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Manley Stereo Variable Mu Limiter / Compressor with T-Bar Mod
Manley Stereo Variable Mu Limiter / Compressor with T-Bar Mod

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Manley Stereo Variable Mu Limiter / Compressor with T-Bar Mod

The Variable Mu T-Bar Mod provides a nearly-identical sounding replacement for the obsolete 6386 tubes featured in earlier Manley Variable Mu® units by adding a custom 1"x 2" circuit board that has two sockets for the more readily available 6BA6 or 12BA6 tubes

The Manley Variable Mu Limiter Compressor is an industry standard for audio professionals, and now with the T-Bar Mod, you get the same great sound using easier-to-find 6BA6 or 12BA6 tubes.

The Variable Mu is a main-stay in recording and mastering studios all over the world. 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 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. 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.

Manley T-Bar Mod

The Manley Variable Mu® has been in production for some years now and was design using the same 6386 input tube as the Fairchild, Gates, and other classic compressors. This tube is the one that actually performs the gain reduction; it, more than any other component, determines the sonic signature of these units.

However, 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 the T-Bar Mod, 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.

Variable Mu® Limiter/Compressor T-Bar Mod Features

  • MANLEY input & output transformers with nickel laminations in mu-metal cases with flat frequency response from 20Hz-25KHz
  • Fully differential ALL-TUBE circuitry
  • Variable Gain vacuum tube: 6BA6 x4 T-BAR Mod
  • Amplifying vacuum tubes: 5751 or 12AX7 x2
  • Output vacuum tubes: 12BH7 x2 current production since 4/2016
  • Output vacuum tubes: prior to MSLC94006 use 7044 or 5687 x2
  • Side Chain Rectifier Diodes: 12AL5 x2
  • Independently regulated B+ and Heater supplies
  • Hardwire 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 )

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