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Homebrew transmitter done! And it works!




 
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Author Topic: Homebrew transmitter done! And it works!  (Read 2533 times)
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K8DI
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« on: September 30, 2019, 04:36:32 PM »

I started this journey into getting on AM a couple years ago now, I saw Heil's pine board project in QST, and thought, that I could build, it'd be fun, I could hang out with my buddy Dan N8ZJV, who is a tube guitar amp junkie, and see what came of it.   Well, the Heil circuit didn't work that well, but it encouraged me to learn. I read and searched and read some more, I built a bunch of circuits on pine boards, I found the flaws in that design, and got a decent working but weak transmitter out of it. Then I went forward.  I "pine-boarded" an RF section with a 6AG7 and an 807, that worked, so I cleaned off the board and built a few modulator versions, eventually settling on line level input 6SN7 driver and a pair of 6CA7 pentodes.  Then I gathered parts, and started making my first chassis. Again, learned a lot. Made mistakes, and worked around them. There's plenty I would change if I were to do it again, but I am not, I am moving up. I started gathering parts months ago for the next one... I'll apply what I learned and then make a fresh batch of mistakes to learn from!  Having Fun!

Ed


* front.jpg (1537.81 KB, 4032x3024 - viewed 198 times.)

* rear.jpg (1277.14 KB, 4032x3024 - viewed 185 times.)

* top.jpg (1367.39 KB, 4032x3024 - viewed 235 times.)
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K8DI
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« Reply #1 on: September 30, 2019, 04:37:15 PM »

More photos


* RF side.jpg (1144.25 KB, 3024x4032 - viewed 181 times.)

* Audio side.jpg (1056.65 KB, 3024x4032 - viewed 157 times.)

* underneath the hood.jpg (1692.76 KB, 4032x3024 - viewed 228 times.)
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K8DI
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« Reply #2 on: September 30, 2019, 04:39:22 PM »

schematics

* AM-40-Oscillator-Section.pdf (30.56 KB - downloaded 109 times.)
* AM-40-Final-Section.pdf (22.09 KB - downloaded 136 times.)
* AM-40-Modulator-Section.pdf (28.74 KB - downloaded 130 times.)
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K8DI
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« Reply #3 on: September 30, 2019, 04:39:53 PM »

more of the schematics

Updated control section with T/R sequencing.

* AM-40-PSU-Section.pdf (32.14 KB - downloaded 84 times.)
* AM-40-Control-and-Meter.pdf (37.83 KB - downloaded 54 times.)
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w8khk
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This ham got his ticket the old fashioned way.


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« Reply #4 on: September 30, 2019, 05:30:59 PM »

Very Nice!  Well thought-out layout and fine workmanship.  Testing on breadboard first pays good rewards.  Except for a few of the newer connectors, switches, and the heat sink, it looks straight out of 40s or 50s vintage, great classic look.  

Please continue to share more.  Glad you are having fun and learning as you go.  That is what it is all about!  73......
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« Reply #5 on: September 30, 2019, 07:53:48 PM »

Looks very nice, fine piece of work. Congratulations
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KK4YY
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« Reply #6 on: September 30, 2019, 08:23:17 PM »

Nice workmanship. I like the audio input isolation transformer and balanced audio chain. Thanks for including the schematics.


Don
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kb2vxa
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I modulate, therefore AM


« Reply #7 on: October 01, 2019, 07:05:28 AM »

The audio input isolation transformer and balanced audio chain is AM broadcast engineering practice, and that's the way uh huh uh huh I like it. Maybe you recognize KC and the Sunshine Band sneaked in there. (;->) That's a cool marriage of old and new, all it needs is something blue. Using 6CA7 modulators is an interesting twist, reminds me of the Eico 730 mate to the unfortunately dirty 720 Novice transmitter.

A friend mad about tube type guitar amps, I wonder if he has that abortion called an Ampeg B-15 with 6 6146s in P-P parallel.
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73 de Warren KB2VXA
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K8DI
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« Reply #8 on: October 01, 2019, 09:46:46 AM »

In reply to some of the comments, thanks for the props!  The goal was vintage look, the HV and mod iron, crystals and socket, and the meter, are old/used/surplus of course. And I used  the octal 6SN7 rather than yet another 12AX7 for the same reason. Tubes are all NOS/vintage, too.

I used 6CA7's because I did not have room on the chassis for a second HV transformer in the 400 volt range to run 6L6s or many other tubes; the 6CA7 can handle up to 800v on the plates while 6L6, etc., cannot -- this way the modulator and final could both be on the 600+ volt HV power supply.

