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HomeBrew - 455Kc Low Level, Plate Modulated Transmitter




 
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Author Topic: HomeBrew - 455Kc Low Level, Plate Modulated Transmitter  (Read 23637 times)
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WZ5Q
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« on: February 04, 2013, 07:20:57 AM »

Howdy Y'all,
I have been asked allot about my HomeBrew Transmitter while on the air. I figured I would post the entire project here on the forum for your viewing pleasure as well as for others to look at in the future.

I have always been disappointed in the Audio Quality of Rice Box AM Transmissions, especially the rigs using DSP to generate the modulated waveforms. To me, their sound lacked Depth, Density, and were just plain Sterile sounding...Lifeless. You might be thinking, "What a minute, this guy sounds like one of those Audiophools!!!" Well Yes, as a matter of fact, I am. Wink  I have been playing with audio for a very long time and can tell minute differences with my ears. I know what I like when I hear it, and I can tell whats wrong with it when I don't like it. I have accumulated allot of test equipment over the years, but nothing as sensitive as the Original Test Equipment the Almighty gave me, my ears. Of course I realize that this particular set of test equipment is very subjective, but I am the one that is listening to it after all.

About 2 years ago, I started on a journey to make the best sounding Low Level Modulator for the Rice Box. After prototyping many, MANY different AM Modulation schemes which included; balanced modulator IC's (yuk spit!!), Trans-conductance Amplifier IC's (better, but still yuk), Differential Amplifier Modulation (better yet), Dual Gate MosFets (even better yet), a pair of Jfet's in Push-Pull - Series Drain modulated by a class AB1 amplifier (best out of the previously mentioned). I just never really liked the way they all sounded, so of course, the next evolution would be Hollow State.

I tested several tubes for the best performance at Plate Modulating Phone at 455Kc. This would include the 6C4, the 12B4A, the 12BH7A, and finally the HY-615. The Hy-615 excelled over all the others and was chosen as the final candidate for this project. I went through allot of different configurations and finally determined that the following was the best performer.

I will refrain from posting all the math involved in calculating the impedance matching and transformers, so I will not bore you with it unless asked.

First off, I needed a stable oscillator and an amplifier that would deliver enough 455Kc Carrier to drive the Hy-615 Triode into Deep Class C. After researching this in depth, I came up with a mixture of different circuits to provide just that. I used a mixture of circuits from David White - WN5Y, Wes Hayward - W7ZOI, & Paul Harden - NA5N. A Pierce Oscillator circuit was used which is tuned thru a 455kc IF Transformer and converted to an impedance of 50KΩ. This signal is attenuated 12.5dB with a PI Resistor Attenuator and then amplified by Q2 & Q3 to 8Vpp to drive Q4 in Class C. T9 changes the low output impedance of Q4 to 50 ohms for the 7-pole low pass filter. Output is 455Kc, up to 1.5 watts into 50Ω with the worst-case Harmonic down by 46dB. The Signal has a very clean & stable Sine Wave.


http://www.wz5q.net/images/455kc%20transmitter/455kc_oscillator_amp_2-13.gif


http://www.wz5q.net/images/455kc%20transmitter/oscillator.jpg


http://www.wz5q.net/images/455kc%20transmitter/carrier_amp.jpg

The next thing needed was a Modulator. I first started out using a 20watt, PP Ultra-Linear Audio Amplifier using a pair of 5881's. Since small size was going to be an important aspect of this project, it was decided to use a 60-watt Solid State Class AB1 Audio Amplifier using the LM3886 IC. In order to save some time, I used a circuit board kit from Chipamp.com. The Modulation Iron is an Edcore 10 watt Single Ended Transformer with a 10KΩ Primary and a 4Ω Secondary, operating in reverse. This is used in a Modified Hiesing configuration thru a 10uf capacitor and a 30H 100mA Modulation Reactor. This set up will modulate 300 volts WAY more then I need, so I got gobs of headroom to play around with.


http://www.wz5q.net/images/455kc%20transmitter/455kc_modulator_2-13.gif


http://www.wz5q.net/images/455kc%20transmitter/modulator.jpg

Now came the RF Deck and HV Power Supply. The HV Power Supply uses a full wave bridge, dual stage choke input filter for +300vDC. I went with a rather large power transformer for improved regulation. The Filament Supply uses a full wave bridge, capacitor input filter using an artificial center tap.

The RF Deck is of course using the HY-615 Triode. The 455Kc Carrier is fed into the grid through an input transformer wound with 1 turn on the Primary and 18T on the Secondary. This equates to 50Ω on the Pri and 12KΩ on the Sec of the BC-73-202 Core. By using Grid Leak Resistance, the 275mW of Grid Drive is providing for -40vDC Grid Bias and 1mA of Grid Current which puts this tube operating in Class C at a little over 2x the Cut Off Voltage. I have +290vDC on the Plate running at 10mA of Plate Current. The output transformer was wound with a Split Winding, Center Tapped Primary to provide an opposite phase signal to the Grid for Neutralization. It was wound on FT-250A-J Core material. The output winding is 2 turns and this feeds the signal into the 7 Pole LPF, then into the 50Ω Load and the variable output control. The reason for using an Aperiodic match was again mandated by size constraints. Can you imagine how big a Tank Coil for 455kc would be?!!


http://www.wz5q.net/images/455kc%20transmitter/455kc_rfdeck_2-13.gif


http://www.wz5q.net/images/455kc%20transmitter/rf_deck.jpg

Performance of the tube is terrific. With 275mW of Carrier Drive, the output is at 325mW of Carrier. When modulated by a 1Kc Tone, the output is 1.3W PEP Sinusoidal. Headroom is also good as up to 200% Positive Peaks can be achieved.


http://www.wz5q.net/images/455kc%20transmitter/tx_mess.jpg

The output of the 455Kc Transmitter is padded down and injected into the 455Kc TX IF of a Rice Box. The only function of the Rice Box is to hetrodyne the 455Kc up to the operating frequency of choice, and preamplify this signal for the High Power Linear. I went through all the stages of the Rice Box to be sure that everything was linear, it was, but only if the Power and Carrier Controls were set to maximum gain, you didn't trip the ALC circuitry, and the AM Signal didn't go thru any IF Filters. This particular rig will pass positive peaks easily and the measured THD is 3% at 100% Modulation, 2.5% at 90% Modulation measured on a Tektronics 2712 Spec An. The Spectral Purity is also great with the worst case spurious signal measured more then -50dB down. Frequency Response is flat from 30cps to 12Kc at -3db points.

