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Groove Tubes GT55, GT57, GT66 & GT67

Large-diaphragm Condenser Microphones
Published April 2003
By Hugh Robjohns

Groove Tubes' versatile new mics offer a range of facilities to suit every budget.

Groove Tubes have been manufacturing a wide range of equipment, mostly involving valve circuitry, for over 20 years. The company also make their own valves at their factory in San Fernando, California. As well as various guitar preamps and amplifiers, equalisers and compressors, they have a growing range of microphones, and six new models were introduced last year. Four large-diaphragm models are reviewed here: the GT55, GT57, GT66 and GT67. The flagship GT67 is a multi-pattern condenser mic with valve electronics, with a fixed-cardioid sibling, the GT66. The GT57 is essentially identical to the GT67, but with FET-based electronics, and the GT55 is its fixed-cardioid relative.


The Groove Tubes GT67 valve and GT55 FET mics.The Groove Tubes GT67 valve and GT55 FET mics.Photo: Mark EwingThe GT66 and GT67 mics are supplied in large cardboard carrying cases, and matched pairs (with adjacent serial numbers) are available for stereo applications. Each kit contains a valve power supply, an 8m dedicated mic cable, an IEC mains lead, a shockmount, a stand bracket, and the microphone itself. A foam-lined and paper-covered wooden box, which exudes a vintage character all of its own, protects the mic. The PSM1 carries a fused IEC mains inlet at the rear, and a pair of XLR connectors at the front. A red LED indicates power (although this is only visible over a narrow viewing angle), but there is no mains switch, which I found unsatisfactory for both safety and convenience reasons. The mic is connected to the PSU via the bespoke cable with seven-pin XLRs (the user's manual erroneously states it has six pins!), and the mic output is provided on a standard three-pin male XLR socket. Phantom power is not required, but can be tolerated without problems.

Groove Tubes GT55, GT57, GT66 & GT67
  • Classic large-diaphragm performance.
  • A fine condenser capsule design.
  • Good standard of construction.
  • Well-judged valve character or clean FET circuitry.
  • Configurations to suit all pockets.
  • Versatile all-rounder mics.
  • No power switch on valve PSU.
  • Frustrating design of mounting brackets.
An integrated range of true condenser mics with large diaphragms, fixed or switchable polar patterns, and solid-state or valve circuitry. These mics look professional and deliver impressive sound quality, with finely judged characteristics.


The plastic stand bracket is fitted with a 3/8 to 5/8-inch thread adaptor (hurrah!) and the microphone sits in a cup on this bracket, retained by a threaded collar. Unfortunately, though, this is not captive and fits from underneath, making it easy to drop and lose. It is also possible to fit the bracket upside-down, making it difficult to tighten the screw collar sufficiently to support the mic properly. For anyone mechanically minded the correct assembly is obvious, but I'm sure many users will be caught out.

The elasticated shockmount consists of a metal inner frame suspended from eight loops of elastic attached to an outer ring circling the mid-point of the frame, to which the stand bracket is also attached. As with the bracket, the screw collar retaining the mic in the shockmount is not captive — if you can leave the mic mounted in its bracket this won't be of much concern, but for anyone who needs to disassemble the mic and cradle on a regular basis, the collar can become a major frustration.

GT66 & GT77

The Groove Tubes GT66 valve mic with its PSM1 power supply. The Groove Tubes GT66 valve mic with its PSM1 power supply. Photo: Mark EwingThe GT66 is a chunky microphone with a matt nickel finish — it weighs about 530g and measures 47mm in diameter and 190mm in length. The side-firing condenser capsule contains a 1.1-inch evaporated-gold diaphragm which is just three microns thick. A 'disk resonator' apparently interacts with the diaphragm at high frequencies (above about 14kHz) to help increase the HF sensitivity. The front of the microphone is indicated by the GT logo, and two miniature toggle switches select a 10dB input pad and a 75Hz high-pass filter, the latter intended to help reduce rumble and proximity effects.