The balanced audio and transformer is from my time in broadcasting, or in recording studios, or in professional audio/live sound... I don't remember the last time I connected two audio things together with unbalanced cable!

Ed
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« Reply #9 on: October 01, 2019, 10:03:32 AM »

Kudos on your first try, Ed! Cheesy

73DG
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Carl WA1KPD
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« Reply #10 on: October 01, 2019, 10:11:03 AM »

Nice work!
What is the output range with the variable pot in the osc?
Thanks
Carl
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Carl

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K8DI
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« Reply #11 on: October 01, 2019, 04:25:14 PM »

What is the output range with the variable pot in the osc?

Not sure exactly what you mean -- the screen voltage can be lowered to zero; a bit before then, the tube stops oscillating. So output is nothing. It can be brought up fairly high; I don't monitor it, but in testing, 100-120 volts on the screen was about the limit plate-current wise, which I do monitor with the meter. It allows me to adjust grid drive on the 807. The 6AG7 is decidedly bandwidth limited. I had hoped to make this a three band rig; 80-40-20, but the 6AG7 can't give me enough output on 20. It would work to do 160-80-40 but the problem there for me in particular is I don't now and will not ever at this house have space for a 160m antenna, and I didn't have enough coil stock to make the final pi network tune 160m, so I did not even try to accommodate it. All that aside, the level adjust is to pull back the grid drive on 80m, and crank it up on 40m, to deal with the lower output level on 40m due to the tube's inherent bandwidth limitations. The 807 wants to see at least a couple mA of grid current to work; 5mA is the limit, so I meter that and adjust the drive down as needed.

807 datasheet: https://frank.pocnet.net/sheets/049/8/807.pdf

Ed
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Carl WA1KPD
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« Reply #12 on: October 01, 2019, 04:34:22 PM »

Hi Ed,
I was just wondering what sort of output you got betwen the osc. point and 5 ma of grid drive. How looooow can you go?
Carl
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K1JJ
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« Reply #13 on: October 01, 2019, 05:49:48 PM »

Great job, Ed!

All self-contained.  Nice.

About grid current on the class C final:  The 807, like any class C plate modulated final, needs as much grid current as it can handle to produce the biggest audio peaks under modulation.  Yes, you can get a full carrier with low grid drive, but when you put an audio tone through and look at it carefully on the scope at 100%+ modulation, you will find there is a sweet spot minimum where the tone waveform cleans up and the peaks are the highest with no flat topping.  I find with my plate modulated rigs that I often need to go 10% to 20% ABOVE the recommended tube grid current to find that point. (140%+ positive peaks, etc)

Loading, tank Q, stable HV and the modulator's ability to produce clean power at maximum will have an effect too.

The best test is to put an audio tone thru (sine and triangle) and adjust all rig parameters until you find the cleanest and highest peaks. Be careful to use some form of negative peak limiting, especially when testing, to protect the mod iron.

Good luck on the next rig.  BTW, place your small parts (underneath the rig) at perfect right angles for the best aesthetics.

Tom, K1JJ
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« Reply #14 on: October 01, 2019, 05:52:45 PM »

Hi Tom,
Looking at the circuit as a driver, so the plate modulation is not a factor in ther 807 stage. Just wondering wht the range of drive I could get would be
Carl
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Carl

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« Reply #15 on: October 01, 2019, 05:53:49 PM »

The slab Bacon had a small QTH; check out his antenner fer ideas.


AKA kb3ahe


KLC
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KC2ZFA
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« Reply #16 on: October 01, 2019, 07:31:22 PM »

nice work !

what is the role of D1 in the plate feed to the oscillator ?

Peter
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« Reply #17 on: October 02, 2019, 12:39:24 AM »

congrats Ed, 6CA7's are one of my choices for modulator tubes as well.

Phil - AC0OB
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K8DI
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« Reply #18 on: October 02, 2019, 08:39:13 AM »

what is the role of D1 in the plate feed to the oscillator ?

I wanted a regulated supply for the oscillator for stability. The 400v supply is regulated, but the voltage is too high for the tube's ratings. If I drop it with a resistor it will vary with the tube's plate current draw; it will not be regulated.  A reverse-biased zener will have a fixed voltage drop, it will act like a variable resistor self-adjusting to always have that drop. So the 100v zener means I have a regulated 300v supply for the oscillator.  Using a zener to regulate this in the conventional way (resistor in series, diode as a shunt) would mean I need a 300v zener or a stack of several totaling 300v. Using it this way only takes one, that I already had a few of on hand because I used some in the 400v regulator circuit. It's a 5 watt unit, the typical tube current in operation is 20mA, for 2 watts dissipated.

