McIntosh MC240 Vintage Tube Amplifier - Serviced Collector Piece

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Single owner!

Purchased new in Dallas, TX back in 1967.

Our favorite vintage tube amp from McIntosh. A bit more musical than its big brother the MC275, and a better choice if you have efficient speakers.

Well cared for and serviced by Audio Classics to be super reliable and quiet.

Includes a rare manual, schematics, and Audio Classics service receipts.

Service Notes:

Serviced by Audio Classics in November 2004. This service receipt indicates that some of the main electrolytic filter capacitors were replaced, the negative bias supply capacitor was replaced, controls cleaned, and a 12AX7 tube was swapped. Investigation of the inside of the unit confirms this in additional to the following. All of the power supply filter caps were replaced, and RCA jacks were replaced with gold plated versions with a copper ground bus.

At SkyFi we ran the unit through our routine bench tests and it tested to the rated spec of 40WPC with no issue. All of the controls were cleaned and the chassis was detailed. We also took the opportunity to freshen up the tubes with period correct NOS tube sets fully tested on our Amplitrex AT1000 modern professional tube tester.

The output tubes are GE 6L6GC, the 12AX7s were upgraded to the ruggedized military version 5751 (GE). The 12AU7s were also upgraded to the military version 6189W. 12BH7s are GE.

It should also be noted that the coupling capacitors are all Sprague Black Beauty Difilm caps which rarely suffer from the leakage characteristics that often plague the colored striped bumble bees.

Serial Number: 45G94

McIntosh MC240 Original Sales Brochure

McIntosh MC240 Owners Manual

McIntosh MC240 Schematic and Parts List

McIntosh MC240 Maintenance Manual

The SkyFi Testing Process for Amplifiers (Tube):

We start with a visual inspection of all internal components to make sure there are no signs of stress or aging. Capacitors are checked for telltale sings of bulging or leaking, resistors are checked for signs of overheating or cracking, and tubes are inspected for signs of stress overheating or cracking.

We then power up the unit carefully and run a simple 1k sine wave while monitoring the output on an oscilloscope for signs of distortion or noise. Finally we run the amplifier to its full rated power into the appropriate resistive load provided by our Sencore amplifier tester. If there is an issue, it will usually be noticeable at this point. If it all looks clean we will often test for THD (Total Harmonic Distortion) measured by our Keithley distortion meter, and compare it against the original manufacturers specifications. The amplifier the remains powered on to "burn in" on our test bench for a few hours to make sure there are no heat related issues. If there are balanced inputs available they will also be tested.

We also test all tubes for output gain and leakage on our calibrated B&K 747 tester against the original manufacturers specifications. If needed, the tubes will be replaced from our vast NOS (New Old Stock) vintage or modern inventory. 

Final testing involves pushing actual music thorough the amplifier. We've learned over time that some issues are only noticeable to a trained ear while listening to familiar source material. Our test bench has reference vintage KEF speakers that we're very familiar with which quickly reveal any discrepancies. Some amplifiers will then move into our listening room where they are tested with our in-house reference speakers of choice. 

Please click here for detailed specifics regarding our specialized packing process that separates us from the rest.