McIntosh MC240 Vintage Tube Amp - SkyFi Restored w/Premium Vintage Tubes

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General:
We painstakingly restored this classic and coveted stereo tube amplifier to its full glory.

Internally we replaced every critical component while carefully maintaining the original sonic quality.

This listing also includes a rare and correct McIntosh tube cover (tube cage).

This is a 40W x 2 (stereo) tube amplifier produced from 1960-1969.

SkyFi Restoration Notes:
This MC240 underwent an extensive electronic restoration in house.

The electronic work included a full rebuild of the power supply using custom made multi-section capacitors that match the exact values and layout of the original power supply. The bias supply was also rebuilt with new parts. The selenium bias diode was swapped to silicon with proper adjustment resistor to maintain the ideal bias level for the unity coupled output section. We also replaced all of the paper and di-film capacitors in the signal path after finding them leaky and drifted.

During the signal path capacitor replacement we checked all of the resistors and replaced any parts that had drifted out of tolerance. By default we replace the phase inverter balancing resistors and plate load resistors with precision metal film parts to ensure trouble free performance.

To finish up, we replaced the power cord and inrush current thermistor.

Tube Notes:
This amplifier is fitted with a set of premium vintage tubes. The output tubes are RCA 6L6 (2) are labeled McIntosh at the base. All four are RCA made tubes. The two labeled RCA have brown colored getter flashes but still test strong and sound great! The 12BH7 are strong matched and balanced GE. The remaining tubes are all strong testing authentic Telefunken 12AX7 and 12AU7.

Cosmetic Notes:
This MC240 was well loved. It has moderate chrome pitting and light silkscreen wear which is common for McIntosh amplifiers from this era. There are also some scratches/markings on the black transformer covers and cage.

Two historic Dave O'Brian clinic stickers flank the McIntosh 240 badge and show years 1971 and 1972.

Brand Background:
McIntosh Laboratory is an American manufacturer of handcrafted high-end audio equipment based in Binghamton, New York. The company was founded in 1949 by Frank McIntosh. The company designs and produces audio amplifiers, stereo tuners and other consumer electronics products.

Ownership:
Unknown

Connections:
RCA Inputs, Screw Terminal Outputs

General Sound:
Smooth, uncolored, undistorted natural and clean

Cosmetic Condition:
6 - See our detailed rating description here.

Working Condition:
Working perfectly and tested in our lab and listening room.

Included:
Unit, power cord, manual, and tube cage.

Packing:
Will be packed using our highly developed in-house process and custom packing materials.
Specs:
Power output: 40 watts per channel into 8Ω (stereo), 80 watts into 2Ω (mono)
Frequency response: 16Hz to 40kHz
Total harmonic distortion: 0.5%
Input sensitivity: 0.5V
Signal to noise ratio: 90dB
Speaker load impedance: 4Ω to 16Ω
Tube  complement: 1 x 12AX7, 2 x 12AU7, 2 x 12BH7, 2 x 12AX7, 4 x 6L6GC

Dimensions:
17-1/4" x 10-3/4" x 8"

Weight:
56 lbs.

Recommended Cables:

Kimber Kable - RCA Interconnects - Better 

Kimber Kable - RCA Interconnects - Best

Kimber Kable - Speaker Cables - Better

Kimber Kable - Speaker Cables - Best

Kimber Kable - Bi-Wire Speaker Cables Best (If Applicable)

Testing Process:
The SkyFi Testing Process for Tube Amplifiers:
We start with a visual inspection of all internal components to make sure that there are no signs of heat stress or damage. Capacitors are checked for telltale signs of predictive failure including bulging, shrunken wrappers, or physical leakage. We also inspect resistors and other passive components for signs of overheating. If tube arcing has occurred in the past we can usually spot discoloration on the output tube sockets. On vintage units we often spot check select capacitors for value and ESR.

If the amplifier passes visual inspection, we move on to a full test of all of the tubes. We use an Amplitrex AT-1000 Tube Tester which is capable of testing both emission and Gm with a high degree of accuracy. We document the results of each tube and replace any weak or suspect tubes before proceeding. When we power on tube amplifiers for the first time we usually use a variac and current limited AC supply and slowly raise the voltage up to nominal mains level while monitoring plate, screen, filament, and negative bias supply voltages where applicable. If everything is in order we feed a low level test signal into the amplifier’s input and monitor its output on an oscilloscope across an 8 ohm dummy load. At this point we are just looking to verify basic function and confirm that the output transformers are not damaged. Once we have verified that the amplifier is safe to operate, we connected it to full mains power. For fixed bias amps we set the bias to manufacturer spec. For cathode biased amps we monitor the plate to cathode voltage to determine if the output tubes are operating in a safe range. Once the output section is verified we move onto bench evaluation.

We start by feeding the input of the amplifier with a low level 1KHz test signal, slowly increase its amplitude while monitoring the amplifier’s output on an oscilloscope for signs of noise, clipping, distortion, or improper channel balance. We continue increasing the signal level until the amplifier reaches clipping. At this point we take an output power measurement and compare it to the spec sheet of the amplifier to verify proper performance. We finish off the bench evaluation with a 1KHz square wave check and a 20Hz to 20KHz sine sweep to assess the amplifier’s frequency response characteristics. This battery of tests will usually reveal if the amplifier has any issues that need further attention.

Before the device leaves the bench, we perform a listening test with actual music using a variety of preferred test tracks. Our benches are outfitted with familiar monitor speakers which help us identify inconsistencies that will not always show up on our test gear. The main things that we are listening for are hum or noise with no signal present, proper center image, clicks, pops, or any other obvious undesirable audio characteristics.

If the unit passes all of these tests it is moved to our long term testing rig where we simulate real word operating conditions for 6-8 hours. For tube amps we like to run this test at least twice. This allows us to monitor the unit for signs of thermal runaway or intermittent issues that only crop up when it has fully come up to temperature. We find this step to be essential, especially for vintage units.