Kenwood KW-550
A tubed FM-stereo tuner that announced itself with a blue light — and backed the light show with a Nuvistor front end and five wide-band IF stages.
Overview
The KW-550 was Kenwood's automatic FM-stereo multiplex tuner of the mid-1960s, and it made its pitch on the front panel as much as the schematic. Contemporary coverage of the KW-55A, its automatic AM/FM stereo multiplex sibling, appeared in the pages of High-Fidelity (April 1965, p.12). Kenwood's own advertising billed it at $169.95 and led with the "FM Stereo Instant Indicator" — a red lamp that switches to blue the moment a stereo multiplex signal is detected. It is a small thing, but it tells you what the tuner is: a piece of furniture meant to sit in a component cabinet and announce its status at a glance, in an era when stereo broadcasting was still new enough that knowing whether you were actually receiving multiplex was half the battle.
The front-panel theater is paired with an automatic relay-switching scheme that hands the changeover from mono to stereo to a set of relay contacts rather than a silent solid-state gate.
Under the hood, the KW-550 is a serious tube tuner. The RF front end uses a 6CW4 Nuvistor in a cascode configuration with half of a 6AQ8, with feedback applied around the first section and AGC voltage fed to that same Nuvistor to tame cross-modulation in strong-signal areas. A local-distance switch on the antenna input introduces roughly 22 dB of loss in the local position — a blunt but effective tool for overload-prone urban reception. The IF strip runs five wide-band stages: two 6BA6s, two 6AU6 limiters, and a 6BL8 that contributes an additional limiter and the muting stage. A panel-mounted variable control sets the inter-station muting threshold.
The multiplex decoder is a balanced diode bridge driven by a 38-kc oscillator, synchronized by a 19-kc pilot amplifier, with the 19-kc pilot also feeding a filter-and-diode network that develops the DC potential to throw the stereo relay. A dimension control — an under-chassis potentiometer — lets the owner trim the amount of L+R mixed with the detected subcarrier to optimize separation. Output is via a low-impedance cathode follower, with stereo and mono outputs plus tape recorder terminals; the separate mono jack stays live regardless of whether the station is broadcasting in stereo, which lets an owner feed a second amp or a center speaker without disturbing the main stereo pair.
Kenwood positioned the KW-550 alongside its KW-220 integrated amplifier — both at $169.95 — as a matched pair for the connoisseur market, and the tuner's proportions reflect that intent: it is a large, imposing panel rather than a shrinking solid-state afterthought, sized to fill a shelf in a proper component cabinet. Collectors today will find it a period piece of mid-sixties tube multiplex design, and one whose relay-based stereo switching and Nuvistor front end give it a character distinct from the transistor tuners that followed. As with any tube tuner of this vintage, condition of the Nuvistor and the multiplex alignment matter more than cosmetics; the schematic in the manual is a genuine asset for anyone contemplating service.
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