The AM Forum

I have been reading a lot of old threads on ART-13 usage. However I still have a few questions.

I have an ART-13 that I plan to use in a 8xx modulated by a pair of 811's. Jack K9ACT used a 810 for the RF tube with good luck. I have not yet decided which one I will use but leaning that way. 812 or 813 is another option. I want to end up with around 150-200W of carrier and just one RF tube.

Here is Jack's schematic (showing a 811 - adding the 810 is straightforward by adding a bias of around -120V for cutoff and a 10V fillament).

see attachment

Anyway, I have a plate transformer that can give me around a 1300V supply being designed. I was using it the way Jack suggested with a choke and a 4uf oil filled cap, however he was using a much larger mod transformer S-22 and no reactor choke. So it was suggested by quite a few here that I use a shunt feed modulator to allow me to use the ART-13 here. As I read the article by Genaille, he moves the reactor from the supply up to being connected to the modulator along with a 4uf oil filled cap, right?

If I start out using just one supply to feed both the modulator pair and the RF tube, will I still get good power supply regulation for the 811's? Should I be planning to keep a 8HY choke in the PS and add a second one for the reactor? I would kind of like to get this running on just one supply for now, but may then add a separate supply to the RF later to get higher B+ and a bit more power.

Also, how do I compute the total amount of power this supply must deliver for the 3 tubes?

John

I know it’s none of my business and hope I don’t offend but what are you going to gain by flogging a seventy year old transmitter just to get another fifty or seventy five watts? What’s the limit of how hard you can hit the modulation transformer in the ART-13 before it fails? Or are you planning on building an external modulator with a bigger transformer? At what point would building a new modulator PA deck be just be as easy?
Don’t get me wrong, it’s your stuff and you can do whatever you want, I just look at this like putting a big block Chevy in a Ford model A and when you add a bunch of horse power on one end often things on the other end tend to fail. Then again a big block Rat Rod can be cool.

I'll add a bit to what Ray has said.

I've heard stories of the ART-13 modulation transformers being badly abused and surviving, so I suspect it's probably a strong point in the transmitter. That doesn't mean all other components are, however. Keep in mind that, while built for reliability with some level of a safety margin (military gear tends to be overbuilt for what is expected of it), it was also built to a specific requirement. Wander much beyond that and you could be asking for problems.

For comparison, I'm currently working to resurrect a 1941-contract Meissner 150-B military transmitter. Configuration? 813 modulated by a pair of 811s. Manual states 150 watts out of the box, but of course the 813 is capable of much more (ask K1JJ). When these transmitters hit the surplus market after the war, of course they were snapped up. And of course, they were pushed for more output.

Not a lot of info out there, but I have read several accounts of users trying to get 200-250 watts or more out of them and succeeding - briefly. The problem not being the tubes, but the other components used. Seems today users are content to get 175 watts or so out of them and play it safe.

So you may be fine with the 813 and a bit of tweaking, testing, and so on. But also as Ray says, it's your transmitter. Experiment away. ART-13s are anything but rare, many parts are still available. No one is likely to miss it or cry if you blow it up, except you. ;)

You've got the right approach - ask a lot of questions and move forward slowly. Chances are you can make it sing somewhere in the 150-200w range without a lot of trouble.

You are not offending me, however, I need you to explain your comments further. This is my first effort into AM tube transmitters and the concepts are fuzzy. I currently run a homebuilt SDR with 50W peak on AM and virtually no one can hear me. I thought it might be fun to get into some of this older technology and learn something. I could go buy a big linear and be done but there is no fun in that. I like to build stuff...


John

Even when run stock at nominal 1100 VDC on the 813s and 811s, the  ART-13 stock mod tranny has a significant amount of distortion below about 400 Hz. Fine for milcom audio, but don't expect anything like broadcast quality from it , especially if you try to push more power through it.

Jack got a respectable 125W out this simple transmiter using an 811 for the RF tube. Maybe since I am using a weaker mod transformer, I should drop back to a more conservative tube pair in the modulator perhaps 807's or something similar. Or perhaps chalk this ART-13 purchase (NOS $75 bucks) a beginner's mistake and put it back on the 'bay and go looking for a better modulator.

another question - I am going to use 1N5408 diodes in strings for the bridge. Should I get rid of the choke based power supply and the oil filled cap and use a capacitor input supply like W8JI suggests in this article. My plate transformer is a 1kv CT model? I am more comfortable with this sort of supply.

https://www.w8ji.com/designing_ham_transmitter.htm

I could use the split supply version and go looking for a modulator circuit that runs at around 700 volts.

Back to the supply, when I build the bridge, do I use ceramic standoffs or can they be built on normal terminal strips. Never built KV level supplies before so a bit afraid of doing it wrong. I want the layout to be SAFE!

For the diode strings, will 2 diodes per leg be enough (2KV at 1A rating) or should I go higher?

John

Or, stay on the 811 mod design, and use the art-13 until I find a sweet S-22 or something similar.

Absolutely, John - you've come to the right place. As stated, no offense intended at all. I always ask a lot of questions because I don't know a fraction of what many of the guys on here know, and I hate to re-invent the wheel. I have a family and home, so my radio time is limited and precious.

