Benefits of the T-Bar Mod

Earlier Variable Mu units used a 6386 dual triode for variable gain control — the same vacuum tube used by the Fairchild 670 in the 1950s. When the 6386 went out of production and high-quality samples became hard to source, Manley replaced it with a similar dual triode, the 5670, in 1996. So, do they sound the same? Up to around 6dB of limiting, they do, indeed. But at higher levels of compression, the 5670 tends to sound more “squashed,” and less natural. Some users actually prefer the 5670 version, while others prefer the original 6386, as it can be pushed a lot farther before harmonic distortion builds.

To please all concerned, Manley came up with the T-Bar Mod, which consists of a converter board and surrounding modifications that permit the use of two 6BA6 single pentode tubes — which exhibit smooth limiting curves closely resembling those of the 6386 — wired in triode mode to assume the duties of a single dual triode 6386 or 5670 per channel. The board simply plugs in where the 6386 or 5670 did; however, the mod also involves some additional modifications.

The T-Bar Mod accomplishes three things. If you are purchasing a new Variable Mu, the mod is the optimal method for perfectly matching each per-channel phase-half section and each stereo set. The tubes involved are readily available in quantity, which means Manley can meticulously test and select matched pairs for low noise and low microphonic characteristics. So, you’re getting the most highly optimized version of the Variable Mu ever made, with the smooth, natural limiting characteristics of the original 6386 tube. For owners of 5670 or 6386 Variable Mu units who want or need to upgrade, the T-Bar Mod can be done at Manley’s service center and is also available as a kit for qualified do-it-yourselfers.

From light leveling to heavy limiting

The Variable Mu’s Compress mode is a soft-knee 1.5:1 ratio while the sharper knee Limit mode starts at 4:1 and moves to a more dramatic ratio of 20:1 when limiting over 12dB. Interestingly, the knee actually softens as more limiting is used. Distortion can be creatively employed by turning up the Input and dialing down the Outputs while using very little or no compression.

Perfect for dual-mono or stereo applications

You might notice that the Variable Mu has a ganged input control, but it’s certainly not mono-unfriendly. There are separate threshold, attack, release, and output controls to make compensations with, plus you can always adjust your individual source levels elsewhere. The advantage of the stereo input control becomes clear when you switch to Link mode. And that’s where the Variable Mu excels — on stereo buses and for mastering your final mixes. As one reviewer put it: it’s like pouring “sweet cream” over your mix!