The AM Forum

Timtron has deemed my audio acceptable since I reworked the pertinent bits of my Valiant 1. My new mandate from Tron is to "increase my scrote". I feel I have paid my dues and I am ready to take it to next level. Several options come to mind:

1. Build a transmitter (hard/expensive/long term), likely 2x813 modulated by 2x813. Hardest part is locating iron.
2. Obtain a Johnson 500 (expensive).
3. Obtain a Globe King 400/500 (not so expensive).
4. Find a T-368/BC-610 (heavy-must live in garage below).
5. Find a BC TX. See Item 4.

I do not wish to get an AL82 or similar big amp and drive with the Flex. We're talking totally old school. I also do not wish to go Class D/E (solid state).

Advice? Opinions?

FEELEEP

Advice....Stop talking to Timmy.....

FEELEEP,

AaahhH, the right coast is different from the left. Power/scrote has always been the game on the right. (lived there over 40 years & know) There are more stations on the right coast & the SSB vs AM wars might not be over. This is something the left doesn't understand.

I spend many nights with only the receiver on because the left coast stations live close to each other and get by with 50 watts. Lighting up the filaments on the "500" or Desk KW will not happen for a fading pi** weaker.

#1: Antenna is everything if'n you can do it.

#2:Take care of the little lady of the house & rug rats first!

#3: Be patient and find bigger iron, a GK500 will get the job done.

#4: Had to wait till retirement before the big iron followed me home.

Craig,

It just sort of happens....
Putting the word out here is a great start.
Don't discount BC610 or BC610 iron.

Surely some good man or woman OP out there has a spare mod xformer?
Plate and choke iron easier to find.

Philip,
         May i suggest roll your own in the form of 812A's modded by 811A's? Basically a GK 400. There's really not that much there to have to scrounge for. You already have an exciter sitting by your fireplace as "room decor". Build a P/S for it, get it running, and use it to drive a pair of 812A's in class C. You can use the P-P 6L6's in there to drive a class B 811 modulator as well. If you never get started, it will never get done. I can email some info for you to smoke over if you'd like.

73, Phil

Iron is not hard to find. Don't let that stop you.

I've got lots of iron here, surrounded by three Collins 21E cabinets. All converted and proven on 75/80m by W2INR. You can have it cheap, too. How big is your garage?

I like the 813s x 813s.  Good solid xmtr that if build correctly can last forever.  You should be able to get help here on the forum.  Building from scratch is a lot of work and can take a lot of time (unless you're JJ then you can build one in a week ;D).

This could prove to be easier than trying to keep the Valiant up and running.

Let us know when you plan to get started.

Fred

Anything really heavy will go in the garage directly below the shack, eg. 610/368. We've already done the calculations and talked with my Registered Architect friend who told me "NO WAY". Plenty of power available.

I vote for building one.
You can make it easy or hard, one or two bands is very easy, plug in home made coils, a pair of 4x150's modulated by a pair will do 500 to 600 watts carrier easy, low drive, lower voltages.
Or, don't have or want iron, screen modulate 3 or 4 of them for 300 or 400 watts out.
Drive the grids with a ricebox.

Or fix some over priced old stuff all the time..

Todd's right Philip, with that setup you would be "net control" for the southeast! Nice piece of East Coast Sound to preserve as well. I know if I didn't have so much crap and had the extra room, it would reside here.

Steve, Todd; where you guy's been hiding? Planning on venturing south of Richmond this year?

...Phil

I am the point signal-wise to where I am heard +90% of the time on AM, if I can hear them, that is. I put up the K1JJ recommended limited space dipole, 180' long x 60' high, fed with 600 ohm open wire......so it won't get much better than this aerial-wise unless there is a change of venue.

If'n I put big, and I mean big, iron in the garage below there must be a day or two of cleaning things out. Of course, I'd have to remote the thing from upstairs directly above.

Feelip
Look at this nice monster.

https://www.youtube.com/watch?v=itlSdNherlI

A member or former member of AMFONE KE7TRP has one of these beauties..