Basically you want to make sure all the other components associated with the 813 or whatever tube are up to the increase in power. A simple analogy might be a car with a large engine but tiny, restrictive exhaust system. You can do a lot with an engine, but if it can't 'breathe' well, it won't reach its potential. Using a less restrictive exhaust system (larger diameter pipes, etc) helps to realize more power out.

So you want to make sure you're not arcing over in your tuning network, that chokes, transformers, caps and other components are up to the task. They're probably okay for some increase just as they sit. Clearly if Jack has swapped in the 810 for 813 and not had to change out everything downstream, at least some stock components will be fine.

I'm not well-versed in the ART-13, so I won't try to offer specifics. Others are bound to jump in with more suggestions.

But you're right - 50 watts carrier is okay for local, daytime stuff on 40 & 80m most days but at night? Good luck. Even 100 watts is difficult. If you've really been bitten by the AM bug you'll likely move beyond the ART-13 to something a bit more substantial before long. Many of us have several transmitters for different bands/conditions, from 100 watts to 'legal limit', whatever that means.

To be more clear, Jack started with an 811 and did fine at around 100-125 watts. This was his backup transmitter. He had a larger one that was 8000 modulated by 813. As an experiment, he swapped the 811 RF tube to a 810. He added a bias supply and a 1700v supply. This design was a bit evolutionary for a number of causes outlined on his radio page on his sight.

Sadly he recently has had a shack fire and all of his data has been lost. I have been communicating with him but it has been 11 years since much if this was built and not all of the "data" is crisp in the mind. This just makes it more fun to me. I am not looking for a cookbook. I want to understand the concepts and theory as well.

So, with all of your help I think I will stay on a 811 modulator and a 811 RF tube all on one supply. Perhaps I can ratchet back the modulator power a bit or still plan to use the reactor choke until I locate a better modulator. Hamfest season is upon us here with 2 in the next few weeks. I am getting enough parts from the 'bay to get started but am getting tired of all of the shipping costs to get them here to the shack.

John

Try to find one of the old ARRL handbooks from the late sixties or seventies being they have lots of useful information on designing, building and troubleshooting high voltage vacuum tube circuits. They also have all the safety stuff too, not like the garbage in the handbook today. In all the handbooks from those decades there is at least one project using an 813 to build an amplifier or transmitter.  I would think if you want to do the tube high power thing that’s a good place to start, but that’s just me.

Ray is hip, he knows what cooks.

This is a very good site.
                                  http://www.tubebooks.org/technical_books_online.htm

I particular, check this one out.
                                             http://www.tubebooks.org/Books/orr_radio.pdf

Its the right thing to do.

klc

Man, what a treasure trove of classic radio and electronic engineering references....

Thanks for the links!

Brings back memories of my having the copy of the '64 ARRL Handbook out on semi-perpetual loan from my high school library, and many squandered study halls there reading all of the other ARRL pubs that were part of the reference section and could not be checked out for loan, while listening to Frank Zappa and the Mothers of Invention on the multichannel music distribution system you could use with headphones there.

Don't apologize for your miss spent youth.

https://www.youtube.com/watch?v=1MewcnFl_6Y

klc

I would really like to find some sort of do's and don'ts for wiring practices on high voltage supplies. Things like how to do safe interconnect, how to route wires safely, how to feed HV thru the chassis, etc. For instance, I want to fire up a 2.5kv meter to watch the voltage. I suspect it is very bad practice to run the 1300 Volts voltage up to the meter in the open, and do the voltage divider there. So I am thinking of using a voltage divider under chassis, and then sending up 25V full scale and using a local dropping resistor on the meter that is calibrated for 200uA full scale at 25V. Things like that.

Already read one article about using HV solid core wire from Napa.

Also, what is the best practice for how to feed signals between the mod deck and the RF deck. Should the PS be on it's own deck. Stuff like that.

Perhaps I can find that in the treasure trove of documents listed above.

KLC - I am doing this for the children...and the cats. No unintentionally fried critters allowed...hihi. Especially me!

John

I used many times RG58 for HV upto 5 kV  It is flexible and it is safe, an insulation leak gives a short to ground and blows the fuse.  Inside the cabinet you can use without shielding. Much more readily available than real HV cable

Frank Zappa ... Dynamo Hum ... Must have been thinking about the Art-13 generator?

Watch this
http://slideplayer.com/slide/5731822/

Then  read/download these

http://www.ifwtech.co.uk/g3sek/in-prac/0901.pdf
http://www.ifwtech.co.uk/g3sek/in-prac/0902.pdf
http://www.ifwtech.co.uk/g3sek/in-prac/0903.pdf

As I always say when working around HV, Boom pa-chicka ca-boom boom boom chicka.

This should get you started.

klc

 I have been watching this evolving topic. Quite interesting!

