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McIntosh MC240
40 WPC Tube Power Amplifier
(1960-1969)
Made in USA

Intro   Specifications   Measurements   Sound  

Compared  Usage   Recommendations

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McIntosh MC240

McIntosh MC240 (rated 40 watts per channel, 56 pounds/25.4 kg, measured 145 watts idle power draw, about $2,400 used). enlarge. I got mine at this link directly to them at eBay (see How to Win at eBay).

This ad-free website's biggest source of support is when you use those or any of these links to approved sources when you get anything, regardless of the country in which you live. Buy only from the approved sources I use myself for the best prices, service, return policies and selection. Thanks for your support! Ken.

 

McIntosh MC240, rear

Rear, McIntosh MC240. enlarge.

 

October 2017     audio reviews   tube reviews   all reviews

NEW: Why Tubes Sound Better 27 May 2015

 

UC

I'm in the process of renovating one. Below you will see what I've got so far.

The amazing thing is that the old one I got with tubes that dated as far back as 1961 certainly didn't meet its specifications, but for actually enjoying music sounded absolutely spectacular.

My renovation work is to bring this amplifier back into original specifications. As it arrived, it already sounded out of this world.

It turns out that all it needed to meet factory specifications was a new set of inexpensive Russian Tung-Sol tubes from TubeDepot (only about $200 total) and one bad resistor replaced, a 17¢ part.

While I had it open, I replaced all four 12 kΩ 2W resistors, as well as replaced all four major power supply capacitors. I also rebuilt the bias supply with a 1N4007 diode and a 220 Ω resistor to bring bias to exactly -150.0V when the B+ was 435 V, and replaced the original 12 µF electrolytic with a 12 µF 250V film capacitor. The lower voltage supply is 135V (not 140V), which will needing slight adjustment later.

I replaced the thermistor per McIntosh's suggestion, and the new one loses less voltage when hot (1.13V versus 2.5V), giving me a few more watts maximum power.

I raised the drooping 140V line from 135V to 140V by adding a 2 MΩ shunt across the 150 kΩ dropping resistor. The original resistor actually measured about 156 kΩ and the 100 kΩ resistor measures about 104.7 kΩ.

The best tube amps, like McIntosh, have separate feedback windings, so the output taps are completely isolated from everything else, with zero DC offset and no need for fuses to protect speakers against failed tubes.

This lets us series or parallel the outputs to our heart's content for mono or increased power outputs, or driving just about any imaginable load impedance. For instance, the McIntosh MC240, MC275 and MC225 in mono can be wired happily to drive any of 2Ω, 4Ω, 8Ω, 16Ω or 32Ω loads as their optimum impedance. These amplifiers also have higher impedance outputs (typically 150Ω, 600Ω in stereo and many more in mono) for driving very long lines at high powers.

This also means that there is no worry about interference or ground loops, no matter how you wire the outputs.

Contrast this to solid state amplifiers, whose outputs are almost never isolated from anything, and may never be connected in series or in parallel. The mono options on stereo solid-state amplifiers are bridge connections, which tend to lower performance.

 

Introduction         top

Intro   Specifications   Measurements   Sound

Compared   Usage   Recommendations

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My McIntosh MC240 is the best power amplifier I've ever heard. While most great amps do nothing to alter the sound (what comes in is what goes out), and when transistor amplifiers alter the sound it's almost always for the worst, the subtle things tube amplifiers and especially the MC240 do to the music make it sound absolutely fantastic.

Of course the MC240 sounds completely lucid, fluid and detailed as you expect from top-end tube amps, but what you probably don't expect is that it has bottomless bass thanks to its huge transformers and special patented design that offers low distortion and flat response down to 3 cycles! My MC240 has better bass response than many of today's preamplifiers and power amps, along with the astonishingly lifelike midrange and a high end that simply shimmers.

No other amplifiers have McIntosh's Unity Coupled circuit, that along with its huge Bifilar-wound output transformers, lower the distortion to the lowest levels found in tube amplifiers. This unique circuit cross-couples the two 6L6GC output tubes into such a tight a lover's embrace that they operate as class-A at the most important audio levels. The circuit is so linear and independent of tube characteristics that it plays at low levels even with one output tube missing!

The McIntosh MC240 is McIntosh's first stereo amplifier, and their last new tube amplifier design before they gave up on tubes in favor of transistor amplifiers.

The MC240 was first, and it was followed the next year by the more powerful MC275 (75 WPC) and simpler MC225 (25 WPC). This review applies just as well to the MC275 and MC225.

 

History

There are at least four versions.

There are three versions known to most people, and the last version as seen and reviewed here.