Bigger power like around 500 will mean that you will have to heavy up the antenna system to handle the juice.
A few of the guys in 8 land and VE6PG are running a hundred watts and come booming FB into MOPRADIO on 160.............Cover up static and sound nice. I do not mess around with 75M. I'm working on the Extra class to get below 3.7mhz and changing focus on DX and CW operations.

Fred

Checked 3.7 Mc several times over the holiday and heard nary a word. I guess my timing was wrong. And....160M AM is nearly non-existent...where is everyone???? Again, my timing may be wrong.

The cheapest and smallest (physical size/weight) way to go for 160/80/40 is class E.  You can easily build a desktop kW (that's carrier power!) like the one I use at Rattlesnake Island where space is at a premium.

Tube-wise, building is still better than buying (a LOT better), and probably less expensive.

If you have some money to spread around, I would get an Apache Labs SDR and follow it with a big linear.  Unbelievable sound (both transmit and receive)!

You could build the linear and same some money there...

If you want to go with a quad of 813s (2 driving 2), this is a solid design: http://www.amwindow.org/tech/htm/813/813.htm     GORTs innards are made of these.

   The thread from Brett concerning both screen modulated 4X150's where 100 watts carrier per tube is possible, and plate modulated 4X150's at 2X that power is pretty telling to me. In the case of screen modulation, a box the size of a SB200 with a remote supply could run legal limit AM.

http://amfone.net/Amforum/index.php?topic=37242.0

Jim
Wd5JKO

I really like Jim's idea.  I have been following Brett's thread and I plan to build a screen-modulated 4-holer with a variant of the 4X150.  It is the 7035/4X150D, same specs, but 26 volt filaments at approximately 1/2 amp each.  Got a bunch of these on a surplus buy. 

At a small hamfest in Rome, GA a couple months ago, I picked up a "parts unit" Dentron Clipperton L  for $60.00 and the antenna tuner for $50.00.  (I posted a couple pictures below)  He said it was a parts unit because condition was unknown, but it was complete, so I took a chance.  It has a quad of 572B triodes in grounded grid.  I do not need another linear.  My thought was to yank the glass tubes, and mount a small chassis vertical inside the back to hold the four 4X150 tubes, and use the existing muffin fan for cooling. 

The Clipperton runs between 1500 and 1800 volts on the plates.  This could be reduced with an external variac, or the filament winding could buck the plate transformer primary, if needed.  The plate iron checks out OK, should run cool in this mode.  I will pull the entire rectifier/filter assembly and replace with all new components on a far-circuits 8-capacitor board with diodes and equalizer/bleeder resistors; slightly smaller than the module it replaces.

In summary, this approach would eliminate most of the metal work, provide the heavy iron and most of the plate tank components (this one tunes 160 thru 10) and result in a rig that is not too huge or bulky.  All HV would be contained in a single enclosure.  It may or may not be possible to fit the screen modulator components in the main unit;  maybe a separate exciter/driver and audio/screen modulator in a similar size cabinet. 

My suggestion is to research many options, figure out what goal you want to reach, troll the hamfests for the components, then you can see yourself completing the project in a reasonable time frame, and have a rig that you are proud to operate.  GL OM!

"  a box the size of a SB200 with a remote supply could run legal limit AM. "

Be that as it may, I still think that something like a 20V-2 size walk in transmitter is a good thing to have. Someplace to hide from the wife in.


klc

Power supply one level down. Pipe high voltage through conduit to RF deck in shack.

Always seems to be a good crowd from the "mid-west",,, 8 land....I am in Western Pa. on 1880 and 1885, 1945, 1980 just about every evening after 7PM. I hear East Coast stations very well from my Western Pa. location. Antenna and power (legal) is in the formula.
3705 gets tricky with SSB that never used to be there, but now there is cuz AMers showed up. So the Extra class guys go to 3.700 or lower to operate and get away from the SSB.  3.880 or 3.885 in the early evening is pretty nice before the chaos sets in.
Fred

Things to keep in mind:
Screen modulation is easy on the RF deck voltage wise, plate voltage is maximum voltage, so wimpy parts work.
The 4x150 tubes are low voltage tubes, 1500 to 2000 volts is great.
The screen modulator and power supply can be outboard and remote, in a small cabinet with the line level audio equipment, and can work on almost any tube rig at any power level.

