Mandate from the Tron

[N2DTS]

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).

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

[VE3AJM]

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

[N2DTS]

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

Nice rig though..

[VE3AJM]

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

Al VE3AJM

[steve_qix]

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

[ka4koe]

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.

[N2DTS]

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.
 

[W1RKW]

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

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.

[n1ps]

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.

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 .  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

[ka4koe]

"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.

[steve_qix]

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  

[Opcom]

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.

[K1JJ]

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

[John K5PRO]

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

[KC2ZFA]

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 :-)

[ka4koe]

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.

[flintstone mop]

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

[ka4koe]

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.

[KA2DZT]

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

[N2DTS]

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.

[WBear2GCR]

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.

[WBear2GCR]

Phillip, it's very easy to build your own...I'm still apartment bound (so no antenna) but that hasn't stopped me
<snip>  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

Quite buzzardly, sir!

Very high boatanchor appeal.

[N2DTS]

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!

Phillip, it's very easy to build your own...I'm still apartment bound (so no antenna) but that hasn't stopped me
<snip>  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

Quite buzzardly, sir!

Very high boatanchor appeal.

[ka4koe]

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

[WZ5Q]

Phillip, it's very easy to build your own...I'm still apartment bound (so no antenna) but that hasn't stopped me
<snip>  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

Beeeeautiful Work!