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Old 17th Nov 2005, 6:38 pm   #1
Oskar.B
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Default Homebrew transmitter project (Frank vs. Oskar :) )

Hi everyone

I'm just about to start assembly of my 807 transmitter, first mission is to build the PSU, and then test an rf setup on a scrap chassis.

I started this thread so that others can kind of "follow" my progress (or lack of it). Frank N will be joining in and post details about his transmitter project as well. Others are also welcome of course, we can all learn a lot from each other. I will also most likely be asking Frank loads of question all the time , but as soon as some progress is made with the transmitter, I will post link to pictures, schematics e.t.c.

I begin with some pictures of my "raw material":

http://gargnas.net:3000/bilder/dassrull/tx_bits/

/Oskar
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Old 17th Nov 2005, 7:48 pm   #2
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Hi Oskar and the group,

As Oskar just mentioned, then I will also be participating in this little, informal 'build-off', or perhaps 'build-along' is a more appropriate description.

My project is something I have wanted to start constructing for a while, but so far have lacked a good excuse. The design details are not yet clear, even to myself, but hopefully I will have that sorted out before too long. What is known though, is that my aim is a valve based Morse code (CW) transmitter with at least 250W of carrier power at the antenna connector. I may make this a tri-band project (14, 21 and 28MHz), but a single band 28MHz TX is also being considered.

I could choose to hurl a modern transmitter valve or two like the 4CX250B at the project and be done with it, but I much prefer the old school look of clear glass bottles and lots of copper piping. So I will use something else from my stash...

Attached below are a few snapshots of the core chassis and the HT transformer. For a start my project will focus on actually testing many of the critical components. Those being mainly said HT transformer, the output valves I'd like to use and the filament transformer for those. I only need a single output valve for my choosen power level, but would like to make sure I've got a working spare.

Hopefully all the components will work. If not, then it will be time to come up with a plan B.

Parts of today was spent making the HT power supply subassembly, needed for testing the HT transformer. I only need to fit another 20 diodes, then it will be time to run up the variac.

Timeframe to completion: Unknown.

Frank N.
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Old 17th Nov 2005, 9:32 pm   #3
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Hi Frank, how about 4, 807's in parallel for the output. They are still plentiful and cheap as well as running at up to 1KV on the anode.

Ed
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Old 17th Nov 2005, 10:13 pm   #4
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Hi Ed,
Quote:
Originally Posted by Ed_Dinning
Hi Frank, how about 4, 807's in parallel for the output. They are still plentiful and cheap as well as running at up to 1KV on the anode.
It used to be done alot, running smaller valves in parallel for added grunt power. But I must admit I'm not too fond of that arrangement, as I find it too finicky and prone to imbalances and overload of individual valves. Additionally, the total anode dissipation of 'only' 100W would barely be adequate for generating 250W, once you add up circuit losses.

Even a single 813 at 2KV would be pressed to do the job, so I scrapped that one too. No, what I have in mind does have something to do with the number four, but probably in a way few will be able to imagine.

"Once upon a time, faced with a pressing need, They made It. But it was found to be inadequate, so They doubled It. Still not enough, They doubled It again!

And the others said 'Enough! '"

Frank N.

Edit & PS: HT rectifier bridge and smoothing cap bank completed. Awaiting testing tomorrow.

Last edited by YC-156; 17th Nov 2005 at 10:25 pm.
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Old 17th Nov 2005, 10:46 pm   #5
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Good luck guys. I can hear the sirens wailing already!

73's

Aub

P.S. Adibrooke would love this!
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Old 17th Nov 2005, 11:27 pm   #6
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Gaah! Frank is allready way ahead of me!

It's looking really good Frank! I want one of those transformers too, have I mentioned how much I like heavy metal?

I'm still wondering how I should do with the RFCs' for the oscillator and final valves. I only have one proper, section wound, RFC. I was thinking of making my own coil form with sections, I bought a plastic rod for this. But how the heck does one do the math for such a thing? And winding it by hand?