I have since transferred all the bread boarded components to a chassis. Here are a few pics of the almost finished project. I have some Vinyl Lettering that will be applied for the nomenclature. I also have some other circuitry that needs to be installed for indicator lamps.


http://www.wz5q.net/images/455kc%20transmitter/455kc_front.jpg


http://www.wz5q.net/images/455kc%20transmitter/455kc_underside.jpg


http://www.wz5q.net/images/455kc%20transmitter/455kc_rear.jpg


http://www.wz5q.net/images/455kc%20transmitter/455kc_top.jpg


http://www.wz5q.net/images/455kc%20transmitter/455kc_side.jpg


http://www.wz5q.net/images/455kc%20transmitter/455kc_side2.jpg

I hope Y'all enjoyed looking at this as much as I did building it! I am still making improvements and modifications to it so by all means, please feel free to make any comments, observations, or critiques to my design, or build.
Thanks,
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Mike
WZ5Q
From Deep Down in the Dark Dismal Swamps of the Calcasieu
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« Reply #1 on: February 04, 2013, 11:41:35 AM »


"What a minute, this guy sounds like one of those Audiophools!!!"

There is a difference between "pfool" and "phile"


 You've done some fine work. It really louks marvelous.


klc
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W2PFY
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« Reply #2 on: February 04, 2013, 12:25:06 PM »

Quote
What a minute, this guy sounds like one of those Audiophools

I couldn't tell if the HY615 was the all important "black Plate" design? I have a HY 114-B here with no use for it. Free for the taking if you need it. I can't find any data on the HY types.

Kidding aside, where do you operate that transmitter? yes, I am behind the times with this new to me VLF stuff.

Glad to see your having some fun there!
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The secrecy of my job prevents me from knowing what I am doing.
WZ5Q
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« Reply #3 on: February 04, 2013, 01:46:17 PM »

Howdy Y'all,

You've done some fine work. It really louks marvelous.
klc

I truly appreciate the complement, Thanks.


I couldn't tell if the HY615 was the all important "black Plate" design? I have a HY 114-B here with no use for it. Free for the taking if you need it. I can't find any data on the HY types.

Kidding aside, where do you operate that transmitter? yes, I am behind the times with this new to me VLF stuff.

Glad to see your having some fun there!

The HY-615 IS a Black Plate, AND it has that rare "scoop" Getter for really digging into the Sonics and releasing those even order Harmonics!  Wink

Thanks for the offer on the Tubage, but even though the price is right, I will have to decline. When I bought the HY-615, it came as a set of 20 for a price a couldn't refuse. So I am Hytron poor right now.

The transmitter is not a VLF Rig, although I did use some of there Tech in the design.  I am using it to generate, then inject the 455Kc AM Signal into the TX IF of a Rice Box, and using the Rice Box to bring me up to the operating frequency. I am usually in the AM Window of 75, but also play on 15, 40, and 160m.

Take Care,
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Mike
WZ5Q
From Deep Down in the Dark Dismal Swamps of the Calcasieu
http://www.wz5q.net
ke7trp
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« Reply #4 on: February 07, 2013, 02:30:26 PM »

I heard this transmitter today on 15 meter AM.  It sounds awesome.  Very clean, hifi and Better then BC sound.  Good job Mike!

C
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KL7OF
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« Reply #5 on: February 07, 2013, 05:36:06 PM »

I heard this transmitter today on 15 meter AM.  It sounds awesome.  Very clean, hifi and Better then BC sound.  Good job Mike!

C
I worked Mike on 15 M AM today......Wazzat the rig you were using Mike?.....Nice Job!
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WZ5Q
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« Reply #6 on: February 08, 2013, 04:40:53 AM »

I heard this transmitter today on 15 meter AM.  It sounds awesome.  Very clean, hifi and Better then BC sound.  Good job Mike!
C

Thanks Clark, much appreciated! Signals were terrific, you were 40dB over most of the time.


I worked Mike on 15 M AM today......Wazzat the rig you were using Mike?.....Nice Job!

Thanks Steve! Yes, that was the rig you heard on the air. I am really enjoying the performance of this transmitter. It was good to work you again, especially with such an incredible signal.
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Mike
WZ5Q
From Deep Down in the Dark Dismal Swamps of the Calcasieu
http://www.wz5q.net
WD4SCZ
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« Reply #7 on: March 25, 2013, 10:13:46 AM »

Howdy Mike,

All I can say is that having heard your transmitter on several occasions on 3725 kc in the early morning that it sounds great! I am certain I have a much easier time hearing you than you do my pipsqueak Globe Scout. 

Emil WD4SCZ
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WZ5Q
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« Reply #8 on: March 25, 2013, 02:37:58 PM »

Howdy Emil,
Thanks!! It's always good to hear that your stuff is workin.
I enjoy yakin at you on your Globe Scout, it has some great audio too!
Take Care,
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Mike
WZ5Q
From Deep Down in the Dark Dismal Swamps of the Calcasieu
http://www.wz5q.net
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