My 2 cents to add are that a hi mu triode like an 811 is not a great choice to be run class C plate modulated. Low to mid range Mu tubes seem to modulate better and to higher levels. I once tried a pair of 572B's in push pull with cross neutralizing. I could easily run 1KW DC input (2500v @ 400ma) and get somewhere shy of 700 watts RF out. The tubes ran pretty hot, and even when adding additional grid bias (lower conduction angle) for a given grid current, the benefit was little. My theory is you want a tube that will peg the plate current meter with zero bias. That rules out the 811 or 572B with a Mu of 160.

I later changed sockets and filament transformer, and tried some 805's. Those ran so much better, but are only rated for somewhere around 1500v. I eventually got brave (dumb perhaps), and raised the B+ in increments. Ended up running them at 2500V, 400ma, and about 850 out. The plates did not show any red. Used a quad of 808's in push pull parallel for modulators. Fair Radio Sales used to sell 808's for 75 cents apiece! Same thing with them...rated at 1500v again. I used 2250v, and that modulator made an easy 600 watts. I remember load testing it for hours....But that was almost 35 years ago. Memory is not perfect.

It is very fun to go these types of projects.

Jim
Wd5JKO

I read all the posts,  I'm missing something, do you have an ART-13 or are going to build a xmtr from scratch??

The very first thing you need is a proper HV power supply.  That xfmr you keep mentioning is not going to do the job.  You need a HV xfmr that can produce 1200V-1500V DC and at least 300 ma.  A supply like that can power a 813 modulated by 811a's.  The 1N5408 diodes are fine but you need at least six diodes in each leg.  Only use a choke input filter, 8-10 hry and 20ufd cap at a minimum.  I always use a 470K resistor and a .01ufd 1KV ceramic cap across each diode.  Most folks will tell you that you don't need them but I always use them.  You can build the rectifier circuit using terminals strips BUT mount the strips on plastic above the metal chassis.

Another thing, stay away from UTC S series chokes. They're ok but probably the poorest of all the vintage chokes.  The UTC CG series chokes are much better, the earlier UTC PA series chokes are very good if you can find one.  Stancor or Thordarson chokes are good.  Thordarson made many different series chokes over the years.  The earlier ones are better than the later ones (the 21 series was the last and the poorest).  Stancor made pretty much the same chokes through out their history but the earlier ones had a lower HV voltage rating than the later ones that were potted even though they had the same part numbers.

You need to find a local ham that may have a lot of the correct parts you need and can part with them at a reasonable price.

Keep us informed of your progress

Fred

First thing, get the right power supply parts then work forward.

Learn from an OLD HV technician. Bad HV technicians don't grow old..... ::)

Yes, that is what I want to do - build a complete transmitter. I have purchased a modulator that was used in the ART-13. I do not have an ART-13 itself. Many have told me that this modulator is not a very good choice. I will be on the search for a better one but may use this one with a shunt reactor choke if I get that far by the time I run into a better modulator deal.

Yes, I want to start by building the power supply. I have a 1kw CT plate transformer to work with, as well as a 6.3v at 10A filament transformer for the mod pair. I was hoping I could get around 1300V at 250-300ma from it. There are no markings on it that tell me how much current it can supply. The ham I bought it from said it came out of a old transmitter. This same ham is helping me with some other parts as well.

Thank you so much for the specific information on searching for parts. This can be quite daunting to try to read decades of information and glean specifications from schematics that have only values listed. You have been quite helpful, Fred.

What I want to do is lay out the PS and then put a static load on it and see if the transformer can produce enough power without getting hot. I would use a variac to bring up the mains power slowly as I test it. Does that sound like a plan?

John

Jim Wd5JKO,

Thanks for the tube info. Very helpful. In the progression of this build, I plan to start the PS, then build the modulator. The RF tube is still a open issue, in my opinion. I guess I am struggling with "what works" vs. "what works well"...hihi

Do you happen to have a example schematic of a single 813 rf deck? I understand how the simple 811 triode works in the circuit but not clear on how the extra grids are wired on the 813 and how many different voltages are required. Also want to understand neutralization techniques. I have some air-gap neutralization caps on the way.

John

Power Supply Designer says this transformer can do 230ma at 1150 volts using a FW bridge and a 300uf cap, no choke, with a 5K ohm load. It calculated this from the RMS voltage and the resistance of the primary/secondary. I selected 5K assuming 1250v at 250ma. If I add the choke and the 20uf cap with same 5k load, I get 850V at 170ma. Not good.

Go looking for a new transformer? If so, what should I be looking for. PSUD says 1.5kv at .5A give me 1300VDC at 280ma using the LC circuit.

Why is the choke method better? Seems a lot more inefficient. What am I getting for this level of work - better regulation?

John

So, let's come at this from another direction.

If I use a capacitor only PS and get a reliable 1150vdc at 280 ma - what can I build? Using an 812 tube I estimate that it can put out around 120W of carrier at that voltage reading the data sheet. That gives me (according to W8JI's excellent information repository) a need for around 60W of full sine wave output from the modulator. If I assume that the 812 uses 130ma, I have 150ma or so of power for the modulator. Can I get it to use a pair of 807's? This would also be a good match for my ART-13 mod transformer rated at 50W.