I'll at least be winding the oscillator and 807 grid tuning coil myself. But those are easy.

For those who haven't seen it already, I'll link to my schematics:

http://gordis.mine.nu:3000/bilder/da...5U4GB_REV2.pdf
http://gordis.mine.nu:3000/bilder/da...07_RF_REV2.pdf
http://gordis.mine.nu:3000/bilder/dassrull/ATU_REV1.pdf

/Oskar

edit:
Whoho! One more electrode!
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Last edited by Oskar.B; 17th Nov 2005 at 11:38 pm.
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Old 18th Nov 2005, 9:05 am   #7
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Hi Oskar, if you are wing and RFC on a plastic rod make sure it is "PYE" wound (up to 5 pyes are usual) and that it is would on tghe rod and not in slots in the rod or the Er of the plastic will give you a high self capacitance.

Ed
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Old 18th Nov 2005, 9:15 am   #8
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Hi Ed,

Could you explain "PYE" a bit? Never heard that. The best would be to attach something temporarely to the rod to form the sections, and then wind and remove it.

/Oskar
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Old 18th Nov 2005, 12:14 pm   #9
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Quote:
Originally Posted by Oskar.B
Gaah! Frank is allready way ahead of me!
Not really.

What I have done so far would be the equivalent of wiring a pair of those 47uF caps together, along with their bleeders, and hooking them up to the 5U4GB socket. I just needed the whole circuit, as the bleeders form part of the voltage multiplier for my HT meter. Please see first attachment, still untested.

Note that this is Overkill(TM) for the expected voltage and power level,. Yet I had the components, so I overengineered this with future projects in mind.

Quote:
Originally Posted by Oskar.B
It's looking really good Frank! I want one of those transformers too, have I mentioned how much I like heavy metal?
No, not so far. Then you would probably also have liked the other HT transformer, which I removed from the chassis a few days ago. That chassis was made more than 15 years ago as part of a project I never finished, and would have been a HT power supply.

If the 'small' tranny shown above doesn't work due to insulation breakdown, then I have no option left but to use the larger one. That would mean increasing the expected power level just a hair though...

Quote:
Originally Posted by Oskar.B
I'm still wondering how I should do with the RFCs' for the oscillator and final valves. I only have one proper, section wound, RFC. I was thinking of making my own coil form with sections, I bought a plastic rod for this. But how the heck does one do the math for such a thing? And winding it by hand?
Yes, you forget the complicated Pye technique and wind it by hand. You only need one additional choke, and that is for the PA.

Below I have attached a few snapshots of the common form of RF chokes. The leftmost one is Pye wound, even tapered Pye, which is even better.

A Pye wound choke is not only section wound. The wire in each section is wound very systematically and evenly, you may be able to see this from the photos. Doing this properly by hand would be Hard(TM)! If you didn't and just curled up a bit of wire in each section, then the result would still fall short I would imagine, Ie. the parallel capacitance would be too high.

The rightmost choke is similar to what I would suggest you make for the PA, and then you use your existing choke for the oscillator. Winding 500uH in a smooth layer coil by hand is doable.

I have attached an approximate formula for the inductance I of a single layer, smooth wound coil of length L, diameter D and with N windings, assuming the L/D ratio isn't too outrageous. Dimensions in cm. The end result should be quite good enough for your purpose, the actual inductance of the choke is not that critical.

Note that the rightmost choke is wound on a porcelain tube. At your voltage and power level it probably wont matter that much what you use as coil form, but sometimes even Nylon isn't good enough for high impedance/high power RF work. I have seen a Nylon coil form literally catch fire and burn when subjected to only 100W of power at 7MHz!

If you have a choice, then the proper materials for high power coil/choke forms are Teflon, plexiglass (transparent), ceramics (including porcelain) and glass.

