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Gates FM1 Exciter question

D

darklife

Just looking around in interest for old tube based FM exciters and I guess the FM-1B was popular.
I read it ran 10 watts out and had a separate stereo generator? What did it use as an output tube?
Anyone ever use one of these back in the day and if so what did it work/sound like on air?
Just curious about a little piece of history.

Here's a picture from a different website of the exciter I am asking about..
http://radiolanes.com/GatesExciterafter.JPG

Also interested in other early FM exciters if anyone knows of some.
 
I recently gave one (part of a 1960's Gates 50 W FME transmitter) to a local ham group to play with. I'd previously used one at KUGN-FM in Eugene back in the late 70's, which was one of the first stereo FMs on the west coast, using an early Standard stereo generator.

These exciters used serrasoid and not direct FM, and didn't sound very good. Serrasoid impresses the audio waveform on a modified triangle wave, with resultant narrow band FM at a low frequency which is multiplied many times to get the final output frequency. Stereo was injected not as baseband composite, but separate from the mono as a relatively narrow band input. When we upgraded KUGN-FM to a new Continental direct FM exciter back in 1977, the difference in audio quality was truly amazing.

RW did an excellent write-up on serrasoid FM:

http://www.rwonline.com/article/71272
 
I still have one of those along with the FM1B ipa and final. Took it in payment for a job and it's sat in my storage unit ever since.

I used one for a short time many years ago in Ohio. The premphasis was all inductors and caps, nothing like today and it rang like crazy. That forced you to keep the modulation low even wiuth the best processing available at the time. If anyone ever told you it made 10 watts they are crazy. Seven was more like it. I don't remember what it used as an output tube I'll have to take a look next time I'm out there.

It was pretty stable though. The transmitter had other problems, the 50 watt ipa barley did 40 and it went through 6146's like water. The final had to be reneutralized on a regular basis or it would actually blow a hole through the glass of the final 4-400's if it ran much over 800 watts which mine did. Because of it's low price there were many in use in Northern Ohio from 47 to upgades in the 90's mine went out of use after a fire in 1995. The one I used in Ohio I heard is still in storage.
 
Wow that is interesting about the serrasoid modulation. I never heard of that before until now. Interesting how it resembles digital sampling to some extent.
I always wondered how they got crystal oscillators to modulate with wideband since most crystals can't be pulled far off frequency for self modulation w/o dropping out.
Makes me wonder what other modulation techniques were used for FM that I don't know about.
 
I think this unit used a 6360 output tube. From what I can remember the crystal frequency was around 110 to 120khz and was multiplied about 832 times. This thing used what I think was a type of phase modulation of the crystal and any change in frequency was also multiplied 832 times. I think I have the book on one of these if you need any info from it let me know and I will try to look it up when I get time.
 
The FM-1 sounded pretty good in mono however that is where it stopped. The exciter held power very reliably with a good 6360 output tube and there was never a problem with frequency stability. A lot of Class D stations used this exciter early in their life. The frequency nonlinearity and baseband bandwidth did not favor stereo. It had to be carefully aligned to sound good in mono.

If you look at the picture posted in this thread, I built a copy of the last stages of the exciter (right side from the 7MC point on up ) and drove it with a varactor-modulated 7MC oscillator. It made a nice "experimental" transmitter in the late 60s. I used a Motorola MV850 and a cascode-buffered silicon transistor vfo. It did not take much deviation of the oscillator to get +/- 75KC swing at the carrier frequency with a 13 times frequency multiplier. It took some experimentation to find the most linear bias point of the varactor which required changing the L and C values of the oscillator circuit to compensate.

Fond memories of a fun time period during school time. Thanks for the reminder.
 
You ask about other early forms of FM modulation. The most interesting one I came across was found in the Early GE FM transmitters that came out in the late 40's after the band changed to 87.9 to 107.9. The exciter that they came out with used a thing called a phasatron tube.

