RCA 11850-C
A lacquer-cutting head that held its curve to within 2 dB from 30 to 10,000 cycles — and needed a thermostat to do it.
Overview
The 11850-C is not a piece of home audio. It is RCA's high-fidelity magnetic recording head — the cutter — built as the heart of the RCA 73-B Professional Recorder and sold separately as an additional or replacement head. Where most of the gear in this archive is meant to be listened to, this one is meant to carve. It is a precision instrument that sits on a carriage over a lacquer blank and physically engraves the groove, and the numbers RCA published for it are the kind that make a cutting engineer sit up: frequency response held to within plus or minus 2 dB from 30 to 10,000 cycles per second, with the head's own heater and thermostat working to keep it there.
That response figure is the whole story of the 11850-C. A magnetic cutter's coil heats up as it works, and as it heats its characteristics drift — the very thing that ruins a lacquer side halfway through. RCA's answer was a self-contained heater and thermostat built into the head, with the discrepancies caused by temperature variation eliminated by that arrangement. A visual indicator, switched from the base, shows when the heater is in the circuit.
The electrical side is equally specific. The head's impedance is 15 ohms nominal, and sensitivity is quoted at +30 dBm — one watt — referenced to a groove velocity of 6.3 cm/sec (.00079 inch peak-to-peak) at 1000 cycles. RCA's own documentation is candid that the nominal figure is just that: in a 15-ohm cutter, the actual impedance varies from 2 ohms at 50 cps to 30 ohms at 10,000 cps, a swing that any driving amplifier and equalizer network has to be designed around. Audio (April 1949, p.13) walked through exactly this point, noting the same 2-to-30-ohm spread across the band and warning that the flat-response position of an equalizer should be checked during setup because some networks start their bass boost too high. That is period operating advice for this head, not marketing.
Styling is either sapphire or steel, and RCA listed both: a 90-degree sapphire (MI-4878-D), a 70-degree sapphire (MI-4842), and a steel stylus (MI-4879-A). The 11850-C also appears in RCA's accessory catalog as the additional high-fidelity recording head for the 73-B, alongside the standard cutter head (MI-11853), the fine-groove conversion kit (MI-11882), and the automatic equalizer (MI-11100-A). It was a system component, in other words, not a one-off — but it was the component the rest of the system was built to serve.
The Cutter in the System
The 73-B's carriage and feed-screw mechanism exist to move the 11850-C across the blank, and RCA's manual describes the head's own refinements: an improved lowering device to prevent stylus damage and overcutting, and stylus angle and depth of cut adjustable while cutting. Spiralling pitch runs approximately 14 lines per inch at 78 rpm and 6 lines per inch at 33⅓ rpm, with the pitch continuously variable while cutting from 96 to 152 lines per inch in detents of 8 — and the MI-11882 conversion kit pushes fine-groove work up to 300 lines per inch. Changes from inside-out to outside-in are made by turning a knob, without changing belts, feed screw, or driving gears. The turntable it all rides over is 17½ inches in diameter, rim-driven through rubber idler rollers from two hysteresis synchronous motors, handling blanks up to 18¼ inches across and up to ⅜ inch thick, at 33⅓ or 78 rpm with speed accuracy of plus or minus ½ percent. Wow is quoted at 0.14 percent rms at 33⅓ and 0.07 percent rms at 78.
The head did not work alone on the electrical side either. RCA developed dedicated filter networks for the 11850-C, and the manual is unusually forthcoming about why: the NAB lateral recording standard assumes a cutter with a 500-cycle transition frequency and a high-frequency tip-up with a 100-microsecond time constant, and the difference between that ideal and the 11850-C's actual lacquer-cutting response defines the filter that has to sit ahead of the driving amplifier. The required network turned out to need a tuned circuit resonating near 500 cycles, a dip at about 1000 cycles, and a broad hole around 4500 cycles to equalize the top. The resulting filter, rated at 600 ohms in and out, was designed so that several could be bridged across a program line without much disturbing line impedance — a practical touch for a station running multiple cutting rooms. Audio (June 1949, p.21) reproduced the response-versus-ideal comparison and the filter design that followed from it.
What the Period Press Made of It
The 11850-C got a proper technical airing in the pages of Audio. H. E. Roys's article "Lacquer Disc Recording Head" (Audio, February 1949, p.15) presented the head as a new cutter with excellent characteristics, complete with photographs of the head and its heater assembly mounted in place. The same magazine's April 1949 issue (p.13) returned to the practical side, discussing how the head's wildly varying impedance across the band interacts with the equalizer — and noting the built-in heater and thermostat as the answer to thermal drift on program material. Audio (June 1949, p.21) then carried the crossover-filter follow-up, working from the head's measured cutting curve to the compensation network. Three pieces in five months is not a product launch; it is an engineering community taking a cutter seriously.
Living With One
The thermal stability is the headline — a cutter that holds its curve through a side is worth more than one with a prettier spec sheet — and the adjustable stylus angle and depth, controlled from two knobs atop the carriage, are what make the head usable for both wax and lacquer work. The advance ball kit (MI-11851) is the accessory to look for if wax recording is on the agenda; without it, the head is a lacquer tool. The heater circuit is the other thing to check on any example that comes up — a dead thermostat turns a precision cutter into a drift-prone one, and the visual indicator on the base is the quickest way to confirm the heater is live.
Original pricing for the head alone is not documented in the available material, and production years have not surfaced; the 73-B recorder it served was sold through the late 1940s and into the 1950s, and the head appears in RCA accessory listings from that period.
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