Does this make sense? Recall that my initial goal was to build a transmitter between 100 and 200 watts with a lower HV supply. I did not want to go whole hog on a pair of 813's modulated by a pair of 811/13's. If I go shopping for a bigger transformer, then I am definitely heading away from my initial goals.

The next questions is whether a 120W transmitter is worth the effort. I am guessing that is a judgement that I will have to make. So far I am into this for around $150.

Maybe Borgi's K9YQQ recommendation to me of starting with a DX-100 or Valiant is not a bad suggestion at all!

John

Playing with PSU Designer, tried a voltage doubler circuit. It shows around 1840Vdc at 360ma. Can this be used? See attached...

Two things, old ARRL handbook on how and what to build if your going to build and start building with what you got, although there is a lot to be said for starting with something that’s already together. Building a high powered AM transmitter from scratch is no easy task. I have been doing this stuff forever both as a hobby and professionally for all my life and I would not attempt it. Way easier to find and adapt.

Learning starts with experience, ask ten hams how to do something and you will get twenty five different ideas. It’s a wise idea to ask direction but beyond a point too many cooks ruin the soup.
HV, high current or AC lines won’t hurt you if you treat them with respect and it sounds like you got the right idea. The old Handbooks have the method, its time to put it into practice. If you don’t have everything you need start with what you got.
Software simulations are no substitute for solder. If you want to play with power supplies invest in some big 100 watt 100 K resistors and remember the one hand rule.

If this keeps up I will start my rant on analog VOM being far superior to the digital LCD next.

You may need to have that rant. I just looked and my digital vom only goes to 1kV. Wonder if an old trusty Simpson 260 can do any better...hihi

I played with adding the choke and got similar results. I think I will go build this and see what I can get.

Sorry about the series of back to back posts here. Kind of a stream of consciousness thread as I was exploring things on a rainy Saturday morning.

John

PS - wish I had paid more attention to this stuff back in the day when I was getting my EE degree. Back then, digital and micro-programming was the rage and that is the direction I went. Spent 35 years in operating system development and let my analog side get rusty. This is bringing a lot of it back.

To get a good AM xmtr you need a solid HV supply.  You are not going to make it with that xfmr you have.  Forget the voltage doubler and the cap input filter.  You can only get so much VA from the xfmr.  When you use a cap input filter you must reduce the current load on the xfmr by about 30%.  You can build a nice xmtr with a supply at around 1000V-1200V and a current load of around 300 ma.

A single 813 modulated by 811a's is easy to build.  You'll get 100-120 watts out of it.  Both the PA and the modulator can run from the one supply.  You can use the ART-13 mod xfmr.  That mod xfmr can handle a lot more than 50 watts.

We can help you through every step of the build, but first find a better HV xfmr.

Also probably better to but away the power supply computer program.  I've been building power supplies for more than 50 years never needed a computer.

What other xfmrs, if any, do you have on hand??  I have a small collection of over 1000 xfmrs and chokes here.

Fred

Way back when I was in the US Army I acquired a ART-13 through MARS. I built a PS for it and had a lot of fun with it. One thing that was very noticeable was the talk back from the mod transformer. I was using a carbon mic, I think it was a T-17. T-17 mics come in two forms, one has only about 5 holes drilled in the cover over the microphone for high noise environments such as in a B-17 or a B-29. The other is exactly the same but has about a dozen holes drilled in the screw on cover. Perhaps this was for ground environments, to let more sound in? I guess if you have one of the 5 hole covers you could drill more holes in it for a better response?  Well anyhow attached is a recording of W2JBL's ART-13 mod transformer that is in his ARC-5 mobile rig. I know that his finals tubes are 1625's and I assume he is using the same tubes for his modulator since it is a 12 volt system?  1625's are the 12 volt equivalent of an 807 for those who don't know that. Chris has been using ART-13 mod transformers in quite a few home brew and modified ARC series transmitters and he produces excellent audio from them. If I were going to build a transmitter with said transformer, I would use the shunt form combination to keep DC off the secondary windings. I think I read somewhere that is you use this modified form with the heising modulation choke, you can get about three time the audio power through it, but as stated above, you may get talk back? If you do get talk back, the cure may be to add more inductance? It worked for me ;D ;D  Yes they are great transformers but you can't run 100 KW through it :D :D :D      

Yes, yer 260 is OK. Its still being made and sold, so I guess its a solid design........

But......

   I got a GriefKit VTVM at a hamfest for $10.00. I have the rocket ship HV probe, so its a nice set up for the HV measuring follies. You cans till find these things at hamfests, cheep.

FWIW, I use a orange colored adapter for working on equipment.  This cuts down on the "Is it unplugged?". One should still check to see that the caps are discharged, and 'it' is not on, before poking yer fingers in it.  Take off the watch, ring if you made that mistake, so you don't have to try welding yer finger to that fil. supply.

