HomeBrew - 455Kc Low Level, Plate Modulated AM Transmitter

Force them Rigs to accept True Voodoo!
Post Reply
User avatar
Voodoo Guru
VooDoo Site Admin
Posts: 718
Joined: Wed Feb 23, 2005 8:47 am
Location: Down there on the right...

HomeBrew - 455Kc Low Level, Plate Modulated AM Transmitter

Post by Voodoo Guru » Sat Feb 09, 2013 10:03 am

Howdy Y'all,

As some of you know, about 2 years ago we started on a journey to make the best sounding AM Modulator for the TS-950sdx. 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). We just never really liked the way they all sounded, so of course, the next evolution would be Hollow State.

Several tubes were tested 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. Allot of different configurations were gone through and the following design was the best performer.

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

First off, a stable oscillator and an amplifier was needed that would deliver enough 455Kc Carrier to drive the Hy-615 Triode into Deep Class C. After researching this in depth, We came up with a mixture of different circuits to provide just that. A mixture of circuits from David White - WN5Y, Wes Hayward - W7ZOI, & Paul Harden - NA5N were incorporated for the final design.

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.

Image
CLICK HERE TO SEE HI REZ PIC

Image
CLICK HERE TO SEE HI REZ PIC

Image
CLICK HERE TO SEE HI REZ PIC

The next thing needed was a Modulator. We 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, a circuit board kit from Chipamp.com was used. 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 what is need, so there is gobs of headroom to play around with.

Image
CLICK HERE TO SEE HI REZ PIC

Image
CLICK HERE TO SEE HI REZ PIC

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. We 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. We 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?!!

Image
CLICK HERE TO SEE HI REZ PIC

Image
CLICK HERE TO SEE HI REZ PIC

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.

Image
CLICK HERE TO SEE HI REZ PIC

The output of the 455Kc Transmitter is padded down and injected into the 455Kc TX IF of the TS-950sdx. The only function of the SDX at this point is to hetrodyne the 455Kc up to the operating frequency of choice, and preamplify this signal for the High Power Linear. All the stages of the SDX TX IF were tested 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 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.

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

Image
CLICK HERE TO SEE HI REZ PIC

Image
CLICK HERE TO SEE HI REZ PIC

Image
CLICK HERE TO SEE HI REZ PIC

Image
CLICK HERE TO SEE HI REZ PIC

Image
CLICK HERE TO SEE HI REZ PIC

Image
CLICK HERE TO SEE HI REZ PIC

Hope Y'all enjoyed looking at this as much as We did building it! We're still making improvements and modifications to it so by all means, please feel free to make any comments, observations, or critiques to our design, or build.
Thanks,
Voodoo Guru
From Deep in the IDD of the Sub-Harmonix Realm

User avatar
w5kai
Voodoo Audio Head
Posts: 33
Joined: Sat Mar 19, 2011 7:33 pm
Location: Salty Swamps of Mississippi

Post by w5kai » Mon Feb 25, 2013 5:00 am

Hello Mike,
I've heard you on AM with this unit and it sounds great. I think you've found the right design for the vmsdx. I was blown away at the AM modulation of the vmsdx compared to the heavy irons you were talk'n to. Not to mention it Purty too. Hope your have'n a blast AM'n.
VMSDX # 24
Special Zombie Edition

Dave457
Voodoo Audio Head
Posts: 27
Joined: Tue Jan 24, 2012 1:45 pm

Post by Dave457 » Tue Mar 05, 2013 3:32 pm

Do you have any audio samples ?

User avatar
Voodoo Guru
VooDoo Site Admin
Posts: 718
Joined: Wed Feb 23, 2005 8:47 am
Location: Down there on the right...

Post by Voodoo Guru » Tue Mar 05, 2013 4:05 pm

Well, here is one from 15 meters, but the signals were not that great.
Also the recording station had some local QRN.
But you can get the jist of it...


http://www.wz5q.net/mp3/wz5q_from_ve6cq ... -22-13.mp3

Here is another one from a few days ago, again the band conditions were not that great.

http://www.wz5q.net/mp3/wz5q_from_ve6cq ... 3-4-13.mp3

Can't wait for 10m to open up again!!
Voodoo Guru
From Deep in the IDD of the Sub-Harmonix Realm

Dave457
Voodoo Audio Head
Posts: 27
Joined: Tue Jan 24, 2012 1:45 pm

Post by Dave457 » Wed Mar 06, 2013 8:29 pm

Great sound, heard you on 15 meters today as well, smooth sounding audio :D

Post Reply