 

Now, correct me if I am wrong (no worries on that point), but if you implement screen modulation, doesn't that limit one to 100% modulation and no more????

FEELEEP

Absolutely not!

Screen modulation, properly implemented will yield broadcast quality audio. The positive peak headroom is limited only by how you adjust it.

The problem with screen modulation is the efficiency is very poor, but no worse than AM linear operation.  Also, most people really don't implement it correctly.

The screen is a varying impedance load; a nonlinear load also.  Therefore the screen must be modulated using a low impedance device.

The best device for the job is a MOSFET operating as a source follower.

If I had a reasonable power tube RF amplifier lying around here, I'd build a screen modulator as a demo and then document the circuit.   Come to think of it, I do have an old Invader 2000 kicking around (pair of 4-400s) - no power supply for it, but I suppose something could be clip leaded together.  I'll give it some thought.

Duplicate Frank's (AHE's) 4x1 circuit driving your 4 by 1/2's.
 He had to turn down the wick when Riley walked towards us at Gathersburg.  ;D

You can adjust it that way.
Or not.

I can get all kinds of positive modulation at 95% neg mod, and my screen setup seems to want to run a bit over 4 times carrier, 250 watts carrier gives 1200 watts pep with typical adjustment.
I could crank up the audio and hit much higher positive peaks if I could limit the negative ones.
I could adjust things to limit the negative modulation but that would be distortion.

I do not think you want over about 120% positive, you just distort most receivers running more, its better to increase the density then the peaks.

My screen mod setup seems to be just like running AM through an amplifier, but you have no rig to limit anything or distort.
You can modulate 100% with a 5 Hz audio tone, or a 15 KHz tone using a line level input.

Any way you look at it its the same, running an amp, modern screen modulation, or plate modulation which is like running two amps, one for the RF, one for the audio.
Two amps give more power then one, but you need the combiner (mod iron).

Phil, have you ever built/homebrewed an AM tx before? Do you really want to start out HBing with a high power 300w AM tx as your first tx project? There are so many responses that I have read about here with building a high power AM HB tx, be it plate modulated, screen modulated etc. If you want to start out with a HB tx, why not build a lower power tx, and gain the experience necessary, and work your way up as many of us have done? That takes time. Do you want to take the time? What kind of experience do you have with homebrewing a transmitter? Have you ever done it before? Do you attend local/semi local hamfests, in order to accumulate the parts necessary to HB a AM tx? How about any local AM guys there? Do you network with/befriend them, and let them know about what you're looking for, be it a commercial tx, or parts? They can be a useful resource, and save you time and $$$.

Tim, WA1HLR is a good guy and has good suggestions, but, what do YOU want to do? What about your XYL? Is she onboard with this? This can be an issue for some guys.

I would suggest that you check out the current K7DYY Super Senior AM tx. Its a dual bander, and you can have it ordered for 160 and 80m, or 80m and 40m. It runs 350W carrier, all solid state class D, for max legal power in the USA. So, you could have a max legal limit tx for now, and gain experience in building your own tx in the meantime.

You could always sell the K7DYY rig afterwards, and get your money back out of it.



Al VE3AJM

I do not think the K7DYY rig does 40 meters.
There is nothing on the web site that says it does.

Nice rig though..

Contact the gentleman directly as I have. He does have a working prototype 80 and 40m tx. Hes taking orders.

Al VE3AJM

C'mon Phil - it's time to BUILD something !   ;D  Do screen modulation.  Use the right modulator and it's pretty much foolproof.

My biggest roadblocks are, as I see them, the following:

1. Lack of tools to make custom chassis.
2. Lack of tools to cut big holes.

Working up a predicted BOM to see what it will likely cost, material wise.

I don't see this as a short term thing, unless a GK400 or GK500 comes available, which I am confident I can restore if the unit is not in good condition.

Already researching 813 operating characteristics and decoding K1JJ's design. Even if I don't end up going that route, it's a useful exercise anyway.

Will look at screen modulation as suggested to cover the bases.

You should not have to make many big holes, and home depot sells a very nice Greenlee bearing punch set.
I used all old Radio shack meters because they take a small hole, 3/4 inch I think, I just make up shunts and scales.
I never feel bad about buying tools.