 Looks like this,

I have always thought that if the transformer has a center tap, it should be used as such.  If you have a transformer that is 1000VCT on the secondary, then it will give you somewhat less than 500VDC with a choke input filter. If you want to get, say, 1500vdc, then you either need a center tap transformer rated at about 3100VCT and use a choke input filter, or get a transformer rated at 1500VAC without center tap, and use a bridge rectifier and the filter design of your choice. The bleeder resistor will affect the output voltage as well. I like to use large variacs on the primary leads of my plate transformers. That way I can adjust the output dc voltage over a very large range. Unless you have access to a large stash of vintage parts, building a 1500V@300mA power supply is likely to be a fairly expensive endeavor. Transformers, chokes, and HV oil filter caps are getting expensive. Unfortunately, there aren’t many shortcuts, at least ones that are safe.

Caveat: I have no formal training in electronic theory. These are my observations of 20 some years of building transmitters with some modest success, reading, experimenting, and on occasion, cheating death.  :o Be careful with high voltage equipment!

I have gravitated to using triodes for all my finals and modulators. This simplifies circuit design quite a bit. For the voltages that you have mentioned, I would recommend using 812A’s, 5514’s or 8005’s as finals. Of these, only the 812A’s are easy to find. There are many other tubes that you could use as well. The price that you pay for using triodes is the need for more driving power. I find this easier to deal with than adding a screen supply.

Good luck, Ron W8ACR

Fred,

I have no other HV transformers here. What do I need to purchase? Can I buy one from you?

John

email on qrz if you prefer to take this discussion off-line.

Should I look at ICAS or CCS ratings. A Hammond 724 will to 339ma ICAS at 1500v, but only 226ma CCS. Price is not outrageous for a new transformer.

John

A hundred watts carrier will do well on local circuits.

25 watts, and your in QSB hell, if mother nature isn't completely in your favor.

HOWEVER!!!

It's a lot easier to find a legal limit amp that will absorb 125 watts pep vs one that will take in a hundred watts carrier.

If you are looking at a hundred watts carrier, the next LOGICAL progression is a 3cx3000A7.

Otherwise, build the single 6146 and drive that into the legal limit amp of your choice.

At the end of the day, total watts drawn from the electric company ends up being about the same....  High level legal limit vs low level with amplification.


--Shane
KD6VXI

I use the ccs rating as my carrier power level.  If your into 10 minute old buzzard xmissions, that's what you need.

If you follow that rule, the ICAS ratings tend to fall in line with the peak envelope power made at 100 to 125 pct modulation, give or take.

--Shane
KD6VXI

What is the DC resistance of your xfmr's secondary winding.  Measure the whole winding,  let me know.

Fred

88.7 ohms

Fred,

Are these the specs we are going to follow with the 813 RF tube? Will the PS have to also provide 300V and -160 volts (or close to that) given that we will be at 1100-1150 instead of 1250. Just trying to follow along...

John

John,

The screen (G2) voltage just comes from the plate HV supply.  The screen is modulated from the modulated plate voltage through the screen dropping resistor (very simple).  Look at the specs, it tells you what the screen resistor should be.  Anything close to that 27K will work.  Wattage has to be calculated but probably 50-100 watt resistor.

The grid (G1) voltage is developed from the grid drive power through the grid resistor.  Look at the specs, it tells you what value grid resistor to use.
The grid voltage can also be developed from a combination of (-) fixed bias voltage and a grid resistor.  The value of that resistor (using some fixed bias) can be calculated.  I know you're a computer expert, so, if you forgot Ohm's law we can help you with that (Hi Hi).

If you don't use any fixed grid bias then you need to include a clamp tube circuit on the PA screen voltage (clamp tube 6Y6). Again, this is also very simple to do.

Fred

I'll add some more HV power supply suggestions later today.

Here is what I have so far for a 813 design. I have pulled quite a bit from the K1JJ 813 x 813 design. Goal is to use heavy build techniques that can allow for a later addition of a second 813 and a separate larger RF power supply. I have not done much on the power supply, Fred. Awaiting your suggestions.

John

John

Can you put up a few pics of the xfmr you have??  Are there any numbers on the xfmr??

Fred

Says TP-189. Here is the eBay item. Not at home right now. Let me know if it doesn't work.

https://www.ebay.com/itm/Vintage-Power-Plate-Transformer-for-Vacuum-Tube-Transmitting-Equipment-1KVct-/173201342202

Don't know what you are thinking Fred, but I was comparing the 2 datasheets for the 811 and 813. We could use this trans for the modulator only at around 750v CCS. The sheet says that is good for around 175 watts. Then we could run the 813 higher, say around 1800 or so to get 250 watts of carrier. Would the ART-13 modulator handle that much modulation?

John

John

I took a good look at the xfmr.  Seller probably has the current load about right (200ma)  I don't recognize the part number but I"ll check some stuff later today.  Trying to see if the xfmr can be used in some capacity for your xmtr project.  I'll explain more later, have to run some errands right now.

Fred

Knowing what I know now, I would have never bought it. I put a minimum bid on it and forgot, and it won. So no biggie if we decide to not use it. The good thing is if we use it, we can then buy a cheaper second supply that does not have to give us 400ma or more. Best new 1500V trans I can find is the Ameritron one that is used in the AL-811. $105 bucks. Diodes are cheap. Caps are not that bad either. Whatever you suggest. I am flexible.

