The smaller encoder for fire station dispatch office use (up to four calls plus pulsed alert) was the model G21. A Model G22 was available for use in a car, it was a takeoff on the G21 encoder, powered by 12VDC.
Plectron's version of an emergency backup for their encoders was a thing of beauty. TUNING FORKS sold in the exact tone frequency that was needed to set off your department's alerting receivers. For a single tone alert the procedure was very simple, you struck the tuning fork & held it up to your microphone for 10 seconds or so and then gave your dispatch. Duotone combinations were a little harder (yes, two tuning forks were involved which needed to be struck in the correct sequence). Dispatchers prayed they never had to go to backup if their encoders went down!!
One of the classic encoders of all time was the Plectron Model G2 vacuum tube-type encoder. This was available in either an 8 or 16 button configuration, with a cam and motor for tone timing (either steady or interrupted - pulse) and a LIGHTBULB used to regulate voltage on the tones. It was built like a tank! This encoder was manufactured from the 1950's on into the late 1960's - early 1970's. Another encoder, very rare, was the Plectron G5 encoder, this was a single or dual-tone encoder with an on-off switch that was designed for mobile use (from a Fire Chief's car for instance). I found one on an EBAY auction in November, 2017 which I promptly purchased, the one I have is a dual-tone unit. I'll put up a few pictures of it soon.
Later on in it's history, Plectron (through WEI) entered into business relationships with Federal Signal, their final encoder line was basically built on top of another encoder platform as their G25 encoder - this unit was the replacement for the G8 and G21 series and was internally dip switch programmable. The desktop encoder was available in either a five code (G25-5) or an eighteen code (G25-18) configuration. The G15 console mount encoder was also offered for dispatch centers with console furniture, and was available in either a 20, 40, or 60 code configuration. These encoders were offered starting around 1990 until the remaining business of the company was absorbed by Federal Signal.
I'd bet right now that your first look at the inside has you thinking "OH MY GOD!". Welcome to solid-state tone alerting ala 1972. As you can see, each tone has a separate plug-in circuit board as do the three timing intervals for the tones. Timing on the top row, then "A" tones, then "B" tones. Power supply board on the right, feedback speaker on the left, amplifier boards, alarm board, three motherboards, boards for all occasions in this puppy!
This is a different encoder that was manufactured three years later (1981). This G8 encoder was used with the lighted 20-button remote head, note the extra board to the left of the alarm board. I put this picture in for comparison purposes, the same design was used until roughly 1989-1990 when the G25/G15 series of encoders were released.
The Plectron G2A encoder I've documented for you today comes courtesy of the Cocoa, FL Fire Department, it was one of two encoders their department owned. The network was decommissioned in the very early 1990's, I purchased this encoder for $1.00 at the Cocoa city auction in, I believe, 1991. I'm very glad I saved this example G2 from the electronic junkpile, I had it on display along with a Plectron Sentry receiver in my County office until mid-2006, then I had it stored in a bankers box in secure, climate control storage for the next ten years until 2016. When I dusted it off the other day, I cleaned contacts with Caig Laboratories DeOxit, made sure the mechanisms all worked and were lubricated, then I plugged the unit in on a leap of faith and let it rip! The encoder fired off as well as it did when it was in-service. Considering it had a build date of 1964, 52 years worth of storage and service is something you NEVER see any more. And, when I checked the tone frequency for Channel 1, it was within 1hz of where it was originally supposed to be 52 years ago. If that's not a product testimonial, I don't know what is!!
One other interesting narrative I was email'ed on how this circuit works was from Martin, a highly accredited site viewer... "This is a very cheap, reliable way to build an oscillator. The lamp sets the gain in the oscillator. It is not as well known as it once was, but tungsten lamps have non-linear resistance; the lamp resistance increases as the lamp gets hotter. In this application, the lamp is still very cold when the oscillator is up and running but not as cold as it was when the oscillator was first powered up. The lamp is set up to detect power (and therefore gain) in the oscillator, and provide feedback control to keep the oscillator operating in its linear, low-distortion, highi-stability range. This approach is not perfect. The lamp will exhibit microphonic distortion and respond to vibration or sound that mechanically excites the filament, and it sets lower and upper bounds on the frequencies you can generate. This approach is the one that Bill Hewlett wrote his dissertation on at Stanford, and the basis for the first product from Hewlett-Packard, an audio-frequency oscillator. All the surviving spinoffs of HP (Agilent, HP, and HPE) owe their existence to the money Hewlett and Packard made off that oscillator."
The following are scanned copies of various promotional literature that Plectron gave to interested customers. In some cases this may be the sole remaining copies of information on these product lines available on the Internet. It's very difficult to locate old product brochures, much less get them scanned to make them readily available to the on-line population.
Plectron Alarm Warble, directly off a G8 encoder. Good quality! | Brevard County Disaster Plectron 1985-1995, G8 encoder | Same Disaster Plectron alert test, this time manually encoded | Four Communities VFD, G2 encoder | Bellwood VFD, G2 encoder | Brevard County District 4 Fire Dept., G2 encoder | Melbourne Beach VFD, G21C encoder | Micco VFD, G8 encoder | Titusville FD station alert, G8 encoder | Titusville FD personnel recall (Code 22), G8 encoder | Volusia County FD, alert for Station 6, G8 encoder. | Unknown low-band agency alert 1, G8 encoder | Unknown low-band agency alert 2, G8 encoder | Unknown low-band agency alert 3, G21 encoder |
Plectron's tones and encoding methodology has been confusing for a good many people. Here's a quick & simple compendium on all tones Plectron!
Plectron fast duotone combination: Tone "A" is 750ms, Tone "B" is 250ms. Tone sequences can be reversed if the reversal option was ordered for the encoder, G8 only.
Plectron slow duotone combination: Tone "A" is 3000ms, Tone "B" is 750ms. Again, tone sequences can be reversed if the reversal option was ordered for the encoder, G8 only.
Plectron single tone page: One tone, transmitted for either 3000ms or 6000ms. The old tube type encoders went for 10-12 seconds on calls depending on where the cam cut off the timing motor. Occasionally they stuck running and the tone went on-air until the encoder either quit/blew a fuse, the transmitter timed out, or the finals quit if there was no TOT. Don't ask me how I know this!! Also... when the encoder was transmitting, more often then not the dispatch mike was open as well. A few interesting conversations went on-air during tests! The rotating cam caused a button reset when the code was through with a double "CLUNK", from the mechanical drivetrain releasing the spring-loaded reset mechanism.
Plectron Tones can come in any of the following frequencies (all in Hz): 282.2, 294.7, 307.8, 321.4, 335.6, 350.5, 366, 382, 399.2, 416.9, 435.3, 454.6, 474.8, 495.8, 517.8, 540.7, 564.7, 589.7, 615.8, 643, 672, 701, 732, 765, 799, 834, 871, 910, 950, 992, 1036, 1082, 1130, 1180, 1232, 1287, 1344, 1403, 1465, 1530, 1598, 1669, 1743, 1820, 1901, 1985, 2073, 2164, 2260, 2361, 2465, 2575, 2688, 2807, 2932, 3062, 3197, 3339, and 3487 hz.
Plectron encoders could be ordered with any tone combinations, an example is a Plectron G8 ordered with Motorola Group 2 frequencies as a special order. We had one of those in Brevard County, FL at our EOC that set off Plectron R8000 radio receiver recorders - it was a 100 button lit encoder to boot. With a sequencer. The sound files for this encoder are in the sounds section.
His corporate website is at: Wallace Radio for further information.