Ed
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K8DI
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« Reply #19 on: October 02, 2019, 08:48:02 AM »

About grid current on the class C final:  The 807, like any class C plate modulated final, needs as much grid current as it can handle to produce the biggest audio peaks under modulation.  Yes, you can get a full carrier with low grid drive, but when you put an audio tone through and look at it carefully on the scope at 100%+ modulation, you will find there is a sweet spot minimum where the tone waveform cleans up and the peaks are the highest with no flat topping.  I find with my plate modulated rigs that I often need to go 10% to 20% ABOVE the recommended tube grid current to find that point. (140%+ positive peaks, etc)
...
The best test is to put an audio tone thru (sine and triangle) and adjust all rig parameters until you find the cleanest and highest peaks. Be careful to use some form of negative peak limiting, especially when testing, to protect the mod iron.

Tom, K1JJ

Thanks for the kind words and advice, Tom. I've been testing with sine waves (my audio signal generator is the oldest piece of test gear I own, it only does sine and 'square-ish' waves). I will experiment with higher grid drive and see if the upper levels of modulation clean up a bit. Right now it gets cleanly to about 80 percent and then not as nice. If I crank the audio it will go way past 100% but with plenty of distortion on the positive peaks. Do you have a recommendation for cheap and easy negative peak protection on the transmitter (not audio processing/asymmetrical audio limiting, but mod trans safety/protection)??

Ed
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kb2vxa
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I modulate, therefore AM


« Reply #20 on: October 02, 2019, 09:03:16 AM »

Just one error Ed, the 12AX7 is not the equivalent of the 6SN7, the 12AU7 is. They are both medium mu twin triodes, the 12AX7 is a high mu twin triode. I don't remember if it has an octal equivalent or not.
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K8DI
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« Reply #21 on: October 02, 2019, 09:09:55 AM »

Just one error Ed, the 12AX7 is not the equivalent of the 6SN7, the 12AU7 is. They are both medium mu twin triodes, the 12AX7 is a high mu twin triode. I don't remember if it has an octal equivalent or not.

6SL7 is the high-mu octal.

I wasn't intending to mention a substitute or equivalent, only that the 12AX7 is likely the most common audio tube in existence...

Ed
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« Reply #22 on: October 02, 2019, 09:27:28 AM »

Here is a Soft Knee Limiter I use for my DX-100's, Apache and my Viking II-CDC that was written up by Tom Bonomo K6AD in ER.

Adjusting the values of zeners DZ1-DZ3 determines when the limiting starts to occur.

This circuit protects against negative voltage transients and limits negative modulation.

Phil - AC0OB

* Soft Knee Limiter.pdf (99.69 KB - downloaded 73 times.)
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KC2ZFA
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« Reply #23 on: October 02, 2019, 09:49:54 AM »

Very nice, thanks for this hint&kink.


I wanted a regulated supply for the oscillator for stability. The 400v supply is regulated, but the voltage is too high for the tube's ratings. If I drop it with a resistor it will vary with the tube's plate current draw; it will not be regulated.  A reverse-biased zener will have a fixed voltage drop, it will act like a variable resistor self-adjusting to always have that drop. So the 100v zener means I have a regulated 300v supply for the oscillator.  Using a zener to regulate this in the conventional way (resistor in series, diode as a shunt) would mean I need a 300v zener or a stack of several totaling 300v. Using it this way only takes one, that I already had a few of on hand because I used some in the 400v regulator circuit. It's a 5 watt unit, the typical tube current in operation is 20mA, for 2 watts dissipated.

Ed
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K8DI
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« Reply #24 on: October 02, 2019, 09:54:41 AM »

Hi Ed,
I was just wondering what sort of output you got betwen the osc. point and 5 ma of grid drive. How looooow can you go?
Carl

If you mean oscillator output voltage, it still oscillates down to below 25 volts peak to peak output. If you mean 807 output, you really don't have much room to reduce output before the plate current goes through the roof. This is due I believe to the resistor bias/bias due to the self-rectification of the drive. If you wanted to drop the output power then a bias supply or cathode resistor would probably need to be added to keep the tube at a reasonable DC operating point with low or no drive.  

Ed
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