Luxman MQ68c Tube Amplifier - Serviced

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General:
The MQ68c is a 25W stereo tube amplifier made by the renowned Japanese audio company Luxman.

It was handmade in Japan from the highest quality materials of the time and benefits greatly from the quality of the transformers in particular, which contributes to the lush warm dynamic sound quality.

Luxman tube amps from this era are a rare find in the US. Most were sold overseas and kept in the Asian market. You can buy them from Japanese websites, but it will cost you an arm and a leg and may not be the correct voltage.

This particular amp was just serviced and is working flawlessly.

SkyFi Cosmetic Notes:
This amplifier has a few minor cosmetic issues. One of the wooden guides for the tube cage is missing. One of the input attenuator knobs has some damage, and there are some dings on the back side of the chassis.

SkyFi Technical Notes:
This amplifier was serviced here at SkyFi. We found a couple of leaky coupling capacitors in the driver circuitry. We opted to replace all six coupling caps with polypropylene film types. We also replaced the distortion trimmers and tested all of the tubes. Bias and distortion were then calibrated for optimal performance.

The internal interrupter switch was bypassed which allows the amplifier to operate while the tube cage is removed.

Brand Background:
The history of the LUXMAN audio brand began in 1925, at the birth of radio broadcasting. LUXMAN has paid particular attention to the world of audio and has gained high reputation as a quality brand of high-end audio products both domestically and internationally.

Ownership:
Single Owner

Connections:
RCA Line inputs, A pair of speaker screw down terminals

General Sound:
Smooth, uncolored, undistorted natural and clean

Cosmetic Condition:
6/10 = Fair. Three to four minor scratches, or one major scratch. See our detailed rating description here.

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

Included:
Just the Unit

Packing:
Will be packed using our highly developed in-house process and custom packing materials.Specs:
Power output: 30 watts per channel into 8Ω (stereo)
Frequency response: 10Hz to 60kHz
Total harmonic distortion: 0.05%
Damping factor: 14.5
Input sensitivity: 0.88V
Signal to noise ratio: 105dB

Dimensions:
15.75" x 6" x 6.5"

Weight:
30 lbs.

Approximate Age:
1980

Recommended Cables:
Kimber Kable - RCA Interconnects - Better

Kimber Kable - Speaker Cables - Better

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.