Lots of fun and pride in building, but you could save up and get a Globe King or Johnson 500.
The WRL stuff was marginal, poor iron, wimpy components, but easy to work on.
Sometimes you can pick up an RF deck and build a power supply and modulator.
If you have the mod iron, modulators are VERY easy using zero bias triodes.

An sdr and a big amp is cleaner/smaller and has resale value, homebrew is worthless junk for the most part.

A pre made class E rig does not seem like it would be much fun long term, plug and play, like a rice box.
 

Modular design is always a good way.  do it in steps.

http://amfone.net/Amforum/index.php?topic=13330.0         see a few posts down.

I purchased aluminum chassis's to make life easier.  A hole saw back by hard wood to cut the holes for the 813s. A drill press worked best but can be done by hand. Make sure all is clamped down.  A clean cut can be made.  cost is minimal

Let a machine shop make your chassis pieces.  Sketch up what you want and get quotes for aluminium sheet from your local machine shops.  Use al angle from HD and connect the plates together by drilling and tapping the angle. 

I personally prefer making class E transmitters...no high voltage to worry about.  But building is fun and and good way to learn, so I agree with Steve on that.  If you let some smoke out...well...that is part of the process ;D ;D.  On this forum or others and I am sure at least regionally, there should be good support.  Go for it and build. 

My ,02

Peter

"Phil, have you ever built/homebrewed an AM tx before? Do you really want to start out HBing with a high power 300w AM tx as your first tx project?"

No. I have a buddy, however, that lives in VA who did just that. He homebrewed two (2) 4-1000 rigs modulated by a pair of 833s and it was his first.

Being a poor machinest with few tools, I like to recycle old home brew equipment.

Often you can pick up complete chassis with front panels already punched out and sometimes with meters still there!

I have built many, many transmitters and have *never* built a custom cabinet, chassis or anything like it.

Every rig is in something recycled  ;)

A friend uses HK-54's modulated by 808's with a BC-610 modulation transformer. The modulator drive is something like a 6L6 type output transformer driving the grids via a small solid state PA amp. It sounds super.

Hi Phil,

I agree with many of the posts - just start the project. Plan it, build it and make it known what parts you need. The parts will find you.  813s X 813s are a perfect power and starting level.

I posted some pictures of a metalworking style I use that is easy and inexpensive.  Go to a local metal shop and have them cut out a 19" wide aluminum panel  X  24" high. Get two pieces.  Get some aluminum angle and use nuts/screws to bolt the two sheets together as a chassis and front panel. Use side struts to make it strong.  Put angle on the bottom chassis sides to make it stiff. (as shown in the pic #1)  Add castor wheels and you're all set.

Using 813s, you can build both the RF final and modulator on it. The mod xfmr and Heising reactor will likely fit too.  

Make a box for the final and modulator tubes air sub-chassis. Use angle and screws.  I usually blow air past the 813s for better cooling.

Home depot has inexpensive hole saw bits that can cut your tube and meter holes, etc.  You can pull it off with simple power tools.

You can either mount the rig in a 19" rack or build a cabinet around it as I did using aluminum sheet. That cabinet slides off in seconds for easy rig servicing.

Good luck.

Tom, K1JJ


Pic #1  is 4-1000A grid driven linear

Pic#2  is rear view of cabinet for class C final.

Pic#3 is  4-1000A class C final

Steve, -QIX,
I used to have an Invader 2000 sans power supply. I wonder if that is a common configuration of that thing?

At workplace, we screen modulate a pair of 4CW250,000B tetrodes in parallel, using a single 4CX3000A cathode follower. That thing can swing 22 kV and supply up to 200 Amps. It is pulsed however, plate modulator for a big RCA triode for Megawatts of RF (for linac).

KA4KOE, I have plate iron and modulation iron that would be suitable for an 813 sized rig. Its essentially capable also for replacement supply for a Johnson 500. Contact me PM if you are interested.

