Variable-mu compressors are renowned for their soft, gooey compression. A few of the heavyweights that come to mind are the Fairchild 660, RCA BA6A, and Gates Sta-Level. All of these come with a hefty price tag to go along with that sound, but what about a sub $1000 variable-mu? There used to be the Dizengoff D864, but that only exists on the used market. Enter Locomotive Audio's Weight Tank, a variable-mu compressor that lives up to its name.

The layout of the black powder coated enclosure matches the simple workflow of this compressor. On the Weight Tank's faceplate are four variable control knobs for Input gain, Output gain, Attack, and Release. There are also three switches: Power, Bypass (true bypass), and Round/Drive. The Round/Drive switch changes the 12AU7 tube's bias points to enhance the harmonic structure. An orange, jeweled LED illuminates when power is engaged, a backlit meter displays gain reduction and on the back there are XLR input and output connectors plus a stereo link jack. The true bypass still has the signal being loaded by the input transformer, so dB reduction will be shown on the meter even when the compressor is bypassed. On the inside there are three steel Edcor transformers for the input, interstage, and output as well as through-hole components on a printed circuit board. The Weight Tank utilizes both a 6BC8 and a 12AU7 output tubes. The 6BC8 tube is a medium-mu, semi-remote cutoff twin triode that was initially intended for television receivers. In this review unit there were RCA and JJ tubes, respectively.

All of Locomotive's products are made by hand in St. Louis, MO, and are built with American components. This unit borrows concepts from Locomotive's own 14B [Tape Op #107], Universal Audio's 175B, and Altec Lansing's 436A compressors – all wonderful inspirations for such a design. This is the second revision of the Weight Tank, and though the differences may be subtle, they are worth mentioning. Revision B has slightly more harmonics with a fixed ratio of 3:1 versus Revision A at 4:1; however, a variable-mu compressor, by design, changes the ratio as the input is pushed further past the threshold. Revision B's faceplate has also been updated with a different logo and is flat black compared to the glossy black finish of the original. The balance feature was moved internally, requiring adjustment only with new tubes or if there is another issue.

In the studio I found the compressor complimentary to most instruments, but specifically acoustic guitar, piano, bass, some vocals, and room mics. With all variable-mus the results vary widely depending on the source material and how much signal you feed them. It can result in the same source being overdriven, grabby, or smoothed depending on the character of each model. I found subtle compression with Drive mode enabled really opened up the top end of the piano without exhausting the dynamics. On acoustic guitar the performance was smoothed out while adding a little euphonic top end to help the rhythmic elements of the performance pop. With gritty-sounding vocals, I first sent the signal through a Universal Audio 1176-style compressor to catch just the transient peaks before going to the Weight Tank. With the signal reigned in by the 1176, I enabled drive mode to further saturate the vocal, which yielded a totally different type of tone. I have found many creative uses for the Weight Tank in my studio, The Grey Room, while recently working on a lot of electronic music projects – using it to pump rhythmic elements to help the bounce of songs. Another use was adding sustain to 808 hits and long release time drum samples. This isn't limited to electronically derived sounds either – the added sustain on electric and acoustic bass is fantastic! I would definitely use a pair for busing duties on certain instrument groupings, but would proceed very light handed if using a pair on the mix bus.

While remakes and famous compressors are available, there is not much in the variable-mu department under $1000 – I'm only aware of the Grove Hill Liverpool [Tape Op #115]. The Weight Tank has tapped a niche with its original design and price point, operation is simple, and the layout is clean. It's solidly built and can handle lots of different source material. This may not replace some of the high-end variable-mus that offer a polished, silky sound, but the Weight Tank appropriately provides heft to the source material by adding harmonic color while smoothly clamping down the dynamics and extending sustain. The Weight Tank compressor is worth having in your collection during tracking and mixing; I'm excited to check out Locomotive's other offerings.

Tape Op is a bi-monthly magazine devoted to the art of record making.

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