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Manley Stereo Variable Mu® Limiter Compressor (T-Bar Mod)  From Manley

The Variable Mu Limiter Compressor is the GLUE that holds the mix together while crystallizing it into a professional, final product. All-tube, all transformer, an all-American legend. Specially modded to enable usage of two 12BA6 or 6BA6 single pentodes wired in triode mode to have the same limiting characteristics of the original 6386 for smoother operation. 

Manley Laboratories Inc


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Manufacturer's Description from Manley

The MANLEY VARIABLE MU® LIMITER COMPRESSOR has been our best selling product for many years. It is one of the very few compressors that has become a real standard in Mastering studios and contributed to most hit records over the last decade and probably the next. "Mu" is tube-speak for gain, and Variable Mu® is our registered trademark for this limiter compressor. It works by using the "remote cut-off" or re-biasing of a vacuum tube to achieve compression. The precious vintage Fairchild 670 also uses this technique and is one of few all-tube compressor to do so, that we know of. Even the side-chain has glowing rectifier bottles. How’s it work? The unique 5670 dual triode is at the center of the peak-reducing and compression action constantly being re-biased by the vacuum tube rectified side-chain control voltages which cause this tube to smoothly change its gain. Just like that.

The COMPRESS mode is soft-knee 1.5 to 1 ratio while the sharper knee LIMIT mode starts at 4 to 1 and moves to a more dramatic ratio of 20 to 1 when limiting over 12dB. Interestingly, the knee actually softens as more limiting is used. Distortion can be creatively used by turning up the Input and turning down the Output while using very little or no compression. See the gain reduction curves here!

You might notice that the Variable Mu® Limiter Compressor has a ganged input control, but do not jump to conclusions that it is mono-unfriendly. Track away! There are separate threshold and output controls to make compensations with plus you can always adjust your individual source levels elsewhere, right? The advantage of the stereo input control becomes dramatically clear when you switch to LINK mode, and that’s what our Variable Mu® Limiter Compressor does better than anything else: final mix, 2-track, or mastering limiting and compression. Like one reviewer put it: “It’s like pouring a bowl of sweet cream over the mix.” Mmmmmm. Yummy. Give your music a big hug.


PURPOSE: To provide a replacement for the obsolete 6386 tubes in earlier Manley Variable Mu® units or convert later 5670 units to have the same limiting characteristics of the original 6386 for smoother operation.

METHOD: To install a converter board and surrounding modifications to enable usage of two 12BA6 or 6BA6 single pentodes wired in triode mode to take over the duties of a single envelope dual triode 6386 or 5670 (per channel).

UNITS AFFECTED: Serial numbers for units that will benefit from this update are (all). See the Version Identification page for more info.

INTRODUCTION: The Manley Variable Mu® has been in production for some years now and has become an indispensable tool in studios since its inception. These units operate on the same principle as several top-notch vintage tube limiters, and the original Manley version indeed used the same 6386 input tube as the Fairchild, Gates, and others. This tube is the one that actually performs the gain reduction; it, more than any other component, determines the sonic signature of these units. HISTORY: The 6386 dual triode was designed as a cascode RF/IF gain-controlled amplifier, and as such, had many characteristics ideally suited for use as a gain-controlled audio amplifier. However, it seems this tube was used more in commercial/industrial applications rather than high volume consumer applications (like in radios and TV's). Also, it appears that none of these have been manufactured in recent years. The combination of these two factors means that present supplies of 6386 tubes are quite limited, expensive, and in many cases unusable due to poor matching, high noise, or microphonics (which may not have been issues for the tube's original intended use). Manley decided in June of 1996 to begin using a type 5670 dual triode which were similar, importantly sharing the same pin-out as the 6386 and available in sufficient quantities to allow for testing and matching.

ENTER NEW TUBE TYPES: The 5670 equipped Variable Mu's were well received, and retained many of the characteristics that made the original version of the Variable Mu® popular. They did sound different than the original, especially when pushed past 6db or so of limiting. Even though some people even preferred the new version with the 5670, it was felt that it would be worthwhile to give the users the ability to return to the traditional sound without resorting to use of an esoteric (or unavailable) tube. We also wanted to furnish an alternative replacement for the 6386 in older units. The goal was to find a tube that was 1) nearly identical to the 6386 in performance 2) manufactured (or being manufactured) in large quantities 3) able to be retrofitted into existing units (in the field, if at all possible). This was a tall order, as other available remote-cutoff dual triodes (such as the 6BC8 and 6ES8) are not close enough in characteristics to be used as a direct replacement. Enter the 12BA6, which is a remote-cutoff single pentode. Wired as a triode, this tube has characteristics very close to the 6386 as confirmed by a Tektronix 570 curve tracer, and by A-B listening tests. These tubes were manufactured in large quantities for use as gain-controlled IF amplifiers in common radio receivers. The hitch: the 6BA6 (or 12BA6) is a single-section tube; the 6386 is a dual!