 

Specifications         top

Intro   Specifications   Measurements   Sound

Compared   Usage   Recommendations

 

Rated Output Power

40 WPC (16.02 dBW) stereo into 4Ω, 8 Ω, 16Ω, 125Ω (70.7V)  or 600Ω (140V) from  20-20,000 Hz with less than 0.5% THD.

80 watts mono into 2Ω, 4Ω, 8Ω, 16Ω or 32Ω depending on output connections.

This is a very serious output power specification, I know of no tube amplifier today that specifies output power at such a low level of distortion over such a huge frequency band. Modern high-end tube amplifiers lack the low-end guts of the McIntosh and can't put out much power at all at these low distortion figures at low frequencies.

 

Inputs

Three switch-selectable inputs, all 250 kΩ:

 

MONO

One mono input jack with a level control.

500mV minimum to 30V maximum input for rated output.

 

TWIN

Two input jacks, each with its own level control.

500mV minimum to 30V maximum input for rated output.

 

STEREO

Two input jacks.

No level control, but there is a balance control to fine-tune the system's balance between the two channels.

Level fixed at 2.0V input sensitivity for rated output.

 

Frequency Response

16 ~ 40,000 cycles +0, -0.1 dB at rated power.

16 ~ 60,000 cycles +0, -0.5 dB at rated power.

10 ~ 100,000 cycles +0, -1.0 dB at half rated power.

These again are serious specifications not matched today, even by McIntosh. Today's tube amps cover a similar range, but at -3 dB.

All other tube amplifiers, including new ones from McIntosh, have much sloppier specifications, and they certainly aren't rated at full power.

 

Phase Shift

± 6º 20 ~ 20,000 cycles.

 

Signal to Noise Ratio

≥ 90 dB below rated output.

This is ≤ -73.98 dBW or ≤ 566 µV or ≤ -64.95 dBV.

 

THD

< 0.5% from 20 ~ 20,000 cycles at rated power.

≤ 0.3% typical.

 

IMD

< 0.5% for peak powers up to twice rated power for any two frequencies from 20 ~ 20,000 cycles.

 

Damping Factor

> 10, regardless of selected output impedance.

Output Connectors

Barrier block, 3/8" spacing for low impedance loads.

Octal socket for high impedance loads.

 

Tube Complement

(4) 6L6GC or 7027A for output.

(3) 12AX7.

(2) 12BH7.

(2) 12AU7.

 

Quality

Made in Binghamton, New York, USA.

 

Construction

Sheet steel chassis.

 

Power Input

105-130 VAC 50 ~ 60 cycles.

Negative temperature coefficient thermistor for inrush current limiting for soft turn-on.

 

Rated Power Consumption

145W at idle.

270W at full power.

3.2A slo-blo fuse.

3A internal B+ fuse.

See also Actual Power Consumption.

 

Size

8 x 10-3/4 x 17-1/2" HWD.

 

Weight

56 pounds.

65 pounds as shipped.

 

Price, USA

May 2015: about $2,400 used if you know How to Win at eBay.

A really shiny one will sell for about $2,800, and a rusty one will sell for about $1,800.

$288 list price in 1960.

 

Measurements         top

Intro   Specifications   Measurements   Sound

Compared   Usage   Recommendations

These measurements are made with an exotic Rohde & Schwarz UPL laboratory analyzer. The traces from the Rohde & Schwarz UPL laboratory analyzer are color coded for the Left Channel and for the Right Channel. When they don't lie on top of each other, it's due to channel imbalance. When they do lie on top of each other, the trace turns blue.

Unless otherwise specified, all measurements are RMS at 1 kHz at 1 W into 8Ω from the 8Ω tap, both channels driven. I used a 117 VAC supply voltage.

 

Input Levels    Gain

Frequency Response  Distortion   Noise

Actual Power Consumption

 

Input Levels     measurements        top

 

STEREO Input

475 mV for 1 WPC output.

1.502 V for 10 WPC output.

3.030 V for 40 WPC output.

 

MONO or TWIN Inputs

83 mV for 1 WPC output.

524 mV for 40W output.

 

Gain     measurements        top

 

STEREO Input

15.533 dB when set to level both inputs.

16.123/15.156 dB at center.

 

16.182/15.107 dB at center.

16.543/15.543 dB when set to either side.

 

old tubes:

15.500 dB left, 15.500 dB right, at balance S.

15.714 dB left, 15.272 dB right, at balance center.

16.092 dB left, -50.22 dB right, at balance left.

-53.71 dB left, 15.979 dB right, at balance right.

 

Twin Inputs

30.845 dB left, 30.708 dB right, at maximum.