I loved the suggestion of having a big BC rig to hide from the XYL in. Problem is, what if she knows how to throw the switch?
John
K5PRO

Phillip, it's very easy to build your own...I'm still apartment bound (so no antenna) but that hasn't stopped me from cutting metal at the balcony, and constructing (with stone knives and bear skins) in the kitchen alcove. All I have are very basic hand and power tools, two hands, and two thighs to hold pieces when drilling is required. Parts come from here and a few other BBS's...I also repurpose stuff, my avatar tx is being built in a TU-5B box...

My avatar is wholly hand-made, knob placement, and other holes, is basic geometry with a ruler, a right angle and a compass tool. Here (and more at the link) is some progress on the innards https://imageshack.com/i/0cimg1754nrj

You should add to your New Year's resolutions :-)

Funny thing is, a Gates BC1-G is available within 4 hrs drive....but getting it home would be problematic. Affordable too......probably more than me and my meager talents could handle. Eastwood..."A man's gotta know his limitations."

Tom, if there is any backup documentation for the design calculation-wise, I'd love to see it. The original napkin design is likely insufficient. I've already pulled up application notes for the 813s.

Steve, good hearing you last night. The past three evenings, as mentioned, turned simple two-three person chats into big roundtables. Propagation was bizarre to the A index value of 18 last night. I don't think I'm gonna go with Class D/E.

I do not want to do a prefabricated exciter from K7DYY. There is no pride in simply laying down one's moolah.

I built a nice vacuum tube theremin and have maintained this Valiant for 2 years....comfortable around test equipment.

Decisions decisions, Feelip
I get all fired up myself, when a building project is in the dreaming process....THEN when the rubber meets the road and it's time to start, and gather the parts, and drilling holes, and messing with metal........
The good responses in this thread are from veteran builders, who have access to nice junque boxes, or are very resourceful to find the hardware and make beautiful things.
If there was a "Heathkit style" Class E kit; I would be the first order.
Another member in AMFONE mentioned that going down the road to build a class E transmitter will limit the number of Elmers available to get you, and definitely me, through the process of bringing the TX to life.
It might be better to get an 813 rig built. And nothing wrong with screen modulation. There seems to be some tricks involved to make it happen, but the end result is a really great sounding, powerful TX.
80 meters and up is not so critical to be legal limit. 160M is a lot of fun, but antenna and QRO is the best way to enjoy that band.
I see you like to make strange eeerie sounds with your theremin.
Fred

Thanks, FM.

The engineer in me wants to KNOW how this thing works. For instance, I notice the lack of a clamp in JJs design. I feel that a simple design that does the job and is somewhat forgiving is best. High parts counts do not equate.

I'd love to see photos of similar rigs......things to consider.....

1. Safety. Multiple power supplies, overcurrent protection, cages.....master cutoff/contactor.

2. Wiring. Routing dangerous voltages between the chassis. Spark plug wire??? Insulators. I don't think the usual terminal strips are rated for 2-3K and are exposed, dangerous besides.

3. Ventilation: open rack ventilates well but has exposing, killing stuff. Close up the sides and we need to ventilate the rack with a fan on top, perhaps??

I chatted with KC9PCP last night and he was on a screen modulated 813 transmitter. Not bad.

Philip,

Really nice job on the theremin.

I would go with the 813s x 813s  Get a mod xfmr and go with plate modulation.  First item you need is the mod xfmr, from that you can then figure what xmtr you can build.  It's the mod xfmr that is the limiting factor.  Don't worry about the high voltage.  A rig like this can run on 1800-2000 volts which is no big deal to work with.  My xmtr has some screw terminals in use with over 1500 volts on them without any problems.  There are many ways to route higher voltages.  I also have Cinch plugs (the small size) with 700 volts DC unfiltered over 1100 peak without any problems.

You're right about the clamp tube (usually a 6Y6) to protect the finals.  You can also use some protective bias on the finals.  My single 813 HB xmtr uses the clamp tube.  Possible with a pair of 813s you may need two 6Y6s in parallel.

My HB rig is a complete xmtr from the vfo and only runs on 40M and 75M.  The vfo and the lower level stages can be harder to get working right than the final PA.  I designed the whole xmtr including the vfo but this was a lot of work and took a lot of time.

