FM MPX Decoders

12AU7 - ECC82 6BL8 6KE8 6AG11 - Compactron 6M11 - Compactron

 


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Last Edited:
12-Sep-2004


Scott was ready to begin manufacture of FM (outboard, self-powered) Multiplex Decoders in April of 1961 when the FCC approved FM Stereo Multiplex broadcasting. Production units of the H.H. Scott Type 335 Multiplex Adaptor were shipped in May, before the first FM (WGFM-FM, WEFM-FM, and KMLA-FM), stations went on the air with multiplex transmissions on June 1,1961. Score another industry-first for H.H. Scott.

This 1st-generation, 335 multiplex decoder used a three-tube decoder lineup that was also used on the 350/LT-110 integrated FM-MPX Stereo tuners. The basic three-tube Scott multiplex decoder line-up consisted of:

6BL8 combination composite buffer and 19 kHz amplifier
12AU7 38 kHz oscillator, driving a germanium diode switching matrix
12AU7 audio matrix tube to adjust the ratio of (L+R) and (L-R) signals, used to insure full stereo separation

A stereo AF line-level audio amplifier delivered the final output. And the outboard 335 had it own power transformer an 6X4 full-wave rectifier.

The original decoder circuitry had no active indicator to display the presence of a pilot carrier when the listener tuned into a stereo broadcast. A "Stereo Guide" light kit (C-55/350-A) was introduced, but sometimes provided false indications of pilot carriers. An improved "Sonic Monitor" (a stereo "beep") appeared on 350-B units and while more reliable was greeted with user apathy. Soon after Scott switched over to the industry-standard "Stereo" light indicator, operating a 6KE8, auto-"magically." (310-E, 4310, 340-B, 350-D, 380).

In the Scott Compactron tuner designs (370-series/LT-111), the multiplex switching matrix consisted of 4 vacuum diodes, 2 in each Compactron. The only other Scott tube tuner to not use a germanium diode switching matrix was the 4310, which also used vacuum tube diodes in its switching matrix.

 

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