Luxman MQ-88uC Stereo Power Amplifier

Cause for Celebration

Equipment report
Categories:
Tubed power amplifiers
Luxman MQ-88uC Stereo Power Amplifier

In the pantheon of Japanese audio companies, Luxman holds a place of honor for significant contributions to the audio arts over many decades. In fact, several years ago it celebrated its 90th anniversary. What is not so well known is that Luxman has been manufacturing tube amplifiers continuously since 1961. I don’t know of any other audio company that can equal that track record. Luxman had the foresight in the mid-1970s to hire Tim de Paravicini as a design engineer. In particular, one of his designs, the MB-3045 monoblock, is one of my all-time favorites. 

The new MQ-88uC stereo power amplifier reviewed here was fully designed and engineered in-house. The lead engineer was Kazuyuki Doi, a 40-year veteran at Luxman, who is also responsible for the MQ-300 single-ended triode amplifier and the upcoming CL-1000 preamp. All of the final sonic tuning was performed by Mr. Nagatsuma, head of R&D. In the past, tube amplifier introductions by Luxman have typically been a cause for celebration, and the MQ-88uC is no exception. My first impression was that it exemplified Luxman’s vacuum-tube heritage and design acumen. 

The chassis evokes a classic Luxman retro look, and is the work of industrial designer Mr. Kenji Hagiwara, who was also the lead industrial designer on several recent products, including the L-509X and NeoClassico II integrated amplifiers and the PD-151 turntable. Removing the tube cage exposes two pairs of KT88 beam power tubes. Don’t let that fool you. Unusually, this is not an Ultra-Linear (UL) or beam-power amplifier. This Lux was designed from the ground up as a push-pull triode amp. In the 1950s, the push toward greater power and higher efficiency, coupled with the availability of new power tubes such as the EL34, KT88, and 6550, ousted filamentary triode designs from mass market applications. Here each KT88 is wired as a triode by having the screen grid connected to the plate via a current-limiting resistor. Some have referred to such a connection as a fake triode. Not so—transfer characteristics don’t lie. A triode-connected KT88 measures and acts like a triode, and most importantly, sounds like a triode.

The MQ-88uC is modeled after the vintage Luxman MQ-60 amplifier, which dates back to 1969. The MQ-60 was a triode version of the classic Mullard 5-20 circuit with the substitution of the NEC 50CA10 power triode for the EL34 pentode. The Mullard 5-20 push-pull topology achieved significant popularity in the early 1960s, and in my estimation, is still more than satisfactory today. Think of the MQ-88uC as a modernized version of the MQ-60. Instead of the EF86 small-signal pentode favored by Mullard, the input voltage gain stage was modified to use a JJ Electronic ECC83S dual triode connected in parallel for lower noise and increased transconductance. Although it is a 12AX7 type, it features rugged construction similar to that of a Telefunken ECC803S. The input stage is directly coupled to an ECC82/12AU7 dual triode configured as a long-tailed pair phase splitter.

The output stage uses fixed bias obtained from a dedicated power supply. Bias adjustment pots and test points corresponding to each output tube are located in front of the output transformers. Luxman apparently did not intend these adjustments to be performed by anyone other than a qualified technician. Hence, no biasing instructions are included in the owner’s manual. However, seasoned audiophiles who are accustomed to biasing power tubes should have no problem with this. Although this amp does not require frequent bias checks, it is good practice to check and tweak the bias after the initial installation since the bias may vary with the local AC mains voltage, which in many localities runs higher than nominal. I would recommend a multimeter with long leads capable of measuring DC voltage below 1V. There are three test points for each channel, with the center point being ground, while the left and right test points are for the left and right KT88s. An insulated Phillips head screwdriver should be used to adjust the corresponding potentiometer as needed. And unless you have asbestos fingers, I would caution you to use a cotton glove to protect your hand from accidental contact with a hot tube. The test points allow you to measure the voltage drop by the idle current across a 10-ohm cathode resistor, a very common test scheme. The recommended voltage range for each test point is 0.46V to 0.48V, which implies an idle current of 46 to 48mA. In my listening room, the initial bias voltage checks conducted after an hour warmup were in the range of 0.50V to 0.53V. And after being tweaked to within the recommended range, the bias stayed rock-solid over a period of many weeks.

The power tubes are run conservatively. With a plate voltage of 460V and an idle current of 48mA, plates dissipation is only 22W, about half of the rated KT88 maximum dissipation of 40W. The entire tube complement is sourced from JJ Electronic, a choice made by Luxman not only on the basis of reliability and long life, but also due to sound quality. In particular, this KT88 is known for its warm and natural harmonic structure. JJ Electronic has come a long way since it was founded in 1993 by Jan Jurco as a small garage business, and now manufactures more than 35 preamp, power, and rectifier tubes. Noteworthy is the fact that every component used in its vacuum-tube production is manufactured in-house.

As we all know by now, the output transformer is an integral part of a conventional tube amp and plays a critical role in its performance, and ultimately sound quality. Luxman’s classic amplifiers had always used OY15-type output transformers that featured interleaved windings for superior bandwidth, neatly packaged in die-cast aluminum housings. These have been redesigned for the MQ-88uC and reproduced with the original shape. Impedance taps are provided for 4, 8, and 16-ohm loudspeakers. In addition, two sets of RCA inputs are provided on the back panel. One set bypasses the front-panel volume control for a direct path to the input stage. Should you wish to connect a DAC directly to the amplifier, be sure to use the variable inputs. 

Featured Articles

Lists