Edit: The smooth wound choke in the photos has an inductance of around 125uH, and is made from 0.3mm copper wire. The dimensions are 4,3cm long by 2cm in diameter. So you only need about twice as many windings, which would make the final choke about the same height as an 807!

Quote:
Originally Posted by Oskar.B
Whoho! One more electrode!
Say what?

Frank N.
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Last edited by YC-156; 18th Nov 2005 at 12:29 pm.
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Old 18th Nov 2005, 12:36 pm   #10
Oskar.B
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Ok, I will try winding my own RFC this weekend and do some measurements to see if I will get troubles with self-resonances and such.

It's nice to have access to all the lab aquipment at work. I can check both L and C of the coil there. If I can get away with it, I'd rather use my standard hard-paper rolls for coil form. I like them better than plastic. I should get my 1,6mm² copper wire for the coils today.

By the way, I was glad to become a Tetrode ...

/Oskar
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Last edited by Oskar.B; 18th Nov 2005 at 12:43 pm. Reason: spelling .. again!
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Old 18th Nov 2005, 12:54 pm   #11
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Quote:
Originally Posted by Ed_Dinning
Hi Oskar, if you are winding an RFC on a plastic rod make sure it is "PYE" wound (up to 5 pyes are usual)
Ed
Hi Gents,
I believe the correct term for coils and chokes consisting of several sections is "PIE" winding, not "PYE". Just to avoid any confusion with the famous maker of Black Boxes...... .

Regards, Mick.
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Old 18th Nov 2005, 1:31 pm   #12
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Quote:
Originally Posted by Oskar.B
Ok, I will try winding my own RFC this weekend and do some measurements to see if I will get troubles with self-resonances and such.
If you have access to a grid dip meter, the technique is to temporarily connect a short piece of wire from terminal to terminal, and then search for resonances near the operating frequency and a few of the harmonics. This will reveal the troublesome series resonances.

If you dip the choke without the short, you will find the parallel resonances, and those happens to actually increase the effectiveness of the component.

Quote:
Originally Posted by Oskar.B
If I can get away with it, I'd rather use my standard hard-paper rolls for coil form. I like them better than plastic.
Paper?!

Paper and wood are quite lossy at RF frequencies, as those materials absorb moisture from the air. I wouldn't even waste time trying to use those for coil forms at any significant power level. In the oscillator a paper coil form would be way too unstable as well. Even PVC tubing would be a better choice, and that is saying something.

In the other thread I mentioned that the oscillator in particular should be built like the proverbial battleship, and I wasn't joking. If either the chassis or the components can be deformed or deplaced by pushing with a finger, then it isn't strong enough. You want to build something, which is mechanically stable to within a tiny fraction of a percent.

Below 28MHz the bandwidth used for amateur transmissions on AM is less than 6KHz (3KHz sidebands), so your transmitter oscillator must stay put to within a fraction of this if you want it to be practical for real communication with other amateur stations.

Edit: To illustrate just how sensitive the oscillator components are, then imagine you secured the oscillator coil with some type of glue. If you were unlucky, then the presence of the glue itself could be enough to render the oscillator useless, simply because the electric or mechanical properties of the glue might have some odd temperature dependence.

Quote:
Originally Posted by Oskar.B
By the way, I was glad to become a Tetrode ...
Oh! OK, congrats! You had totally lost me there for a moment.

Frank N.

Last edited by YC-156; 18th Nov 2005 at 1:37 pm.
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Old 18th Nov 2005, 1:32 pm   #13
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Hi Mick,
Quote:
Originally Posted by mickjjo
I believe the correct term for coils and chokes consisting of several sections is "PIE" winding, not "PYE". Just to avoid any confusion with the famous maker of Black Boxes...... .
You are almost certainly right, but I wasn't sure one way or the other.

