McIntosh MC30 Vintage Tube Monoblock Amplifier Pair - Electronically Restored by SkyFi

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General:
With regards to sound quality, the MC30 is considered to be the most desirable McIntosh model among audiophiles and collectors. It was only produced from 1954 to 1962, and unlike other McIntosh Tube amps there were never any later model anniversary or commemorative editions produced.

The restoration goal for this pair was to make these classics as reliable and as cosmetically clean as possible, without disturbing their special vintage sound quality.

In addition to the restoration, we fitted a brand new tube set including current production and NOS (new old stock) vintage tubes.

The end result is they sound fantastic!

SkyFi Cosmetic Notes:
This pair of amplifiers was carefully cleaned and polished. There is some light chrome pitting and corrosion to both chassis, but the chrome overall is above average  especially considering these are nearly 70 years old. New capacitor cans were installed. Transformers show some wear. One amplifier had its barrier strip replaced, the new barrier strip has a Cinch Jones logo where the original on the other amplifier does not.

Overall they are in great shape and present well. 

SkyFi Technical Notes:
This pair of amplifiers underwent an extensive electronic restoration here at SkyFi.

This work included the replacement of all capacitors, including a custom chrome replacement for the multi-section filter on the top of the chassis. The amps were fairly far apart in serial number and had slight internal differences.

We brought both pieces up to the configuration of the later serial number, and both are now identical electronically. Select resistors were replaced as needed. We also loaded in a fresh tube set featuring current production Tung Sol 6L6GC, along with a mix of NOS and strong-testing pre-owned vintage tubes for the rectifier and driver sections.

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:
Single Owner

Connections:
RCA, Speaker Binding Posts, AC Power Outlets

General Sound:
Smooth, uncolored, undistorted natural and clean

Cosmetic Condition:
8/10 = Very Good. Excellent front faceplate, one minor flaw on chassis side or top. 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.Specs:
Power output: 30 watts into 8 Ω (mono)
Frequency response: 20 Hz to 30 kHz
Total harmonic distortion: 0.3%
Damping factor: 12
Input sensitivity: 0.5V
Signal to noise ratio: 90 dB
Speaker load impedance: 4 Ω to 16 Ω
Tube complement: 2 x 6L6GC, 2 x 12AX7 1 x 12AU7, 1 x 12BH7, 1 x 5U4GA

Dimensions - Each M30 Amplifier
13" x 8" x 8"

Weight - Each MC30 Amplifier
30.5 lbs.

Approximate Age:
1954

Link to Manual:
Click Here

Recommended Cables:
Kimber Kable - RCA Interconnects - Better

Kimber Kable - RCA Interconnects - Best

Kimber Kable - Speaker Cables - Better

Kimber Summit Series Monocle XL Speaker Cables (PAIR) - Best

Testing Process:
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 a modern Amplitrex AT-1000 Professional 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 1 KHz test signal, slowly increasing 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 1 KHz square wave check and a 20 Hz to 20 KHz 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 minimum.

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.