You can just use another small xmtr to drive the 813s.  You can simplify the rig if you limit it to just the bands where the AM activity is, mainly 40M ,75M and possible 160M.  You don't need high power on the upper bands.

Fred

The Bill Orr (west coast) handbooks are the best place to start.
Lots of homebrew rigs with pictures and details.
I like the yellow and red years.

A nice dropping resistor on the screen voltage supply works well, more current, more drop through the resistor, less voltage to the screen, self regulating to a point.
An overload relay can just be a relay coil in series with the screen with a pot across the coil.
Too much current flows, relay opens, when it opens it connects the screen voltage to a resistor to hold it open. Break the circuit to reset.
Many relays will work, 813's may not need it.
I have a pair of 813's that use a fixed supply (variac set) a big wire wound pot dropping resistor, and a choke to self modulate. I do not think the choke is needed with the resistor.
I also have the overload relay but never tripped it in 25 years, the dropping resistor prevents excessive current.

For 2 or 3 bands, a plug in tank coil made of copper tubing makes things very easy to build, build it in a rack cabinet, the power supplies can be remote or in another cabinet.
813's need no air, or a small fan will do.
They do not seem to screen modulate real cleanly, but a pair runs cool at 2000 volts and 400 ma input, about 600 or 650 watts carrier out plate modulated.

For screen modulation, go with a pair of 4-400's, a 4-1000, or some 4x150/4cx250b type tubes.
If you need small, its hard to beat the little ceramic tubes, very small tubes, low voltage means small parts.

My 4x150 RF deck has loads of extra space, and is about dx100 size, and much less then 1/4 the weight, despite the fact its built to support plate modulation with bigger parts.
It can run an easy KW carrier out at ccs ratings.

Harbor Freight, or better, Grizzly or Jet sell decent drill presses not too expensively. Craigslist has 'em too, as does ebay. You can do a whole lot with a drill press.

Thrown out bed frames happen to be made of angle iron.

Hole saws make nice large holes when the work is clamped and you have a drill press.

The 813 x 813 rig works really well and is pretty easy to build.
Both KC2FXE, John and W2TRH, Steve have built them.

But anything you build is good. Go with what you got or can get!!

Looking at ur theremin, no doubt you have the construction skills to pull it off.

Quite buzzardly, sir!

Very high boatanchor appeal.

:D

Yes, REALLY nice, right angle drives and everything.
I never built a complex band switched anything because of the complexity and here we have it.
Very nice indeed!

I have begun my warp field calculations.....now to find the correct set of curves to draw a diode line. The numbers reflect one 813.

FEELEEP

Beeeeautiful Work!

One 813 does 300 watts carrier out easy.
2000 volts at 200 ma.
10 K ohms.

I have a single 813 with about 1900 volts at 200ma but only get about 220 watts out.

Fred

380 watts in.
220 watts out, 160 plate dissipation more then you should have on an 813 that is rated at 100 watts ccs or 125 watts icas.
Very inefficient.

2000 volts and 200 ma is 400 watts in, 300 out, 100 watts plate dissipation.
That is about 75% efficiency.

Thanks guys, I'm hoping within a year I'll be on the air.

I want to thank Brett, Phred, 'INR, Ron, Dennis G, and others who have sold me stuff over the last couple of years I've been tinkering.

Peter

I was thinking the same, should be more output.  I think the plate voltage is not 1900.  I have to take an accurate measurement.  Also, I have a mod reactor in line that is 280 ohms, at 200ma that's a drop of almost 60 volts. The screen current is from a dropping resistor, so it runs through the mod reactor also. Then the current runs through two plate chokes that have some resistance where the plate voltages drops more.  Another thing is that I'm measuring the cathode current which includes the screen and grid drive currents.

I just reworked the plate supplies so I need to do some measuring.  Could be the plate voltage is around 1825 and plate current alone is only about 180ma.

1825 volts at 180ma is about 325 watts input with 220 watts output leaves about 105 watts dissipation.  This could be about right.

Fred

Class C 813s really come to life when the plate voltage is above 2000V and screen / grid currents are adjusted accordingly to match specs.  Even 2500V is not too much.  

