a bit here...
PW and Solid State ain't gonna fly in proposal, so let's get away from that stuff.
I am old school in the sense that achievable design elements for yours truly can
be obtained. I should take time here to emphasize that I seek a more classic approach
to this design criteria. I want to build a kick ass transmitter with tubes that are
obtainable, reasonable replacement cost and "Less is more" in mind with the design.
I would like to be able to achieve 375 watts carrier and legal limit output audio without
having to strain both the structure of the transmitter and my brain/bank account.
So far, the 813/813 and 4-400 designs sound to me like a good start, but...
Where do we land on that platform where you have no regrets? Yeah, I'm happy with this
and it will do the job nicely? When I think about these 3000 watt tubes and associated
hardware, I see the costs going through the roof! I mean, this is a hobby, not a business!
I think I can build a kickass transmitter for 2000 dollars or less.

A pair of 813's or a single 4-400 certainly looks nice in an old school setup.

But what is the difference between those two options, when they have to be bandswitched?

Martin, PA4WM

They work very well.  I have used the 6DQ5 as a pulse width modulator for a pair of 6146Bs - Worked great!  You couldn't use a 6DQ5 for something like an 813 - too much average DC voltage for the modulator.  I used a 4-400 as a pulse width modulator for an 813 - again, it worked wonderfully.  No forced air needed on the 4-400, either - because the modulator was so efficient.

The discussion about alternative methods of AM is not necessarily trying to convince the original thread creator to use such methods, it is a "side" discussion, so to speak  8)

Regards,

Steve

Steve
It is interesting and worthy of discussion even if I don't have a clue of how pulse width modulation works.
If it does the job of modulating the RF deck without the associated iron and costs, then by all means point us in a direction where this information exists. You have cracked the lid on my pushback Steve.

Might I suggest this:

It is pretty simple, good modulation without solid state or big iron.  All the tubes listed are available easily, and you could even make the final stage out of 810's as specified without breaking the bank.

Plenty of them around, they made a lot of them.

(They don't make 813's any more, either. :P)

To clear it up, the series mod suggestion was only that, something different. I have no tried/working circuits, fiddled with it for a converted 807 TX once, blew up the PWM (555 and some transistors, the 807 mod tube lived), I did not get good results re; mod % with the 555 anyway, and gave up.

I was shown a 5K Collins BC TX and the engineer said it was a series PWM, I think 3CX2500's? - so nutty me I had to try it at home. Never mind - use what others say works!

That sounds like an expensive ouch!
I love challenges. I just took a 68 year old Collins ART-13 and restored it to great working condition. This includes building a kick ass power supply that matches the period styling but with modern components mixed with old school circuit topology. It was an epic journey and might make the pages of ER soon. The thrill of bringing that transmitter on air for the first time was a treat that money cannot buy although I spent quite a lot on the project, Hi! This got me into the idea of building a transmitter from the ground up, one that I could be proud of and never regret the choice of platform and performance. I think it's a good plan to ask other's for opinions because once you start on something like this, there is no turning back, at least for me. I want to make sure I am embarking on the "Right Stuff". I have built plenty of guitar amplifiers to know that planning is essential for optimal results. Nothing is more frustrating than wasting time... you never get that back folks, the meter is still running. You get my drift.
Thanks for all of the kind suggestions so far.