 

30.86 dB left, 30.62 dB right, at maximum.

Old tubes: 30.453 dB left, 30.825 dB right, at maximum.

 

MONO Input

30.823/30.68 dB.

 

30.824/30.580 dB at maximum.

MAX
+30.7 dB
4 o'clock
+30.1 dB
3 o'clock
+27.3 dB
2 o'clock
+22.6 dB
1 o'clock
+19.0 dB
12 o'clock
+16.9 dB
11 o'clock
+14.7 dB
10 o'clock
+11.3 dB
9 o'clock
+7.2 dB
8 o'clock
-9 dB
MIN
-53 dB

 

Old tubes:

30.432 dB left, 30.855 dB right, at maximum.

16.734 dB left, 17.17 dB right at 12 o'clock.

 

Frequency Response      measurements    top

McIntosh MC240

Frequency response.

 

McIntosh MC240

Frequency response, expanded scale.

 

McIntosh MC240

Phase response.

 

McIntosh MC240

Infrasonic response, cursor at 18 cycles at -0.1 dB.

 

McIntosh MC240

Infrasonic response, cursor at 8.5 cycles at -0.5 dB.

 

McIntosh MC240

Infrasonic response, cursor at 6 cycles at -1 dB.

 

McIntosh MC240

Infrasonic response, cursor at 3 cycles at -3 dB.

 

McIntosh MC240

Infrasonic response, cursor at 3 cycles at -3 dB.

 

McIntosh MC240

Infrasonic response to 1 cycle.

 

McIntosh MC240

Infrasonic response to 100 millicycles.

 

McIntosh MC240

Ultrasonic response, Stereo input.

 

McIntosh MC240

Ultrasonic response, DUAL input at full level.

 

Distortion    measurements        top

McIntosh MC240

Waveform at 25 watts per channel, old tubes.

 

McIntosh MC240

Waveform after new tubes from TubeDepot.

 

McIntosh MC240

Waveform after new tubes from TubeDepot; a 12k Ω 2W resistor is still out-of-spec and causing the clipping on the negative part of the right channel.

 

McIntosh MC240

Waveform at 20 cycles after new tubes from TubeDepot; a 12k Ω 2W resistor is still out-of-spec.

 

McIntosh MC240

Waveform at 40 watts per channel after new tubes from TubeDepot and replacing a 12k Ω 2W resistor that was out-of-spec.

 

McIntosh MC240

Waveform at 20 cycles at 40 watts per channel after new tubes from TubeDepot and replacing a 12k Ω 2W resistor that was out-of-spec.

 

McIntosh MC240

Waveform at 20 cycles at 44 watts per channel after new tubes from TubeDepot and replacing a 12k Ω 2W resistor that was out-of-spec.

 

McIntosh MC240

Waveform at clipping after new tubes from TubeDepot and replacing all four 12k Ω 2W resistors. Finally, symmetrical clipping!

 

McIntosh MC240

10 cycles burst at 1kc after new tubes from TubeDepot and replacing bad 12kΩ 2W resistor in right channel.

 

McIntosh MC240

5 cycle burst at 50 cycles after new tubes from TubeDepot and replacing bad 12kΩ 2W resistor in right channel.

 

McIntosh MC240

2 cycle burst at 20 cycles after new tubes from TubeDepot and replacing bad 12kΩ 2W resistor in right channel.

 

McIntosh MC240

THD versus power, old tubes, April 2015.

 

McIntosh MC240

THD versus power after testing tubes, putting good ones on Right Channel and then replacing two weak 6L6WGH with two new 6L6GH on Left Channel, 29 April 2015.

 

McIntosh MC240

THD versus power after testing tubes, putting good ones on Right Channel and then replacing two weak 6L6WGH with two new 6L6GH on Left Channel and swapping an old USA 12AX7 and 12U7 for new Chinese tubes, 29 April 2015. Left channel finally meets 40 W at < 0.5% THD; it does it at 0.2%!

 

McIntosh MC240

THD versus power after new tubes from TubeDepot, 08 May 2015. Right channel still out-of-spec due to what turns out to be a bad 12k Ω 2W resistor.

 

McIntosh MC240

THD versus power after new tubes from TubeDepot and replacing bad 12kΩ 2W resistor, 29 May 2015.

 

McIntosh MC240

THD versus power at 20 cycles after new tubes from TubeDepot and replacing bad 12kΩ 2W resistor, 29 May 2015. 42 watts at 0.5% THD at 20 cycles, not bad! What looks like higher THD below 1 watt is actually 60 cycle power supply hum looking like harmonics; ignore it.