You could always put a want add in the wanted section.........

KLC

John

Looked through over 10 pages of xfmrs on ebay.  Didn't see anything that looked good.

Have to try some other sites

Fred

John,
        If you are looking to achieve an output of roughly 100w, why not just acquire a Viking II or DX-100? You can learn a lot about transmitters bringing a commercially built (or kit built) rig back up to speed while building your parts list for a big homebrew transmitter. IMHO, anything less than a pair of 812's modded by a pair of 811A's would probably be a little bit of a letdown for a "first project". For that first 100 watter, about 100-200 dollars for a complete rig with good bones, change some caps, clean some contacts, etc., learn during the process, and get to some "on air" time fairly quickly. Consider the approach.

Just my 2 cents worth...

Phil

Thanks Phil,

That has been suggested by others and is probably a sound suggestion. Just not my way. I would rather spend more time and money building something of my own hands rather than spending time cleaning dust and grime off of an old xmitter. Just how I choose to spend my time. If you look at my qrz page you will see the SDR that I built. That is my only radio. I glean much satisfaction from a homebuilt unit. I thought about building a solid state linear for it but when you get into all the complexity of the supporting circuitry,  it actually is a bit simpler to build a tube rig. Plus way more fun, IMO.

The AM bug has bitten me. I spend almost no time on slopbucket mode. Did some digital for a while but that seems so automated.

I am building this rig robust enough to allow me to add a second 813 later so this first radio is just a stop on the journey, not an end point. A guy has to keep busy in his retirement, right?

John

Fred,

This transformer is still avail on eham. An old navy transformer. Trying to get some specs for it. Looks like a beast! Probably cost more to ship it than it is worth, however. It's located in Tom's River, NJ WA2JHS, not to far from your QTH.

John

Shipping may be a issue but this is a good source of parts.

https://www.ebay.com/itm/E-F-Johnson-Viking-II-transmitter-chasis-PARTS-ONLY/232716440668?hash=item362efb145c:g:M88AAOSwWyRaovoO

Not a big Viking 2 guy so don’t know if those are the plate transformers or modulation transformers.
Agree with you about amplifiers, in order to get the linearity you need from an amplifier it has to run in class A or something similar and the efficiency sucks. No substitute for good old plat modulated AM, just don’t over think it. Math is nice but you would be surprised how far you can get with just bodging things together.

Any idea what the current and voltage is??  Maybe I'll check eham try to see what it is.

Fred

he just said this in a follow up note...

901414-501....grv 80310.  I remember looking it up but I can’t find it either...used the supply to run a pair of 813’s to 400 watts....somewhere around 300 mils...it didn’t even sweat

said it weights around 20 lbs.

This is looking promising. Maybe I should go directly to a pair of 813's? will the modulator have enough kick to keep up?

update - said it would cost around 30-40 bucks to ship. it is 3200V CT and he us only using 1/2 of it at 1600V.

John

I looked on eham but couldn't find the xfmr.  I saw from the pic that he was using only half of the secondary.  A 20lb xfmr is not a big plate xfmr.

Better to use the whole secondary with a FW rectifier.  With a choke input filter the voltage will come down.  You can use a variac to bring the voltage down more or just run it at 1400-1500 DC volt output.

It may be a good one to get,  depends on what he wants for it.  It won't run a pair of 813s modded by a pair.  Probably will be ok for single 813 by 811as

Depends on what he wants for it.

Fred

Get ahold of Gary w1zm at TRS.

I've bought a couple things from him over the years.  All exceeded expectations.

He was selling off a huge pile of old stock transformers a while back.  Might have JUST what you need.

--Shane
KD6VXI

Fred,

$100 bucks for all that is pictured, plus 30-40 for shipping. I think I will hold off for now. Got 2 hamfests in the next 2 weeks and who knows, I may get lucky.

John

Good idea,  I would wait for the hamfests.  At a $100 it's not a bargain.  Should be able to do a lot better for much less at a hamfest.

one of these will provide 1600VAC up to 0.5A. Use two, one for the modulator and one for a 400W output PA.

http://www.antekinc.com/as-8t800-800va-transformer/

Very interesting!

While I await a few hamfests and the hopeful discovery of the perfect transformer on this Easter egg hunt...Can someone teach me about balance between the modulator and the RF tube?

As I read the datasheets, the power of the RF tube is variable based on what I determine the HV to be. So, let's say that I use that spiffy 1600V transformer and get a solid 1450V of power for the RF deck. The data sheet for the 813 lists data for 1250v and 1600v for CCS of a Plate modulated RF amp. That is great. 1450 splits it down the middle. That leads me to believe that the Rf deck will produce 140W of carrier at this voltage. A pair would go 280W. (BTW, a pair is looking more interesting)

Now, looking at the 811, remembering that I have a marginal mod transformer, at 1450V we are in ICAS territory. And the 2 tube modulator will be producing over 330W of audio. I think I see smoke coming from the modulator! I think the 811's should be designed for CCS operation, right?

