Groove Tubes A-series

They may look the same as their predecessors, but inside the a‑versions of Groove Tubes' mics is a new set of circuitry. Paul White's feelin' groovy...

Groove Tubes are a company best known to guitar players for their matched sets of valves, but they also produce valve amplifiers, outboard processors and microphones. Their MD1, 2 and 3 mics have enjoyed success for a number of years, but now they've been given new amplifier circuitry to further reduce noise, increase headroom and lower total harmonic distortion. Though the same capsules are used as in the original models, a new suspension system has been added to reduce handling noise — some performers are actually taking these mics out on the road.

Because these are valve mics, you can't run them from regular phantom power sources, so Groove Tubes produce the mains‑powered PS2A power supply, which can power any two mics (and some other Groove Tubes products) simultaneously. The mic connects to the power supply via a locking 9‑way D connector, and while I have to agree with those people who find these a trifle ugly, they do have the advantage of being inexpensive and easy to obtain, so if you do need to repair or replace a cable the bits aren't hard to come by.

MD1a

Physically identical to the original MD1, the MD1a is a fixed cardioid pattern mic featuring a fairly large diameter capsule with a 10‑micron diaphragm; it can handle SPLs up to 137dB without recourse to cissy pads — and there's no LF roll‑off switch either. The revised valve preamp circuitry, based on a 12AT7 dual triode, is designed for lower noise and lower distortion, but it also extends the high‑frequency response of the microphone. Unlike most cardioid mics, it doesn't seem to have a deliberate LF roll‑off built in, so you have to use a pop shield when recording vocals to prevent popping. There are no presence peaks in the response, and though it has its share of bumps and dips, the nominal response is flat from 40Hz‑18kHz within 4dB. The equivalent input noise is 20dB SPL and the sensitivity is 50.1mV/Pa. A transformer output stage provides a low 30Ω output impedance, with a maximum output level of 1.7V RMS at 1% distortion.

...the MD1a seems to combine the detail of a capacitor mic with the sort of mid‑range punch normally associated with a good dynamic model.

Cosmetically, the MD1a is seriously chunky, with its heavy plated brass case and 'bullet‑hole' ventilation. The tube shines out through a cut‑out GT logo, and the singer performs into the side with the cardioid symbol. An optional heavy‑duty shock mount is available, but otherwise the mic can be screwed directly to the top of a conventional mic stand by means of a threaded insert in the base. An equally tough metal grille protects the capsule.

Subjectively, the mic sounds very similar to its predecessor, though there may be just a hint more air around the top end. I never experienced problems with noise with the older model, not because these were particularly quiet mics, but because microphones of this type are invariably used at close range. This newer version is a little quieter than its predecessors, but on paper the biggest difference is the reduction in distortion. Fortunately, this doesn't seem to have compromised the mic's warm, punchy sound and, like the original, the MD1a seems to combine the detail of a capacitor mic with the sort of mid‑range punch normally associated with a good dynamic model. The low end is also nicely wide and warm, and when the mic is used close up the sound takes on a very rich, intimate character.

MD2a

The MD2a cardioid is, outwardly at any rate, mechanically similar to the MD1a, except that it has a black crackle paint finish and there's a variable sensitivity control peeking out of the end of the case. Unlike a pad, this control affects the capsule sensitivity; talking of which, the gold‑sputtered diaphragm used in this capsule is just 3 microns thick — less than a third of the thickness of that fitted to the MD1a. The result is a much extended top‑end response: this model claims 40Hz‑25kHz ±2dB. The response curve is also a lot smoother than that of the MD1a, with fewer local bumps and dips. The equivalent input noise is 26dB SPL and the sensitivity is 26.4mV/Pa. The transformer output stage spec is the same as for the MD1a.

What we have here, then, is a mic that's a hint noisier than the MD1a, and a little less sensitive, but with a hugely extended upper frequency response. In practical terms, this translates to a mic that's very good at handling transient sounds, making it suitable for percussion, drum overheads, brass, sax and acoustic guitar, as well as vocals. Subjectively, I found it a little less warm than the the MD1a when used on vocals, and the mid‑range seemed to project more. Whether this is a good thing depends on the singer — some people will find this mic gives them the extra cut and projection they need, while others may find it slightly nasal‑sounding. Used for up‑close vocals, this can be a very flattering mic which will really stand out in a mix.

MD3a

While the previous two mics share the same housing style, the MD3a has a striking punched‑metal grille basket finished in polished chromium, the reason being that the pattern of this mic may be adjusted from cardioid, through wide cardioid, to omni by means of a screwdriver. Obviously, a grille open only on two sides would be inappropriate for an omni mic, hence the basket construction. A slotted screw, accessed by means of an aperture in the grille, is used to adjust the mic pattern, and the 'hot' side of the mic is denoted by a Groove Tubes logo. The lower part of the body seems identical to that of the MD2a, and the same variable sensitivity control is fitted.

It seems that Groove Tubes have managed to improve upon an already popular design while retaining the character that made these mics popular in the first place.

Looking at the spec, the MD3a comes closer to the MD2a than to the 1a with a 20Hz‑20kHz response, flat to within ±2dB. The equivalent input noise is again 26dB SPL and the sensitivity is 25mV/Pa. The same transformer output stage is used as on the other two models. Examination of the response plot reveals a better‑controlled low end — the others drop off at 50Hz, then kick back up to a +5dB peak at 20Hz or so. There's also a very gentle dip at around 2kHz, but overall the response is acceptably flat.

Tonally, this model falls somewhere in between the other two. It has the characteristic Groove Tubes intimate vocal sound, and the low end seems just a fraction warmer than on the MD2a but with a similar degree of projection and punch. I have to admit that I don't really like the idea of having to poke a screwdriver into the basket to change the polar pattern, but in other respects this is a really nice mic.

Summary

It seems that Groove Tubes have managed to improve upon an already popular design while retaining the character that made these mics popular in the first place. And there's no doubt that these mics have a lot of character — they're designed to flatter, not to simply pass on what they hear.

Though these are predominantly vocal mics, all three can be used to record acoustic instruments or percussion; in this latter application, though, the extended high end of the 2a and 3a might be an advantage. But, curiously enough, for my own voice I preferred the MD1a — it has a lower‑mid warmth about it that the the other two mics don't quite equal. Of course, different singers will probably find that different models suit their particular voices, and some singers may not feel comfortable with any of them. That's why the present wide choice of 'character' microphones is such a good thing. The obvious competitor at the price is the Rode Classic tube mic and, for what it's worth, I think the Rode has a more natural sound, but then that's a subjective judgement based on my own voice.

It's unlikely that you're going to buy one of these mics without trying it first, but if you like what you hear, you also have the benefit of knowing that you're buying a modern design using readily available tubes and easily obtainable spares — something rarely true of vintage tube mics. What's more, even taking into account the cost of the power supply, the price is very attractive compared with what you'd be expected to pay for a well‑used original tube mic. All three Groove Tubes models represent great value, and though the subjective results of the technical improvements are not that great, the revisions are worthwhile.