Akai GX-F51 (1982–1984)
Akai's two-head deck with a calibration computer that tuned itself in about twenty-five seconds - and a ferrite head built to survive metal tape.
Overview
The GX-F51 is a two-head stereo cassette deck that Akai built in Japan and first sold in 1982, with a recommended retail price of USD $350. It sits in the company's premium GX line. The model name turns up written both ways - GX-F51 in most documentation, GX-F 51 on some listings and reference pages. The front panel is silver. At 440 x 100 x 345 mm and roughly 5.8 kg, it is a full-width component, not a compact deck. The shallow 100 mm height means it stacks without eating much shelf space.
The head is what separates the GX-F51 from the ordinary two-head decks of its day. Akai's Twin Field Super GX head packs separate recording and playback gaps into a single housing - a 4 μ recording gap and a 1 μ reproducing gap. The core is crystal ferrite with high permeability, which lets the deck handle metal tape without chewing through a conventional permalloy head in a year or two of heavy use. One head does both jobs, with a gap geometry designed for each.
Around that head sits a transport built on an FG servo direct-drive capstan motor and a separate high-torque DC reel motor. The capstan motor handles speed stability. The reel motor handles torque. Winding a C-60 from end to end takes approximately 90 seconds. Wow and flutter is quoted at 0.035% WRMS (JIS) - a figure that in practice puts the deck in the same conversation as far more expensive three-head machines.
Frequency response tracks the tape you feed it: 20 Hz to 17 kHz ±3 dB on normal tape, 20 Hz to 18 kHz ±3 dB on CrO2, and 20 Hz to 19 kHz ±3 dB on metal. Distortion at 1 kHz and 0 VU on metal tape measures 0.8%, and the metal-tape signal-to-noise ratio is 60 dB at the 3% THD level, weighted. Dolby B delivers 5 dB of noise reduction at 1 kHz and 10 dB at 5 kHz. Dolby C is the more serious tool, claiming 15 dB at 500 Hz and 20 dB from 1 kHz through 10 kHz.
Key Features
Twin Field Super GX Head with Crystal Ferrite Core
Akai's answer to head wear was to stop using the materials that wore out. The Twin Field Super GX head uses a crystal ferrite core with high permeability. The two-gap design, 4 μ for record and 1 μ for playback, lets a single head serve both functions without the compromise of a shared gap. The ferrite core is also what makes the deck's metal-tape support real rather than nominal. Some listings describe the head simply as a glass ferrite type, but the service documentation is explicit about the Twin Field construction and the two-gap geometry.
Automatic Bias and Sensitivity Calibration in 8 Steps
The deck's party trick is its auto-tuning system. Instead of asking the user to set bias and EQ by ear or by dial, the GX-F51 runs a calibration routine that adjusts bias in 8 steps - the service documentation describes it as a 3-bit circuit - and finishes the whole procedure in about 25 seconds. The tuning stores itself and doesn't change unless the reference button is pressed or the sequence is run again. That matters when swapping between normal, CrO2, and metal stock.
Full-Logic Transport with Auto Tape Detection
Transport control is full-logic, so the buttons respond to the lightest finger contact. A microcomputer governs mode selection, automatic tape-type detection, ejection, and the tuning sequence. The head block lifts automatically when the deck is stopped, which means faster play starts and less head wear over the years. The mechanism is described as quick and quiet, and the auto-detection means the deck reads the tape shell and sets itself accordingly. Belt sets sold specifically for the GX-F51 point to a belt-driven transport, and the belts are the wear item to check first.
Dolby B and C, Switchable MPX Filter, and a 12-Segment FL Bar Meter
Both Dolby B and Dolby C are on board, with the switching handled inside the Dolby IC. A switchable MPX filter strips out the pilot signal so FM stereo broadcasts get proper Dolby processing instead of mistracking on the 19 kHz tone. Level metering is a 2-color, 12-segment fluorescent bar display, which gives a clearer sense of peak levels than a pair of analog needles. The deck also carries rec-cancel, introscan, IPLS (Automatic Head-Out), memory autoplay, an auto fader, timer start, and support for an optional remote control unit. Two microphone inputs on jack connectors are provided for live recording.
Historical Context
Akai first sold the GX-F51 in 1982, and the deck was designed for accurate tape reproduction and strong mechanical performance, which fits the GX line's reputation for engineering-first design. Production is generally given as 1982 to 1984, though some sources place the end of the run in 1983. References disagree on the exact discontinuation date. The service manual for the model also covers the GX-F31 and GX-F71, and the auto-tuning section uses the GX-F71's symbol numbering to explain the GX-F51's circuit, which is why GX-F71 references appear throughout the GX-F51 documentation. That manual runs 133 pages, and a factory photocopy covering all three models circulates among collectors.
Collectibility & Value
Pricing data for the GX-F51 is thin and scattered. An eBay listing has shown the deck at US $249.00 or Best Offer, while Russian-market listings have shown it at 19,990 ₽, 24,000 ₽, and 25,000 ₽ - figures that suggest it trades as a working vintage component, not a collector's trophy. One Avito listing describes a unit in excellent condition with a serviced tape transport, imported from Japan and set for 100V. A 220V variant is also documented. Contemporary audio press of the period, such as Audio (June 1983, p.96), ran dealer advertisements for cassette decks of this class, reflecting a healthy market for Akai's GX series during its production run. Condition varies widely. One parts-unit listing notes a deck where the tape does not stay in, the cover is loose and needs gluing, and the unit is sold as-is for repair. European belt sets are sold specifically for this model, and the service manual is readily available. No common electrical failure modes have been documented beyond the usual wear items, and no substantive reviewer commentary has surfaced in the available record.
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