I read somewhere that I should design for the modulator to give me 50% of the RF wattage. No more than 100W, and 75W may be better. Is that correct? I am confused... This goes against my understanding when I tune my SDR that can deliver 50W that for AM mode I should set the carrier at 12.5W. That means 75% of the power goes to the sidebands and 25% goes to the carrier. Would that mean that I need to be delivering 400 watts of audio? Huh?

I have this other transformer that can give me 750 volts of power at 200ma. I could use it as a separate supply for the mod deck and get 175 watts out of the modulator. That still seems like too much for the modulation transformer, but that is another discussion.

So, how much audio for carriers of 140W and 280W? using this 1450V example.

Second - we had discussed using a single power supply of around 1100-1200V. Reading the data sheets that shows 120W of carrier and 300W of audio. Is that a good balance?

Help!

John,

You're over thinking everything.  At 1450VDC you'll run the RF and modulators off the same supply.  The 811s will need about (-)4 volts of bias (a few diodes in the 811 filament xfmr CT to ground).  811s might produce more audio than you need.  So, you turn down the audio control to produce no more than 100% modulation.  You always need more audio power than you think.  There are losses that you need to overcome.

Your ART-13 mod transformer was designed for a single 813 and 811s.  While the ART-13 ran on about 1150 volts from the two dyna-motors it used the mod xfmr can handle much higher plate voltages.

Fred

Fred,

What transformer specs should I be looking for? Say 1300V ct to 1600v ct with .5A? Can I do with a bit less current, say .4A or is .5A a solid minimum current spec?

Also, I am going to drive the AF with a audio chain from the small transmitter I am building. After I get it running I will split it and use the RF side for exciting the big transmitter and the audio chain for driving the modulator. Right now the final stage is a single 6V6 running at 300V. It can put out 5 watts. I think I need to redo this stage and make it push-pull. That will give me 14 watts. I will then use a matching transformer to match it to the modulator.

Will 14 watts be enough drive if we are running the mod at 1450V?

John

you need a 2600 vct to 3200 vct (or a 1300 v to 1600 v no ct) transformer to do what you want.

John

Current should be at least 300ma,  400-500ma is better.  I'm driving my 811a modulators with a single 6L6 through a 25watt mod xfmr.  Class B modulators (811a) will draw grid current so you need a step-down driver xfmr.  Hardest part to find is the right driver xfmr.

Plate xfmr should be just what KC2ZFA said.  A xfmr that is only 1300vct to 1600vct will only give you half the voltage you need.  A xfmr like that can be run with a FW bridge rectifier but the current capacity will be half of whatever it's rated for.  In addition not all xfmrs can be run with a FW bridge rectifier.  Some have to have the CT grounded.  Depends on the xfmr.

Another thing is you need to know how to understand the ratings and wording you may see on a xfmr or choke.  Things like RMS TEST voltage, means the  high voltage used to test the unit.  Say you see 5KV RMS TEST voltage.  This dosen't mean the unit is good for 5KV. General rule is you subtract 1KV and then divide by two.  So a choke with a 5KV RMS TEST voltage is good for 2KV.  You may see a voltage rating that says "working".  Say you see a rating that says 2500v working.  That means you can put up to 2500 volts on that unit.  Other times you may see an insulation rating.  No idea what exactly is meant by that wording, but generally you stay way below that insulation rating.

You may see these type ratings on chokes and on xfmr windings.  Most all the standard catalog xfmrs and chokes from the major manufacturers have this info in their catalogs.  Military and special xfmrs and chokes may have more info printed on the unit.  A lot of units you might see may be specials even though you see a familiar manufacturer's name on it.  Usually xfmrs and chokes with 5 digit part numbers are specials and you're not going to find any info about it in any catalog.

Fred

Fred,

What windings are you using on the driver trans. I take the plate voltage divided by the plate current for a 6L6 to get the plate impedance of 3.5K. Is that correct? How do I calculate the input impedance of the mod pair. I don't see input data for the 2 tube circuit. The 813 circuit that K1JJ published takes a 2K winding, as I read in a note.

Thanks for the trans info. That helps.

John

I had a 25watt Merit mod xfmr that has a single PP primary and a secondary that has taps.  Using different taps reflected different impedances back to the PP modulators.  That is how the xfmr is suppose to be used.

I used it in reverse, the pp side I used to drive the 811 grids and used the other side as the plate load for the 6L6.  You need a step-down going to the grids.  Impedances for class B driver xfmrs are calculated from the primary to 1/2 the secondary.  I used all of the tap side for the plate load except for the last end.  So the plate was connected to the tap next to the end of the winding.  I used the end for feed back to the 12AU7 driving the 6L6.  The 6L6 runs with 300 volts on the plate and the screen is tied to the plate through a 100ohm resistor.

I tested this circuit by loading down the grid lines with 130ohm resistors, one on each grid line to ground.  Don't know what the grid impedance is, so I just guesstimated 130ohms. 

I used a audio signal generator and a scope to see how much audio voltage i could get across the 130ohm loads before it started to distort.  The undistorted audio voltage was way above the audio signal needed to drive the 811s to full output.  That grid audio voltage is shown in the tube specs.