It was a special tube that required a series of coils be placed over the glass envelope and then alligned. I never tried the allignment process although I did shift frequency on one.

In the middle 70's I happened to run into a guy who was still running phasatron modulation on a station in Findaly, Ohio. He was very excited about finding a couple of brand new phasatron tubes and exciters that weren't being used and I sold him everything I had for a very good price. By 1980 he had added stereo and a new transmitter and that was the last phasatron tube modulator that I know of in use in Ohio or the midwest for that matter.
 
The Exciter shown on the first post is my 5534, which was the earlier (circa 1960) exciter in the FM-1B. When it was in service back in the early 70's it actually sounded pretty good with a new set of tubes and tuned up.

Lane
 
All about the GE Phasitron exciter:

http://www.w9gr.com/phasitron.html

The Phasitron was capable of greater phase shift than the Serrasoid modulator, so it required one less frequency doubler. The total multiplication factor in the GE exciters was 432, rather than 864 as in the Gates, CCA, ITA and other Serrasoid exciters.
 
Freebird,

Thanks for the link. Those pictures bring back memories. The first station I owned ran on that same 250 watt GE transmitter but with a Gate exciter driving it. I paid $700 for the 250 watter and it looked very much like that in the picture including the stain on the transformer ( must have been a leak in those ) It was reliable and I got 1 KW ERP out of it by using a five bay horozontal antenna. I eventually sold the GE to a noncommercial station in KC.
 
Many years ago (in the '70s), I worked for a station which had one of those old Gates Exciters in a 5G transmitter.
They had the companion Stereo Generator AND the SCA Generator.
The station always had a 9 kHz whistle on their carrier. The whistle changed in amplitude with modulation of the main carrier.
I discovered that the 67 kHz subcarrier generator was heterodyning with the second harmonic of the 38 kHz stereo subcarrier.
The problem was finally resolved when the entire Exciter system was replaced with a new, solid state one.
 
frankberry said:
Many years ago (in the '70s), I worked for a station which had one of those old Gates Exciters in a 5G transmitter.
They had the companion Stereo Generator AND the SCA Generator.
The station always had a 9 kHz whistle on their carrier. The whistle changed in amplitude with modulation of the main carrier.
I discovered that the 67 kHz subcarrier generator was heterodyning with the second harmonic of the 38 kHz stereo subcarrier.
The problem was finally resolved when the entire Exciter system was replaced with a new, solid state one.
Click to expand...
That was a common issue in the early days of FM. Even some of the "big" Cincinnati area FM stations (big in terms of power...in the 60's, none of them were big in terms of ratings) had the 9khz squeal in the background. Turning the Muzak off was not an option...it brought more revenue to the table than the main channel. One station opted to broadcast in mono during the day so they could have more SCA's while a certain chain of stores was open. As the exciters and receivers improved, it became possible to have their cake & eat it too--w/o being serenaded by a 9khz whistle.
 
CaptBob92 said:
I still have one of those along with the FM1B ipa and final. Took it in payment for a job and it's sat in my storage unit ever since.

I used one for a short time many years ago in Ohio. The premphasis was all inductors and caps, nothing like today and it rang like crazy. That forced you to keep the modulation low even wiuth the best processing available at the time. If anyone ever told you it made 10 watts they are crazy. Seven was more like it. I don't remember what it used as an output tube I'll have to take a look next time I'm out there.

It was pretty stable though. The transmitter had other problems, the 50 watt ipa barley did 40 and it went through 6146's like water. The final had to be reneutralized on a regular basis or it would actually blow a hole through the glass of the final 4-400's if it ran much over 800 watts which mine did. Because of it's low price there were many in use in Northern Ohio from 47 to upgades in the 90's mine went out of use after a fire in 1995. The one I used in Ohio I heard is still in storage.
Click to expand...

Owners liked them. Same tubes as the AM and a picture window. WBNL Boonville used one in the 90's.
 
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