H.H. Scott 335 Wide-Band Multiplex Adaptor
A small box that let a mono FM tuner hear stereo - provided you owned the right tuner.
Overview
The 335 is not a tuner, not an amplifier, and not a receiver. It is a Wide-Band Multiplex Adaptor, a standalone chassis that decodes the FM multiplex stereo subcarrier and hands the result to a tuner and amplifier built before stereo broadcasting existed. H.H. Scott's operating instructions say it is designed to provide maximum quality with any H. H. Scott Wide-Band tuner. The company was blunt about the limits. It did not recommend the adaptor for tuners of other manufacture. The 335 might function with some of them, but Scott could not guarantee that multiplex reception would match FCC transmission specifications. The manual says that guarantee held only with an H. H. Scott Wide-Band tuner.
The logic is simple once the problem is clear. Through the early 1960s, plenty of high-quality Scott tuners were already in living rooms, and their owners had no appetite for replacing a tuner just because the FCC had approved a stereo multiplex standard. The 335 was the retrofit. Keep the tuner, add the decoder, get stereo. The manual lists the tuners it was wired to work with - Models 310C and D, 311D, 320, 330A, B, C and D, and 331A, B and C - and the connection notes are specific about each. On the 310C, 310D and 311D, the two low-level outputs sit next to each other rather than where the diagram shows them. On the 320, the owner disconnects the Channel A and B cables from the amplifier, runs them to the adaptor instead, and keeps the tuner's front-panel slide switch in the normal monophonic position. On the 330A and B and the 331A, B and C, the correct outputs are labeled "Binaural." All of them use the MPX output.
The front panel is about as minimal as a component gets. A red light indicates power on. An amber light indicates the adaptor is set for stereo multiplex reception. The Stereo Noise Filter switch, when in, reduces high frequency noise on both the main FM carrier and the multiplex subcarrier, and it does not reduce stereophonic separation. Out removes the filter for normal operation. Then there is the adaptor switch itself, which the manual treats as the single most important habit to learn. For ordinary FM reception the switch stays out. For stereo, it goes in.
The manual spends real space on a problem that would have confused a lot of owners. Broadcasters in this period were using multiplexing to transmit background music to restaurants, offices, and factories, and the FCC-approved stereo multiplex system did not interfere with that revenue stream. A 335 that meets the new FCC specifications will not receive those background music programs. But if a station used a subcarrier in the range of the stereo subcarrier channel for background music, an owner could hear interference effects while listening to regular FM with the adaptor switch in. The manual's example is whistles. The fix is the habit already described - keep the switch out for normal FM. The manual also covers the rarer case of a station transmitting two multiplex subcarriers, one for stereo and one for background music, and the interference between them. It offers four escalating remedies: tune the station more carefully, reposition the antenna, reverse the antenna leads, or use a better and more directional antenna.
Scott also used the 335's manual as a place to talk about the medium itself. Multiplex stereo broadcasting, it notes, was a new experience for most FM stations, and problems could arise because station equipment and operating techniques were not yet developed to the point where listeners could consistently expect the ultimate in performance. Distortion that appears when the selector is set for stereo and disappears when it is turned to Mono, the manual explains, is primarily due to transmitter problems — a common situation in areas with many hills or tall buildings that cause reflections. The same four antenna and tuning suggestions apply. It is an unusually candid piece of documentation: a manufacturer telling buyers that some of what they were about to hear was not the equipment's fault.
The manual also carries the company's standard boilerplate about its own history — a page of "Important Firsts" claiming the first high fidelity AM-FM stereo tuner using wide range AM design, the first successful use of wide-band circuitry in high-fidelity FM tuners, the first to market the Stereo-Daptor stereo control unit, and the first to provide center channel output on stereo amplifiers. Whether every one of those claims survives scrutiny is a separate question; what matters here is that Scott positioned the 335 inside that narrative of being first to solve the stereo problem for existing owners. The manual also includes a short profile of Hermon H. Scott himself — M.I.T. degrees, inventor of the RC Oscillator, RC tuned circuits and filters, and the Dynamic Noise Suppressor, a Fellow of the Institute of Radio Engineers, the Acoustical Society of America, and the Audio Engineering Society, and a recipient of the Potts Medal. It is the kind of founder-worship that filled a lot of component manuals in this era, and it tells you how the company wanted the 335 to be read: not as an accessory, but as the product of a man who had been solving audio problems since before most of his customers owned a hi-fi.
In the Scott line, the 335 is a bridge component. It exists because the company had sold a lot of wide-band mono tuners and wanted those owners to stay in the fold when stereo arrived. The manual's connection notes name the tuners it supports, and the operating instructions are written for someone who already owns one of them. It was not designed to be bought alongside a new tuner, but to extend the life of an old one.
One period write-up that survives in the archives is a tuner shootout published in Vacuum-Tube-Valley (p.9). It touches on Scott tuners generally and describes one unit's sound as musical and warm, with lush, pleasant, rich mids - very listenable, very non-tuner like. It calls the unit a good budget item and the best mono performer in the group, though two of the reviewers felt it was not aligned correctly, and the assessment concluded it was a good value but not up to the Fisher's or Scott's performance. Vacuum-Tube-Valley (p.9) is describing a tuner, not an adaptor. The 335 has no sound of its own beyond what it hands through, and its job is to decode. The manual stresses that the Stereo Noise Filter preserves separation.
For anyone running a period Scott tuner today, the 335 is the piece that makes a mono tuner a stereo tuner without touching the tuner. The catch is the one the manual states plainly - it wants a Scott Wide-Band tuner on the other end. Buyers should match it to the specific models listed in the instructions. They should also learn the adaptor switch habit, because the background-music interference the manual describes is real behavior of the design and not a fault. The amber and red lamps, the filter switch, and the in/out adaptor switch are the whole interface, so there is very little to go wrong and very little to adjust.
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