McIntosh MI350
A 350-watt industrial tube amplifier with output taps for 50 and 64 ohms. Those taps tell you it was never meant for a living room.
Overview
The MI350's spec sheet reads like lab equipment that happens to run on vacuum tubes. McIntosh rated it at 350 watts continuous into 8, 16, 50, and 64 ohms, 325 watts continuous at 4 ohms, and 300 watts continuous at 1 ohm. That load range covers distribution-line impedances most consumer amplifiers of the period never touched. The 50- and 64-ohm taps, plus output voltages of 17.3, 35.8, 53, 75, 132, and 150 volts, show what the MI350 was: a constant-voltage amplifier for commercial sound reinforcement, industrial paging, and institutional installations, where you drive long speaker lines instead of a pair of bookshelf boxes.
The tube complement shows the scale of the thing. McIntosh's parts list shows eight 6LQ6/6JE6B sweep tubes in the output stage (V1 through V8, in four pairs), supported by a pair of 12AX7/ECC83s, two 6DJ8/ECC88s, a 6CG7/6FQ7, and a single 6BL7GTA. That's a lot of glass and a lot of heat. The manual lists AC power input at 117/125 volts, 50/60 Hz, drawing anywhere from 400 to 900 watts depending on how hard the amp is working. The output stage feeds a dedicated output transformer (part 043-766) and an input transformer (159-001), with separate high-voltage and low-voltage power transformers (043-765 and 043-764) carrying the supply.
McIntosh's published numbers are respectable for a tube amp in this output class. Frequency response is quoted at 20 Hz to 20 kHz, +0/−0.5 dB at 350 watts output, and 1 Hz to 70 kHz, +0/−3 dB at 1 watt output. That wide small-signal bandwidth suggests a well-behaved transformer design. Harmonic distortion is listed at less than 0.15% at rated output or below, 20 Hz to 20 kHz, with intermodulation distortion also under 0.15%. Noise and hum sits at −95 dB, input level at 1.1 volts, and output impedance is switchable across 1, 4, 8, 16, 50, and 64 ohms.
There is a known service issue. McIntosh issued a modification titled "Elimination of Self Turn-Off," affecting MC 3500 units from serial 10N01 and above and MI-350 units from serial 10R01 and above. The fix was simple: a single 200-ohm, 2-watt wirewound resistor (part 139-069) added in parallel with capacitor C201 at the circuit breaker, reachable by removing the bottom cover. It stops the amplifier from shutting itself off at maximum output. Any unit that hasn't had this done should have it done, and per McIntosh's own instructions it should be performed any time service is otherwise needed.
Early-production MI350s also differ from later ones in ways the schematic notes spell out. Units with serial numbers below 10R54 don't use inductors L1 through L8, run an 8200-ohm, 5%, half-watt resistor at R114, and use C207D. C6 is present only in early units. A casual listener won't care, but anyone restoring one and ordering parts against the wrong schematic revision will.
Inside, the MI350 is built the way McIntosh built its commercial gear, with a mix of point-to-point wiring and printed circuit boards. The amplifier section is on PCB 043-767 and the meter section on 154-372. The parts list includes a mix of Mylar and electrolytic capacitors, silicon and germanium diodes, wirewound and composition resistors, and a thermal cut-out (S202) on the AC line. The front panel carries a gain control, a meter range switch, and an output impedance switch, with side frames and handles (parts 043-830, 043-831, and 020-026) that suggest the amp was meant to be rack-mounted or at least moved by two people. The output impedance selector (S2) is a rotary switch, not a jumper, which helped installers who needed to change taps on site.
Replacement parts, per the manual, were available directly from McIntosh's Customer Service Department in Binghamton, New York, with common items obtainable from radio parts jobbers. Today the proprietary transformers and the front-panel hardware are McIntosh-only. The tubes, particularly the 6LQ6/6JE6B sweep tubes, are the consumable that will decide whether a given unit is a runner or a project. Eight of them in the output stage is a serious tube bill, and matching them is not a casual afternoon's work.
The MI350 is not a hi-fi amplifier with a commercial badge on it. The 1-ohm tap, the 150-volt output, the 900-watt ceiling on AC draw, and the self-turn-off modification all point to a design brief built around driving distributed speaker systems under load. Collectors who know McIntosh's consumer line will recognize the family resemblance in the transformer-coupled output stage, but the MI350's job was different.
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