McIntosh MC240 Tube Amplifier - Electronically Restored with Tube Cage

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General:
The MC240 is one of the best sounding vintage tube amps from McIntosh. Just enough power (40WPC) to drive most vintage and some modern speakers, yet not too much where it heats up the room unnecessarily. 

It has the traditional look of a McIntosh amplifier so successfully thought out and designed, that it's still being used today in the latest iteration of the legendary MC275. 

The MC240 can be paired up with both a vintage preamp or something modern. Gain controls on the front panel allows you dial in the gain to just the right level while keeping the noise down.  

Heavy-duty screw terminals allows you to use bare wire, small spade lugs, or with the use of an adaptor modern banana plugs. If you need a set of banana adapters simply let us know in your order notes. 

Best of all the tubes don't need any biasing or fussing with. Just plug them in and off they go. McIntosh designs are really easy on the tubes so expect them to last a LONG time.

Technical Notes:
This amplifier was serviced in October 2023 by Fuchs Audio Technology. The service included a full power supply rebuild using CE multi-section capacitors. The technician also replaced all of the paper and di-film capacitors in the audio circuit along with some axial electrolytics. We have inspected and verified all of the work.

Tube Notes:
We have fitted this amplifier with a brand new set of current production Tung Sol reissue 6L6GC STR output tubes. The driver tubes are a mix of strong testing current production Russian 12AX7s and vintage American 12AU7s and 12BH7s.

Cosmetic Notes:
This MC240 has significant silk screen wear and light pitting on all chrome surfaces. One of the terminal blocks had a bit of damage which has been repaired. Scuffs and scratches are visible on all metal surfaces.

This MC240 also includes the tube cage which is often missing.

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:
Screw terminals for speakers, RCA jack for input, captive power cord.

General Sound:
Smooth, uncolored, undistorted natural and clean

Cosmetic Condition:
5/10 = Average - Multiple scratches, including some on the front faceplate. See our detailed rating description here.

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

Included:
Just the unit and power cord.

Packing:
Will be packed using our highly developed in-house process and custom packing materials.Reviews:
Click Here

Specs:
Power output:
40 watts per channel into 8Ω (stereo)
80 watts into 2Ω (mono)

Frequency range:
At rated output
+0, -0.1 dB 16 Hz - 40 kHz
+0, -0.5 dB 16 Hz - 60 kHz

At one-half rated output
+0, -1.0dB 10 Hz - 100 kHz

Signal to noise ratio:
90 dB

Total harmonic distortion:
Less than 0.5% (20 Hz - 20 kHz)

Input sensitivity:
0.5V

Speaker load impedance:
4Ω, 8Ω, and 16Ω

Tube Complement:
Voltage Amplifier - One 12AX7 (Drives Both Amps)
Phase Inverter - Two 12AU7
Drive Amplifier - Two 12BH7
Cathode Follower Drive - Two 12AX7
Output - Four 6L6GC or 7027A

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

Weight:
Chassis Only - 56 lbs 
In Shipping Carton - 65 lbs.

Years in Production:
1960-1969

Link to Manual:
Click Here

Recommended Cables:

Kimber Kable - RCA Interconnects - Better

Kimber Kable - Speaker Cables - Better

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.