 

McIntosh MC240

THD versus power after new tubes from TubeDepot and replacing all four 12kΩ 2W resistor, 29 May 2015. Note matching distortion signatures. Moving the cursor also shows 0.15% THD at 38W, 0.3% THD at 43W, 0.5% THD (spec) at 45.5W, 1% THD at 48W and 4.5% THD at 51W. Of course the maximum power varies as the square of the applied power supply voltage. Run this amplifier from 125V instead of 117V as tested and the clipping part of the curve would move to the right by 6 watts.

 

McIntosh MC240

THD versus power after new tubes from TubeDepot, replacing all four 12kΩ 2W resistors, rebuilding the power supplies and replacing the thermistor, 12 June 2015. Now I get over 45W at less than 0.5% distortion.

 

McIntosh MC240

THD versus power after new tubes from TubeDepot, replacing all four 12kΩ 2W resistors, rebuilding the power supplies, replacing the thermistor and then calibrating all the power supply voltages, 12 June 2015. Now I get 42W at 0.2% THD.

 

McIntosh MC240

THD versus power after new tubes from TubeDepot, replacing all four 12kΩ 2W resistors, rebuilding the power supplies, replacing the thermistor and then calibrating all the power supply voltages, 12 June 2015. Now I get 44W at 0.25% THD.

McIntosh MC240

THD versus power after new tubes from TubeDepot, replacing all four 12kΩ 2W resistors, rebuilding the power supplies, replacing the thermistor and then calibrating all the power supply voltages, 12 June 2015. Now I get over 47W at less than 0.5% THD. HA!

 

McIntosh MC240

THD at 10 mW after new tubes from TubeDepot and replacing bad 12kΩ 2W resistor in right channel.

 

McIntosh MC240

THD at 10mW after new tubes from TubeDepot and replacing bad 12kΩ 2W resistor.

McIntosh MC240

THD at 100mW after new tubes from TubeDepot and replacing bad 12kΩ 2W resistor.

 

McIntosh MC240

THD at 1 W, old tubes.

 

McIntosh MC240

THD at 1W after testing tubes, putting good ones on Right Channel and then replacing two weak 6L6WGH with two new 6L6GH on Left Channel, 29 April 2015.

 

McIntosh MC240

THD at 1W after new tubes from TubeDepot, bad 12kΩ 2W resistor in right channel.

 

McIntosh MC240

THD at 1W after new tubes from TubeDepot and replacing bad 12kΩ 2W resistor in right channel.

 

McIntosh MC240

THD at 1W after new tubes from TubeDepot, replacing all four 12kΩ 2W resistors, rebuilding the power supplies and replacing the thermistor, 12 June 2015. Now the channels match.

 

McIntosh MC240

THD at 10W after new tubes from TubeDepot, bad 12kΩ 2W resistor in right channel.

 

McIntosh MC240

THD at 10 W after new tubes from TubeDepot and replacing bad 12kΩ 2W resistor in right channel.

 

McIntosh MC240

THD at 20 W after new tubes from TubeDepot and replacing bad 12kΩ 2W resistor in right channel.

 

McIntosh MC240

THD at 40 W after new tubes from TubeDepot and replacing bad 12kΩ 2W resistor in right channel. THD went up to 20% at 10 cycles at 40W.

 

McIntosh MC240

Same plot as above (THD at 40 W), except plotted on same scales as McIntosh Audio Clinics used, also after new tubes from TubeDepot and replacing bad 12kΩ 2W resistor in right channel.

 

McIntosh MC240

Same thing (THD at 40W) after new tubes from TubeDepot, replacing all four 12kΩ 2W resistors, rebuilding the power supplies and replacing the thermistor, 12 June 2015.

McIntosh MC240

THD at 40W after new tubes from TubeDepot, replacing all four 12kΩ 2W resistors, rebuilding the power supplies and replacing the thermistor, 12 June 2015.

 

McIntosh MC240

Harmonic distortion content at 1 mW, new tubes and replaced resistor.

 

McIntosh MC240

Harmonic distortion content at 10 mW, new tubes and replaced resistor.

 

McIntosh MC240

Harmonic distortion content at 100 mW, new tubes and replaced resistor.

 

McIntosh MC240

Harmonic distortion content at 1 W, old tubes. Looks same at 10W, just different overall level.

 

McIntosh MC240

Harmonic distortion content at 1 W, new tubes and replaced resistor.

 

McIntosh MC240

Harmonic distortion content at 10 W, new tubes and replaced resistor.

 

McIntosh MC240

Harmonic distortion content at 40 W, new tubes and replaced resistor.

 

McIntosh MC240

19 + 20 kHz difference-frequency distortion at 100 mW after new tubes from TubeDepot and replacing bad 12kΩ 2W resistor in right channel.

