First, you should decide if you really need a compressor (which looks at the average “rms” energy of your waveform) or a limiter (which only looks at the peaks or envelope), or both. Compressors generally sound more musical, because they have a slower reaction time, also called Attack. They usually also have a slower recovery time, also called Release, which is designed to minimize the audible effects of gain reduction like noise floor pumping and the unnatural result when the compressor follows the signal so closely that you start picking up breath and lip sounds on a vocal, for example. Of course, you can have too long of a release time and end up attenuating soft sounds that follow extremely loud ones! Limiters are designed to react and release extremely fast. This makes them typically inaudible when used with restraint (perhaps 2-6 db)? As long as you remember that because of its extremely fast response, it can clip the tops off individual cycles of a bass waveform, causing terrible harmonic distortion. Because of this, they’re usually used more to protect equipment and recording media like tape, optical film soundtracks, and in broadcasting, where channel “splatter” is forbidden. Since you’re probably recording to digital, I feel there’s little use for limiting in small studios. On the other hand, if you want the clipping to be audible, go right ahead and cream the limiter. But, you could probably get the same result by creaming your sound card directly! So, from here on, we’re just talking compressors, since, I don’t know of a single vintage all-tube unit, which does both. The Gates Sta-Level model M5167 is a fully-balanced design from the input to output transformers. The tube lineup uses the 6386, an unusual variable-mu dual triode for input; a 12AT7 for interstage amplification and a pair of 6V6s for push-pull output. The 6AL5 is used for detection and the 0B2 regulator compensates for fluctuations in B+. The 5Y3 is used as a B+ full-wave rectifier. The Sta-Level (I believe it's pronounced "stay level") has begun to have a cult following of its own, since all the "real" recording studio compressors are simply astronomical in price: A Fairchild 660/670 will set you back $25,000 or more. By contrast, a Sta-level runs about $1700 on ebay. Sowter has begun to make all the iron for it, there are modifications floating around on the Prodigy-Pro message board and there's even a commercially-available repro that adds a few creature comforts. Some people seem to think there's something special about the 6386. I'll have to test some of the equivalents: the 5670 and 2C51, to see if the 6386 is really worth the extra money. Some other tube compressors: The Collins 26U-1 has an almost identical tube lineup and circuit, but adds separate attack and release controls The Collins 356E-1 drops the attack/release controls, a stage of amplification and the regulator tube. There's the Federal AM-864/U, which uses two 6SK7s for balanced input, a 6SQ7 (diode-triode) for peak detection and a single 6SN7 for push-pull output. Some message board members say it sounds bad and I have a feeling it was designed for processing 2-way radio communications. The Altec-Lansing 438A was designed for PA use, but Altec claims it was good enough for broadcast and even recording (reportedly the Beatles did!) It used an inter-stage transformer, a 6AL5 detector, and dual triodes for everything else: 12AY7, 6BC8, 6CG7 (for p-p output).