I have run 3KV on my pair. If plate modulated, run deeply into class C, almost class D - and plate loaded to maximum, gives an 800W to 1KW carrier, modulated at 130%.   I usually run spec screen current - and a little over spec for the grid drive. A little more grid current helps the positive peaks.   This assumes your mod iron can handle the extra voltage and power.  Back everything off and you have plenty of headroom and safety factor.

I think 813s, even though officially rated at 125W dissipation, are more of a 175 watt amateur class tube - and can do even more with lantern chimneys and air.  I always add some air to them. The tubes last forever. I've never lost an 813. (Run the carbon plate 813s with NO plate glow.)  Air helps to increase this glow threshold level. The filament is overrated and emission is robust for a "125 watt" tube.

Build well and tweak the tank circuit and operating parameters for maximum efficiency. 813s are incredible tubes in the proper class C environment, just like the 4-1000A.

The 813s X 813s (with modulators triode-connected) are one of the cleanest THD/IMD plate modulated rigs I've tested, even at 3KV.  Be sure to run a clean audio driver or all bets are off.  I prefer the solid state MOSFET design by WA1GFZ.  This way, audio is totally transformerless except for the mod transformer. Add at least 6dB of audio negative feedback and it doesn't get much better than that for tube rigs.

Phil - Sorry, but I have no additional documentation for the 813 AM rig schematic  posted on  the AM Window.  Continue reading the old handbooks and you will eventually learn enough to build this or any project from a schematic. Axe lots of questions here and on the air.

T

Very interesting about the 813 intermod.
I have never run them as modulators, but know they loaf along at 700 watts carrier output for a pair.
I have had the same two 813's in my rig since it was built 35 years ago, and about six good tested spares.
I bet I got most of them at fests for under $10.00 each.

I took some accurate measurements,  plate voltage at the B+ end of the mod reactor is 1800 volts.  Voltage drop through the mod reactor and plate chokes (about 300 ohms x 225ma) is about 70 volts.  So, I have 1730 volts on the 813 plate and loaded to 190ma plate current is about 329 watts input.  I'm getting about 220 watts output (Bird watts).  This leaves 109 watts of dissipation, looks about right.

Fred

Good numbers; thanks. What is the current state of availability of 813's??

Philip

813 are still available but do not buy online unless the person is a known seller of good tubes. I would never buy tubes on Ebay. If you need a good supply of 813's then inquire here and PM from there. You will find them. If I may, here are my words on the subject:

The ART-13 is perhaps one of the greatest transmitters ever designed, and for it's size and weight is unbeatable with the proper power supplies. The 813 tube is one of the reasons, and with that in mind, you cannot go wrong with building a TX with the 2X2 813 format. The iron is out there. Do your homework, decide on your design and parts needs and go from there.
Concentrate on one element at a time, such as power supply, speech amp, modulator, rf deck etc. Keep a log of your project and photograph everything. Ask for advice and help and always work with one hand behind your back when HV is present. The most rewarding aspect of this hobby is rolling your own... and it can also be your biggest supply of disappointment if you don't plan your work, and do so carefully. Triple check everything before you commit, and this is especially true for parts placement, lead dress and building practices. If it's worth doing once, it's worth doing right. Make it a legacy project as it will likely become one of your greatest triumphs that brings you years of enjoyment. Big or small, make the effort worthwhile.
I own 4 major transmitters and a couple of nimble band cruisers. ART-13, GK-500C, BC-610-E, and a Harris BC-1H. I have repaired and or restored all of them with my own hands and nothing has brought me better joy in this hobby (BC-1H is currently being modified) than doing this.
And, like was mentioned earlier, recycling and/or reusing, updating previously built transmitters as built by SK is a great way of gaining useful experience not to mention bringing something back on the air. That is fun unto itself. Whatever it is you decide to do, do it right and make it fun. Be yourself and blaze your own trails.
 :)
Billy N6YW

Yep. Both a power transformer and mod tranny have been available in the For Sale section within the past few days.

Need to get my Bushmaster out of layaway and then we will start collecting parts. I think I will roll my own 813 rig. Lots of good inflammation here!

FEELEEP

Latest shack configuration video....

https://www.youtube.com/watch?v=TI3hOq8TlOk

Nice station