 

 

McIntosh MC240

19 + 20 kHz difference-frequency distortion at 1 W, old tubes.

 

McIntosh MC240

19 + 20 kHz difference-frequency distortion at 1W, new tubes and new resistor.

 

McIntosh MC240

19 + 20 kHz difference-frequency distortion at 10 W, old tubes.

 

McIntosh MC240 measured performance

19 + 20 kHz difference-frequency distortion at 10 W, new tubes and new resistor.

 

Output Noise    measurements        top

McIntosh MC240

Output noise spectrum.

 

A Weighted
Unweighted 22kHz BW
-70 dBV A
-60 dBV

 

 

Damping Factors    measurements        top

McIntosh MC240

Frequency response driving an 8Ω resistor (green trace) and driving a real B&W CDM2 bookshelf loudspeaker (yellow trace), whose impedance varies between 6 and 45 Ω, a typical loudspeaker load. First try with old tubes.

 

McIntosh MC240

Frequency response driving an 8Ω resistor (green trace) and driving a real B&W CDM2 bookshelf loudspeaker (yellow trace), whose impedance varies between 6 and 45 Ω, a typical loudspeaker load. Now with new tubes and four new 12 kΩ resistors. Note lower output impedance.

McIntosh MC240

Same as above, expanded scale.

 

Actual Power Consumption     measurements        top

60Ω thermistor 24 June 2015

Big 60 Ohm thermistor: 12V drop, then 6V after 1.5M. 4.4V after 3.5M. 4.11V after 5M. 5V drop at 40 WPC, then 2.83V drop at idle. Power line reads 3.6Ω DC after running. 11Ω after a minute blowing on it.

 

Turn on at 120V

75W 15s

116W ar 30s

124W at 45S

127W at 60s

129W at 90s

 

 

 

117V

128W at idle.

286W at 40 WPC

 

120V:

135W idle.

144W at 1 WPC

195W (2.01A) at 10 WPC

294W at 40 WPC (3.0 A)

B+: 500 mA idle, 623 mA 1 WPC, 1.04A at 10 WPC,1.7A at 40 WPC

 

-------

Cool Down

15s 3Ω

30s 3.8Ω

1M 4.7Ω

2m 6.2Ω

3m 8.3Ω

4m 10.7Ω

5m 13.25Ω

7m 18.75Ω

10m 26.45Ω

11.5m 30Ω

14:10 35Ω

15m 36.25Ω

20m 41.9 Ω

30m; 47.5Ω

45m 51.3 Ω

57m: 52.9Ω

68.5 Ω 12 hrs 23º C

65.9 Ω 16 hrs 24.3º C

 

75W at turn-on, then 69W, then 135 at 30 seconds

145W (1.52 A) idle at 117 VAC after 90s.

149W (1.54A) at 1WPC

194W (2.01A) at 10 WPC.

286W (2.95A) at 40 WPC.

 

Old tubes

135 watts at idle.

It draws 141 W at 1 WPC.

It draws --- W at 40 WPC.

 

Sound         top

Intro   Specifications   Measurements   Sound

Compared   Usage   Recommendations

Sound is fantastic. Unlike lesser amplifiers, the huge transformers make this amplifier completely unflappable with low frequencies and subsonics.

 

Compared         top

Intro   Specifications   Measurements   Sound

Compared   Usage   Recommendations

 

Versus the new McIntosh MC275

The new digitally controlled (lights and power switching) MC275 is nice, but a very different design.

It uses different tubes and different transformers. The tube complement was changed to use tube types that are easy to find today.

Its design is optimized to work perfectly with crummy modern tubes, even if they only have 25% of the emission that they ought to.

The new MC275 uses RoHS lead-free solder so that it can be sold overseas.

I haven't tested this new amplifier yet. Exciting is how its design has been updated to perform flawlessly with the tubes we easily can buy new today, not harder-to-find tubes used in the original design, which expected great tubes.

 

Usage         top

Intro   Specifications   Measurements   Sound

Compared   Usage   Recommendations

 

 

Feet

 

 

 

Connections

 

 

Level Controls

 

ve that level all the way up and control it from the amp's own control.

 

 

Recommendations         top

Intro   Specifications   Measurements   Sound

Compared   Usage   Recommendations

Get one. It should be the last amplifier you ever need.

Enjoy!

 

Help me help you         top

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If you haven't helped yet, please do, and consider helping me with a gift of $5.00.

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Thanks for reading!

 

 

Mr. & Mrs. Ken Rockwell, Ryan and Katie.

 

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October 2017, August 2015, 15 April 2015