When you build from scratch some of the details have to worked out as you go along.

It's nice to do a lot of calculating and figuring but, non of that is going to do you any good if you don't have the right parts to build what you calculated.

You have to build and make work a xmtr with the parts you have or are able to find.

Fred.

John

I checked my notes on the driver xfmr I used in my HB xmtr.  I used a Merit (A-3109) 25watt mod xfmr.  I have 5000ohms on the plate of the single 6L6 driver and 3000ohms CT on the grids of the 811a's.  I used the 10k end of the primary winding for feedback to the cathode of the 12AU7 that drives the 6L6.

It's the turns ratio of the driver xfmr that is important.  The actual impedances don't mean anything as the load that the 811 grids reflects back to the primary is not known and is not a constant load.  The actual load impedance varies with the amount of grid voltage.

To figure the impedance ratio, 5000ohms/1500ohms = 3.34,  the turns ratio is the sqrt of the impedance ratio.  Square root of 3.34 = 1.83

So the turns ratio is 1.83,  turns ratio of Class B driver xfmrs is the primary to 1/2 the secondary.

2:1 turns ratio is about right for 811a's

This will give you some idea what's needed for a driver xfmr.

Fred

That's what I had calculated ... hihi ... just kidding  ::)  Actually I got 3500 on the 6L6 and 2000 on the grids, which after doing the math give a turns ratio of 1.87.

Got a schematic for that circuit? I wrote out a design for my 6L6 audio amp and would like to compare them. I don't have a feedback line. I got my 6L6 circuit from an audio amp and joined it up to my mike preamp/amp. The article described a grid tap of 43% on the audio transformer for ultra-linear mode. I don't know what that means.

Off to the ham fest this morning. Taking my grandson with me. He is a bit young (9mo old) but we will have some fun anyway. At this age he is a bit of a YXL magnet ... hihi

John

An ART-13 can be made to sound as good as a stock commercial ham transmitter (Apache, Ranger etc...)at the 120 Watt out level, with no modifications other than in the speech module, and by adding some feedback. The output transformer is not the limiting part in terms of fidelity, it is more related to the input transformer and the anemic and restricted driver circuit. So get a spare speech module and start playing. There is a dandy article in 73 that talks about running 500 Watts input power with an ART-13 in CW mode. The limiting factor to AM output power are the 811's and the mod transformer, not the 813.

Went off the reservation. No transformers other than a Dahl for $350. Ouch. Did buy a Drake L4B wo PS for $200. I still have a PS to build...

Maybe the next fest you'll find a good xfmr.  When you're at the next fest tell the sellers you're looking for xfmr.  Lot of times the guys don't want to drag out heavy xfmrs but may have something they willing to sell.

There is a Denton clipperton xformer in qrz for 75 bucks right now I believe.

I offered 50, he balked.

--Shane
KD6VXI

Look under the tables and under the cardboard boxes. You never know what lurks there.
 

klc

I saw that Shane. You think that would make a dandy replacement/substitute? I think it has the right voltages for the hi/low voltage doubler circuit but could not determine if it has the same current ratings. I was going to add up the current demands of the 4 572B tubes and see how they compare to the pair of 3-500's in the Drake. I would use a heathkitshop L4 board also.

Interested in your thoughts...

I am also investigating doing the grounded grid and zero bias mod. The bias mod would eliminate the requirement for the 120V line.

John

This was a real small fest. Probably a dozen tables. I asked around and only one guy brought any transformers. That was the Dahl guy. If anyone wants that level of iron, he had the goods. He had that monster 3500v transformer, a matching choke and a 6000v oil filled cap. Makings of a maul for sure. He wanted good money, though. $350 for the transformer, $150 for the cap and I did not ask about the choke. I can pass on his name if anyone is interested. Was way out of my league in cost and scope.

I did dig into every cardboard box I could find. And struck up quite a few conversations with the old iron guys, but no firm leads. Will go to another fest this next weekend. Half of the fun is the hunt.

John

Shane,

Dahl lists replacements for both amps and give specs. The Dentron direct replacement is S1) 0-640-1010 VAC @ 600 MA CCS. The direct replacement for the Drake is S1) 1065/780 VAC @ 0.81A CCS.

I am guessing this will "work" but be even more anemic than the reported drake PS.

The Heath SB-220 is a closer fit - S1) 860/1171 VAC @ 0.7A CCS, but they sell for around $175 one the 'bay. $100 more for 100ma. hmmm...

There are a lot of other options listed for the Drake. One that is very interesting is this -

DRAKE L4B-X2B-SC
FOR USE WITH BRIDGE REC & SWINGING CHOKE
CORE) A-0518/ C-CORE         STYLE #) 10  HIPERSIL C-CORE

PR) 120/240 & 48/96 VAC 60 HZ 1 PH

S1) 3200 VAC @ 1.2A CCS

The guy I bought the amp from suggested this sort of mod. He said that the 3-500 tubes can certainly handle it and the rest of the amp is built to take it as well.

Interesting...

John