Frank N.
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Old 18th Nov 2005, 1:44 pm   #14
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Ok, no paper then . I'll drive off to Malå (40km) to see if the hardware store has some suitable plastic plumbing to use for coil forms. I plan to use something with about 4cm diameter.

We have all sorts of L, C meters, but no Grid-dip meter. I was hoping I could measure the coil's capacitance, to calculate self-resonant frequencies. It might be difficult to measure though.
/Oskar
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Old 18th Nov 2005, 2:41 pm   #15
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Quote:
Originally Posted by Oskar.B
I'll drive off to Malå (40km) to see if the hardware store has some suitable plastic plumbing to use for coil forms.
If you are forced to use PVC, then try to find something white.

Another long shot: If you know someone, like a local jeweler, who could cut and smooth the edges, then spare lamp glasses for petroleum lamps might be an option. The long glass tube would have about the right diameter, and the material is about as good as it gets. They aren't expensive either.

Once cut down to size, you could attach fittings using two component epoxy glue (Araldite).

Frank N.
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Old 18th Nov 2005, 4:22 pm   #16
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Quote:
Originally Posted by YC-156
If you are forced to use PVC, then try to find something white.

Another long shot: If you know someone, like a local jeweler, who could cut and smooth the edges, then spare lamp glasses for petroleum lamps might be an option. The long glass tube would have about the right diameter, and the material is about as good as it gets. They aren't expensive either.

Once cut down to size, you could attach fittings using two component epoxy glue (Araldite).

Frank N.
Would test tubes do the job

Mike
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Old 18th Nov 2005, 4:29 pm   #17
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Test tubes are good, especially for chokes.


Paper can be used succesfully, but it requires the paper to be completly dry, and then impregnated with varnish - Paxolin is only resin impregnated paper, that is an excellent coil former material.

Jim.
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Old 18th Nov 2005, 10:16 pm   #18
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

I dug deep into my treasure-boxes and found two additional RFCs.

http://gordis.mine.nu:3000/bilder/da...s/PB180005.JPG


I measured their inductance and the one without "legs" are 940uH. I think this will be perfect for the oscillator plate RFC. The other one, with legs on it, measured 870uH. I think I can use this for the oscillator screen grid. Unfortunately I had nothing to test for self-resonances, but these should be low capacitance coils.

Hopefully I can try making the 807 plate RFC tomorrow. Maybe I should go for something with even bigger radius, that way I don't have to wind as many turns as I would have to do with a smaller diameter coil form.

/Oskar
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Old 19th Nov 2005, 5:52 pm   #19
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Since I'm making my project in colaboration with the University I have to set date for the comletion and presentation, I have to hand in the documentation then also. Earlier we had settled on 12 December, Im not going to make it!

I have negotiated a new deadline with my tutor at the University, 23 January. More realistic than 12 December . He was very understanding, so it was no problem to move the presentation .. phew

/Oskar
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Old 19th Nov 2005, 8:01 pm   #20
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Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Hi Oskar,
Quote:
Originally Posted by Oskar.B
I have negotiated a new deadline with my tutor at the University, 23 January. More realistic than 12 December . He was very understanding, so it was no problem to move the presentation .. phew
Excellent, that does feel a bit more realistic. Especially since you are still waiting for some key components.

Speaking of components, then I just did the math 'properly' for your 807 PA stage, just to get an idea of how far off/close in our previous estimate of the RF load impedance was.

At 450V for B+ and Ia = 80mA, I get Rl = 2860 ohm for class C, which is pretty close to our previous estimate. Carrier power output is 25W before circuit losses, which usually runs around 10%.

If we use the commonly used value of 12 for the loaded Q and an operating frequency of 3.7MHz, I get the following component reactances/values (parasitic/stray capacitances not included, of course):

Xc4 = 238 ohm => C4 = 181pF.
Xl1 = 268 ohm => L1 = 11,5uH.
Xc5 = 31,4 ohm => C5 = 1370pF.

Frank N.
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