ENTER THE ADAPTER BOARD: So why not use two 12BA6 tubes to replace one 6386? There is enough room inside the unit to accommodate two tubes; the pair will directly replace a 6386 in an older unit or a 5670 in a newer unit with only minor modifications. The other advantage to this approach is that the tubes can be tested and installed as a matched pair. We have numerous examples of 6386's where the two triode sections inside do not match each other within a usable range and there is nothing we can do about it because the two triode halves are locked inside the bottle. So what we did was to design a 1"x 2" circuit board that has two sockets for the 6BA6 or 12BA6 tubes ("6" or "12" depending on the heater voltage) that plug in where the original tube went. Of special importance in using two single element tubes to replace a dual element tube is the ability to match and select very well matched tubes not only for each channel's two phase-halves, but also for both channels. This factor alone, especially when considering that there are really no decent 6386 tubes left, allows the "T-Bar Mod" units' performance to easily exceed the older units. There are four major versions of the stereo Variable Mu® that have been built since 1994, and consequently three different versions of the adapter boards. Refer to the Identifying the Different Versions of the Variable Mu® page for specific details as to which board you'll need.

INSTALLATION: Version 1 adapter boards require no modifications to the existing circuitry, only re-calibration. Versions 2, 3, and 4 require minor circuit modifications as well as re-calibration. Instructions for Version 4 units can be found here.

We can perform the installation at our Manley Service Center for a nominal charge, or it can be done by a technician familiar with repair and modification of audio gear under our direction. An audio signal generator and AC voltmeter are also required to calibrate the unit. Reading the "Installation Instructions" will give you an idea of the work involved for the different units.

SOUND COMPARISON: In direct A-B comparisons, the 6BA6/12BA6 version sounds nearly identical to the 6386. Comparing to the 5670, the 6BA6/12BA6 sounds less "squashed" as the amount of limiting is increased; consequently, it is possible to obtain more limiting with fewer artifacts than before. Performance at faster attack times is enhanced as the 6BA6/12BA6 tubes can be better matched than before with the dual-triode tubes.

COST AND TURNAROUND TIME: The conversion kit installation can be done by Paul Fargo in our Arizona Service Center for $500 plus shipping, and includes all necessary parts and labor plus re-testing and calibration. This usually takes 2-5 days depending on existing workload. It is recommended to schedule in advance with Paul for fastest turnaround. We can also supply just the kit with installation instructions for $300 plus shipping (and sales tax as necessary) for installation in the field by a qualified technician.

Manley Laboratories Inc

About Manufacturer

The MANLEY brand has been around since the late 1980's. Manley's first commercially available products were upper echelon high-fidelity vacuum tube amplifiers and preamplifiers constructed at the old Vacuum Tube Logic of America factory.

In this history lesson, we'd like to direct special focus on our people at Manley Labs. The people whose years of dedication, service, and expertise bring you people, our customers, these products. It's all about the people...

On January 30, 1989, a 20 year old EveAnna on sabbatical from her studies at Columbia University met the crew and was hired on to the production team. Baltazar Hernandez, already with two years experience, taught her how to solder in those first few days. It was in 1989 that the first Manley professional products were developed, starting with the original 1u Reference 60dB Microphone Preamplifier and soon followed by our Manley Enhanced Pultec and Mid Frequency Equalizers. Baltazar and EveAnna, busy building gear, were soon joined by Saulo and Ramiro. Just next door, Elias was employed by a printed circuit board maker later brought into the Manley Labs fold when in 1993, Manley Laboratories, Inc. became a separate entity apart from VTL. The new Manley Labs factory at 13880 Magnolia Ave. in beautiful and bovine Chino, California was outfitted to house the Manley HiFi and Pro Studio manufacturing as well as the Langevin production lines, our newly acquired machine shop, and printed circuit board facilities.


  • MANLEY input & output transformers with nickel laminations in mu-metal cases with flat frequency response from 20Hz-25KHz
  • Fully differential ALL-TUBE circuitry using one each 5670, 5751 (or 12AX7), 7044 (or 5687), & 12AL5 per channel
  • Independently regulated B+ and Heater supplies
  • Hard-wire BYPASS switch
  • Stepped switch INPUT attenuator as of 10/2011. (older units use Silent dual-ganged potentiometer)
  • RECOVERY 5 steps: 0.2s, 0.4s, 0.6s, 4sec., 8sec.
  • Variable ATTACK: 25msec-70msec
  • Continuously variable THRESHOLD
  • LIMIT (4:1 to 20:1) or COMPRESS (1.5 to 1)
  • HP SC: High Pass Side Chain Filter -3dB @ 100Hz
  • Large ILLUMINATED Sifam METERS (older units before serial number MSLC61642 shipped before 12/2003 use: 26V 1.2W FESTOON LAMPS; Manley's Part Number: VAR016B) Order spare bulbs using our parts order form. (newest units after serial number MSLC61642 shipped after 12/2003 use white LED lighting)
  • Several units can be linked for Surround (custom order )
  • Maximum gain: 35dB
  • Max. output: +30dBu (26Vrms) 26dB Headroom
  • <0.1% THD @ 1KHz Noise floor: -85dB typical
  • Power Consumption (120/240VAC): 80 watts
  • Unit is factory set for 100V, 120V or 220-240VAC operation for original destination country's mains voltage.
  • Operating Mains Voltage changeable with power transformer changeover switch and fuse value change.
  • Mains Fuse Value for 100~120VAC operation: replace with 1.25A as of 10/2011
  • Mains Fuse Value for 220~240VAC operation: replace with 0.6A (600mA) as of 10/2011
  • Mains Fuse Type: MDA or MDL SLO-BLO Time delay 1 1/4" x 1/4"
  • Mains Voltage Frequency: 50~60Hz
  • Dimensions: 19” x 3 1/2” x 10” (chassis occupies 2u) Power transformer protrudes 3.5" out the back of the chassis.
  • Shipping Weight: 23 lbs.