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Plectron Corporation encoder information and tone archive

Dedicated to Plectron users nationwide by keeping the memories alive!

Last Update - 5/23/2018

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!!

The inside of the unit. Note the two vacuum tubes AND the light bulb. The light bulb never lights, it's there solely as a part of the circuit. The service manual specifies this technical description of the unit (reproduced verbatim);"A 12BH7 dual triode operates as a Wien bridge oscillator. This circuit consists of a two-stage resistance coupled amplifier, utilizing both positive and negative feedback... The operating level of the oscillator is determined by the 5K feedback control, R4 single tone, R4 and R21 duotone. A part of the feedback network consists of the three watt lamp, V3 single tone, V3 and V5 duotone, which operates as a non-linear resistor so as to hold the output voltage of the oscillator at the constant value determined by the feedback control. Frequency selection is accomplished by manually switching the correct precision capacitors in the oscillator circuit."

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."

Plectron G2A encoder, singletone alert generation, steady tone

Plectron G2A encoder, singletone alert generation, pulsed tone


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 dealer application package, circa 1990.

Plectron Encoder line brochure circa 1978.

Plectron Encoder line brochure circa 1981, including price list.

Plectron general products line brochure circa 1979.

Plectron general products line brochure circa 1990. This is after Woodson took over the company.

Plectron alerting and general product line brochure circa 1991, this is around the time Federal Signal acquired interest.

Plectron's general warranty circa 1978.


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.


Wallace Radio LLC., 526 Waynoka Drive, Sardinia, OH 45171. Bob Wallace is the proprietor,"Plectron@Frontier.com" for EMAIL.

His corporate website is at: Wallace Radio for further information.

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