McIntosh MR55 FM/AM Tuner (1960–1963)

Eighteen tubes, a six-gang tuning capacitor, and a capture ratio McIntosh's own advertising claimed was "near unity." The MR55 was the company's first serious try at pulling clean FM out of the air.

Overview

The MR55 is a tube tuner - a "Tube Tuner (Receiver Component)" in the catalog language of the day - and one of McIntosh Laboratory's first dedicated tuners. It has no amplifier stage and no speaker outputs. It has an 18-tube FM/AM front end, a high-impedance output meant to feed a tube amplifier's inputs, and the overbuilt RF chassis that built McIntosh's reputation in the early 1960s. Production is documented as 1960-1963.

The headline number, if a tuner has one, is sensitivity: reportedly around 2.5 μV for 20 dB quieting on FM. That figure comes from a single source page, so treat it as approximate rather than lab-verified. No independent test bench numbers have surfaced in the available record. It fits what the MR55 was built to do, though. FM was gaining recognition for better sound than AM, stereo broadcasting was starting to take hold, and high-fidelity tuners were becoming essential components in audiophile systems. The MR55 was McIntosh's entry into that market.

What separates it from the integrated receivers of the era is the approach. The MR55 functioned solely as a tuner, with no amplification on board. The preamp, the power amp and the speakers were somebody else's job. That left the tuner with one responsibility, signal purity, and the design reflects it.

Key Features

18-Tube FM/AM Chassis

The tube complement runs 6AQ8, 6AU6, 6U8, 6BA6, 12AT7, 12AU7, and a 5U4GB rectifier. That's a lot of glass for a tuner. RF amplification, mixing, IF stripping, detection and audio output are all done with discrete stages, no shortcuts. The service documentation reads like a technician's workbook: alignment procedures for the 6BE6 mixer, the 6AB4, the 6BZ7 and the 6AU6 IF strip, each with its own dummy antenna coupling, dial setting and adjustment point. This is not a tuner you align with a screwdriver and a prayer.

Six-Gang Tuning Capacitor

The manual identifies a six-gang tuning capacitor, with AM sections listed as antenna 10-508 mmf, RF 7-495 mmf, and oscillator 4-156 mmf. Six gangs on a consumer tuner is serious RF engineering. More gangs mean tighter tracking between the antenna, RF and oscillator stages across the dial, which gives more consistent sensitivity from one end of the band to the other. It also explains the MR55's reputation among restorers: there's more to go wrong, and more to get right.

Unity Coupled® Output Transformer

McIntosh's Unity Coupled circuit is best known from the company's power amplifiers, but the MR55 uses a Unity Coupled output transformer too. Its job here is impedance matching and clean signal transfer to external amplifiers. The output impedance is high, explicitly intended to drive tube amplifier inputs rather than modern line-level gear. Pairing an MR55 with a solid-state preamp requires attention to loading. Pairing it with a period McIntosh tube amp is the easy path.

Slide-Rule Dial and Analog Metering

A front-panel analog meter and a slide-rule dial handle station selection. The meter does double duty - the manual lists separate FM tuning and sensitivity meter functions. The finish is a black chassis with a brushed aluminum faceplate, the McIntosh house look of the period.

FM/AM Switching and Sensitivity Control

The control set includes a rotary FM-AM selector, a rotary AM sensitivity switch, a lamps Hi-Lo slide switch, and an antenna/AM AntLoop DPDT slide switch. The AM sensitivity control is a real functional feature, not a trim. The alignment procedure calls for setting it to minimum for one adjustment step and maximum for another, which suggests it changes the RF stage gain in a meaningful way. There's also an FM-Tune position on the band switch used during alignment.

Historical Context

The MR55 arrived in 1960 and ran through 1963, positioned as one of McIntosh Laboratory's first dedicated tuners. It laid the groundwork for later high-end tuners like the MR61 and MR70, and the family resemblance in the later models is not accidental. FM radio was gaining recognition for better sound than AM, and as stereo broadcasting took hold, high-fidelity tuners became essential components in audiophile systems.

Contemporary advertising leaned hard on one specification. McIntosh's own copy, run in the audio press of the period, billed the MR55 as the product of over $100,000 and two long years of development, and singled out a capture ratio of near unity as the standout feature. According to the company's advertising, that near-unity capture ratio meant more interference-free stations, reduced distortion on all receivable signals, and improved multi-path conditions. The same ads also touted ultrasonic muting, zero time constant limiters, and what they described as the lowest total noise and distortion and lowest hum level in the category. Those are marketing claims, not independent measurements. The capture ratio figure is the one that mattered technically, because capture ratio determines whether a tuner can lock onto a strong station and reject a weaker one on an adjacent frequency. Audio (April 1958, p.53) carried the same McIntosh advertisement making these claims.

Collectibility & Value

The MR55 carries a collectibility rating of 7/10. It's not as rare as some limited-production McIntosh amplifiers, which keeps prices from going stratospheric, but it's a respected early tuner with a real following. Fully restored units in excellent cosmetic and operational condition typically command prices between $300 and $600. Originality, tube complement, and provenance, such as service records and original packaging, can push a unit toward the top of that range.

Buying one unrestored is a project, not a plug-in. The known failure points are predictable for a 60-year-old tube tuner. Electrolytic and paper capacitors degrade over time and should be replaced during restoration. Tubes such as the 6AQ8, 12AT7, and 5U4GB may need replacement. The IF and RF stages may require realignment to restore peak sensitivity and selectivity. Older power transformers can develop issues, so check for overheating or low output voltages during servicing. Tube sockets, switches, and potentiometers should be cleaned with appropriate contact cleaner. Dust and corrosion are the quiet killers.

Restoration advice from people who've done it: perform a full recapping before powering up a non-working unit. Use a variac when powering up after long storage to avoid inrush current damage. Verify proper grounding and chassis integrity to maintain safety and reduce hum. Store in a dry, temperature-controlled environment to prevent future degradation. The MR55 is not a hard restoration for someone with tube experience, but it is not a beginner's project either. The alignment alone needs a signal generator, a scope, and patience.

One note for shoppers: you'll see "MR-55A" or "MR55A" listings floating around, sometimes for literature rather than the tuner itself. The relationship between the MR55 and the MR-55A is not documented in the available record, and no source confirms they're the same product or explains the difference. Treat them as separate until proven otherwise.