Here's an e-mail sent by Steve Hemphill to Paul McLane, to explain some of the background information on the creation of the phasitron reproduction:
Saturday, April 27, 2002 3:24 AM
Subject: Cool Stuff Award for GE BT-11B / Steve Hemphill
To: radioworld@imaspub.com
Dear Paul,
I thank you for the honor you have given me by selecting me to receive a Radio World Cool Stuff Award for my GE BT-11-B transmitter reproduction.
I must admit that I was totally surprised at the attention that it had generated in Arno's booth this year at NAB. I thought that maybe just a few really old-timers might recognize it. Erich (Arno's son) and Dwight Macomber (one of Arno's staff engineers) had each telephoned me during the show to inform me of the attention that the BT-11-B was creating. I'm so glad that it helped Arno get more traffic into his booth. He's a good friend and he was very kind to host the transmitter on the show floor.
I had originally planned to be present at the show this year, but with my dad's passing in just the week before, I thought it appropriate to be home with my mom and brother at that time.
I am sorry that I was not there personally to meet with you.
I'm getting long-winded here, so let me try to answer your questions as briefly as I can.
The reason that I decided to build the transmitter was that I was completely enamored with the Phasitron FM modulation concept, which was developed by Dr. Robert Adler of the Zenith Radio Corporation in the 1940's. It was later commercialized by the General Electric Company, who used the design in their early FM broadcast transmitters.
Dr. Adler's idea of producing a phase-modulated carrier by utilizing a highly focused electron beam (produced by a special electron tube, the Phasitron) , and then influenced by an external magnetic field produced by a deflection coil placed around the Phasitron tube, totally mesmerized me! Crystal controlled wide-band FM with a minimum of components (for 1946!) WOW! What a concept!
My high school radio station, WHHS-FM in Havertown, PA., in my home town, originally went on the air in 1949 with a GE BT-11-B transmitter. The original unit had been replaced (due to the obsolescence of Phasitron tubes) in 1969 with a Gates (predecessor of Harris Broadcast) model BFE-10C 10 watt transmitter by the time I got there. I remember the fond memories of our technical adviser, Charlie Higgins, about the GE unit and it's robustness and quality of construction. Additionally, I believe that WHHS-FM was the first non-commercial high school FM licensee on the air in the U.S. Buy the way, I have restored the Gates BFE-10C transmitter and it sits proudly in my exciter collection!
One of the stations' alumnus, Dave Weston, had very kindly provided me with copies of some 35mm slides which he had taken as a student in 1968, of both the front and rear views of the original GE transmitter. These photos allowed me to recreate the panel layouts, as all original documentation from GE had been lost.
I had been working on the project for about 6 months. Bill Gillman sent me an original GE instruction manual for a model BT-1-A transmitter, which is a 250 watt version that used the same Phasitron modulator panel as the 10 watt version that I built.Thanks to Bill and his donation of the manual, I was able to replicate most of the original circuitry and layouts.
I was able to find original tubes and some components from suppliers on the Internet.
One local supplier in my area, Fred Chassey Electronics, actually had an old GE 1KW AM rig which he allowed me to part-out. I was able to obtain many original parts used in the FM version of the GE. Fred also supplied me with the power supply components that I needed.
Arno, at Belar, allowed me the full use of his in-house sheet metal shop where I did all of the chassis and panel construction work. I actually made production templates and silk-screens for the project. Talk about getting carried-away! It seems the farther along I went, the more fanatical I became!
I had custom coil shield cans made and I also had a special Allegheny MU-metal shield can constructed for the Phasitron tube so that the mechanical design would remain original. I designed and hand-wound all of the RF multiplier coils and transformers. No details were provided by GE on these items, so I had to do them from "scratch". I even had to hand-wind the Phasitron modulation coil. It's over 4,000 turns of #36 wire! I had to review Maxwell's equations on flux density to calculate the design of this coil. It was a lot of fun and a very good learning experience!
The project has cost me in excess of $7000.00 to date and if I had known that it was going to go to this level of expense, then I am sure that I would not have started it. You can see in this experience that love and good sense are mutually exclusive!
Here is some of my background - Mario Hieb had asked me about it.
I started out as a design engineer working for Gately Electronics in Havertown, PA. which manufactured audio consoles and equipment for the professional tape and disk recording industries. (Remember those ?).
I always had an interest in high quality FM broadcast audio and I started my own company, Solid Electronics Laboratories, in 1973, originally to market FM exciters,stereo and SCA generators and related accessories.
My designs were ultimately used by Singer Broadcast Products, who later was acquired by CCA in Georgia.
I now market and sell an FM broadcast composite processor and low pass filter, the Model DCP-1A, which I have exhibited at NAB in the past, and I do design and consulting work for Arno at Belar in Devon.
Thanks, again, Paul. I hope that I have answered your questions without going overboard!
Sincerely yours,
Steve Hemphill SOLID ELECTRONICS LABORATORIES
Here's another e-mail from Steve Hemphill, where he reports on his STEREO equalizer success for the Phasitron:
Wednesday, May 08, 2002 8:52 PM
Subject: The Phasitron goes STEREO!!!!
Hi, Dave,
I also wanted to let you know some more good news - I have made a working adapter for my BT-11-A which allows it to operate in full stereo. I had tried this early on in the project when I had originally made the transmitter multiplier bandwidth excessively wide. It worked fairly well except for the crystal oscillator sideband issue. Adjacent channel users ( and the FCC) wouldn't have been too happy with that version!
I made up a high-pass response to compensate for the falling multiplier stop-band loss. It is a slightly modified Chebyshev 4th order with some L+R summed-in to provide the mono signal path thru the filter. I have also provided an adjustable low-frequency tilt corrector to improve the somewhat - irregular Phasitron frequency response at and below 50 Hz.
I didn't curve-fit the loss response exactly ( I would have to model the transmitter stop-band characteristics more accurately), but I think it performs very well as I now have it. It really sounds great! I can re-broadcast another station's composite thru it and, by A-B comparison, you can't tell which signal is the original and which is the re-broadcast!
Here are the measured stereo separation numbers :
50 Hz -27.5 dB
500 Hz. -38 dB
5000 Hz. -28 dB
10K Hz. -28.5 dB
15K Hz. -19.75 dB
There is some truncation of sidebands at 53 KHz. due to my imperfect loss response matching, but I think it will be good enough for now.
Thanks for the story about injecting the 19 KHz into your college station's Phasitron transmitter. I really enjoyed that! Believe it or not I had done the same thing when I was a youngster fooling around with modulated oscillators on the FM band and broadcasting to my neighborhood.
I didn't know how to generate the stereo baseband signals in those early days, but I knew that just sending out the 19 KHz signal would at least light up the stereo pilot lights on my listener's radios. I thought that I might fool the listeners that I was actually a real stereo basement broadcaster!
I also wanted to update you on the history of my high school's GE Phasitron transmitter.
It was originally listed on the station's FCC Construction Permit File # BPED-99 dated the 15th of February, 1949. The station went on the air with it in that year and it remained in continuous service until it was replaced with a Gates Model BFE-10C in October of 1968. That's 19 years of constant service! Not too shabby!
It might interest you to know that I have the original GE Model BY-1-C Single Ring antenna that was supplied by GE to my school with the Phasitron transmitter. It's in fairly good shape and still works great! I couldn't bear to see it get scrapped and I'm glad I could save it.
Regards,
Steve H.