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Locomotive Audio WT-Comp

Vari‑mu Compressor By Neil Rogers
Published April 2023

Locomotive Audio WT-Comp

This characterful valve processor offers vari‑mu compression at a very attractive price.

I’m a paid‑up member of the vari‑mu fan club. I own, and have owned, a number of this type of valve compressor, and it seems to me that their only downside is that the really good ones tend to be rather expensive!

I was intrigued, then, when I first learned of Locomotive Audio’s WT‑Comp (aka the Weight Tank): it seemed to tick many of the boxes I look for in terms of features, while being priced very competitively. Locomotive are based in St Louis, Missouri, from where designer Eric Strouth began to release his interpretations of classic recording gear in 2015. Hannes Bieger reviewed the Model 14B in SOS May 2017 and was full of praise. Whilst it’s by no means expensive (in vari‑mu terms!), the 14B is priced more in the ‘normal’ realms for high‑end valve gear. So I think it’s fair to say that the WT‑Comp is Locomotive’s attempt to make this style of tool more accessible.

Locomotive say the WT‑Comp takes its inspiration from various classic compressors, including the Universal Audio 175B and the Altec 436A (and its British mod), along with some aspects of their own 14B, and the goal here is clear: thick, characterful compression that can not only control the dynamic range but, to a degree, change the personality of the audio that passes through it. The literature reinforces that idea: it seems keen not only to describe the quality components that our audio will encounter in the signal path, including three (input, inter‑stage and output) all‑steel transformers and two valves (a 6BC8 and a 12AU7), but also to offer up the sort of colourful adjectives that might encourage people to stray outside their DAW when mixing.

With its sizeable vintage‑style input and output controls, large VU meter and chunky switches, this compressor feels reassuringly well built and pretty hefty too. Power comes in on an IEC inlet with integrated fuse holder; the voltage is switchable between European and US mains. With a fixed 4:1 threshold, the front‑panel controls are nice and straightforward: gain reduction is achieved by playing with the large input and output dials and there are controls for attack and release, along with the option of using the WT‑Comp in Smooth or Drive mode. Locomotive explain that the Smooth setting produces a “smooth, sweet tone” whilst the Drive setting will produce more harmonically rich, overdriven‑style results, due to a change in how the 12AU7 output tube is biased. Finally, we have the welcome addition of a true bypass switch, which allows you to audition the effect of the compression — it’s a feature not always found on this style of compressor.

The Weight Tank can run on US or European mains voltages — to swap, you just need to flip a switch and change a fuse in the rear‑panel IEC housing.The Weight Tank can run on US or European mains voltages — to swap, you just need to flip a switch and change a fuse in the rear‑panel IEC housing.

In At The Deep End

The WT‑Comp saw plenty of action at my studio during the review period, and for the first week or so I put it to use during some busy tracking sessions. My first impressions seemed to match my preconceptions and I quickly found putting this compressor in the chain of drum room mics or bass guitar seemed to add a little weight and ‘heft’ without being over the top. I got a little nervous when tracking vocals, as I could clearly hear saturation and harmonics being introduced. This was not unpleasant, but something I wasn’t always ready to commit to when tracking, so I generally found myself using quite subtle settings during this period.

It did a super job of ‘pinning down’ a clean electric guitar part, whilst also adding some upper mid‑range harmonics that helped the guitar to sit more confidently in the mix.

In the more controlled environment of a mixing session, I was able to evaluate the WT in a more structured, detailed way, and as I did so I began to learn more about its personality. Starting with what turned out to be my favourite application for this device, it did a super job of ‘pinning down’ a clean electric guitar part, whilst also adding some upper‑midrange harmonics (in the Drive setting) that helped the guitar to sit more confidently in the mix. It did a similar job on acoustic guitar, though I was aware that I was trading off some of the nicer high‑end content of a strummed part, in exchange for a more consistent dynamic.

I found myself striking a similar compromise at times when using the WT on vocals. If a voice benefited from a little harmonic ‘edge’ it was fine to get stuck in, but I typically found I had to use another 1176‑style compressor in the chain to get a vocal sitting as I wanted it to against the music. Perhaps, not surprisingly, I found it suited male vocals best — it felt a bit heavy‑handed for a couple of the female voices I tried it on.

It was great to hear how the WT performed on bass guitar. It was possible to dial in large amounts of gain reduction before the compression became overly audible, and I was also impressed with its ability to shape the initial transient of a bass part — the attack knob seemed to give me plenty of fine control over how the bass sat with the other instruments.

On drum room mics, and indeed any source that will happily take a dose of attitude, the WT performed very well indeed. I was impressed by how much I could shape things like cymbal decay — a combination of varying the release times and changing how hard the signal hit the compression circuit.

Weigh To Go?

While it’s easy to get up and running with a vari‑mu like this, I’ll admit it took me a little while to figure out just what type of tool I was playing with here. I find that this type of compressor can take a little getting used compared with the more common VCA or FET compressors. Everything is that bit slower, and it’s common when looking at the meters to see quite large amounts of gain reduction being applied initially, before then observing the meter hovering around the low dB range, before being released when the music stops. The conclusion I eventually arrived at was that it sits somewhere between being a classic Altec‑style compressor and a dedicated tube‑driven ‘colour’ box.

The WT’s key strength, to my ears, is that it does what it says on the tin: it adds weight, colour and harmonic content to things like bass, guitars, drum room mics and the right sort of vocals. On the right material, it’s a lovely, seductive effect. As I’ve suggested above, it’s often not subtle, but if handled with a little care and used in the Smooth setting it can also do the more conventional job of compression. Does it deliver ‘that’ high‑end vari‑mu sound? Well, yes and no. On many sources, such as bass guitar and the right guitar or vocal part, it will get you very close indeed. Unlike some of the higher‑end (and rather more expensive) tools I have and have used, it wasn’t always the right tool for the job and judged on these terms I was occasionally aware of a slight ‘sonic compromise’.

Judged against other units costing around the same, though, it’s very attractive. The WT‑Comp does exactly what it promises to do, then, and would be a superb addition to tracking or mixing setups in need of good value, characterful hardware options.  

WT-Comp Audio Examples

It’s sometimes hard to convey all this in words, so I created a few audio examples to help you hear what I’m writing about. You can find these on the SOS website at


  • Very solid build quality.
  • A quality variable‑mu compressor at a very good price.
  • Characterful compression.
  • Capable of some great saturation/distortion effects.
  • Smooth and Drive controls a nice option.


  • Perhaps a little heavy‑handed for more delicate sources.


The Weight Tank sets out to impose itself on your audio, and it does just that, with its multiple transformers and valves adding colourful, weighty compression via a simple but effective set of controls.


£999 including VAT.

KMR Audio +44 (0)20 8445 2446.