Unscrewing the base collar enables the body tube to be removed to access the valve and other components. A label carrying the mic's serial number is also revealed on the output transformer. The military-grade GT6205 miniature pentode valve is supported on the underside of a small circuit board carrying full-sized components, and is used in a Class-A triode configuration to achieve low noise and a wide dynamic range.

Both the GT66 and GT67 use the GT6205miniature military-grade pentode valve .Both the GT66 and GT67 use the GT6205miniature military-grade pentode valve .Photo: Mark EwingThe review microphones were not supplied with any specifications or test reports, but the UK distributor's web site provided some basic specifications (and more information than the Groove Tubes site, strangely). The maximum SPL the mic can accommodate is 135dB, increasing to 150dBSPL with the 10dB pad switched in — I know the sums don't add up, but that's what the specs say! The frequency response is claimed to extend between 20Hz and 18kHz within a narrow ±1.5dB tolerance, and every mic is apparently matched to within ±1dB of the company's 'golden microphone' specification. The cardioid polar pattern of the GT66 is fairly wide, but with reasonable front-back rejection over most of the frequency range. The directional characteristics are typical of a large diaphragm capsule, with a progressively omnidirectional bottom end, and a narrowing towards a hypercardioid pattern in the HF region.

The multi-pattern GT67 is essentially a dual-diaphragm version of the GT66, with the same basic construction and internal circuitry. It is the same size but weighs a little more, at 550g, and has a different body shape, with a slightly barrelled body and chisel-shaped capsule grille, resembling the classic Neumann U87 form. Pad and filter switches occupy similar positions on the front of the mic, while two further toggle switches to select the polar pattern are on the rear. The first is a three-way switch providing omni, cardioid or figure-of-eight options, while the second switch enables the cardioid mode to be changed to a supercardioid pattern. All the patterns work as advertised, albeit with the typical distortions associated with large-diaphragm capsules. The cardioid pattern seems wide, and the supercardioid is a useful option where tighter control is required. Although it has a rear tail, this is fairly insensitive and shouldn't cause any problems in normal use.

GT55 & GT57

These two microphones are identical to their tube counterparts in size and shape and employ the same capsules, but have FET solid-state electronics. The GT57 is a multi-pattern mic based on the GT67, and the GT55 is a fixed-cardioid model based on the GT66. The most obvious visual difference between the two ranges is the matt black bodies on the FET designs, but these mics are also priced significantly below their valve siblings.

The Groove Tubes GT57 microphone.The Groove Tubes GT57 microphone.Photo: Mark EwingThe GT55 & GT57 are both supplied in a compact cardboard outer case enclosing the same paper-covered wooden microphone boxes used by the valve models. Matched stereo pairs of mics are also available, if required. A stand bracket (with thread adaptor again) is supplied as standard, this time equipped with a black plastic screw collar in place of the silver metal version supplied with the tube mics. Class-A FET electronics are used in place of the valve circuitry, driven by standard 48V phantom power, and construction is to the same high standards as the valve models, with almost identical performance specifications. However, the GT57 multi-pattern mic does not have the supercardioid switch found on the GT67 — only the three-way omni/cardioid/ figure-of-eight switch is present.

Listening Tests

First impressions are that these are very nice mics and I enjoyed making use of them in a variety of applications, from recording a small choir and various orchestral string and wind ensembles, to a range of solo instruments, hand percussion and voices. I started my listening and recording tests with the FET versions and found them to be well-behaved overall and with a typical large-diaphragm character. They both produce a rich, full, but generally well-balanced sound, which is clean, neutral and reasonably quiet. However, the mic is susceptible to mechanical noise through the mic stand and cable, and although the filter helps considerably I would definitely recommend purchasing the optional shockmount here.

The GT57's switchable patterns make this a very flexible mic, but I suspect most people rarely use anything other than cardioid, so the GT55 is probably the better buy given that its performance is indistinguishable from that of the GT57 in cardioid mode. As with all large-diaphragm mics, the variations in polar response with frequency and incident angle mean that small changes of position relative to the source can be used very effectively to tailor the captured sound.

The two valve microphones supplement the fine sound quality of the solid-state models with a nicely judged thermionic character. They certainly have a subtly fuller and richer sound quality which is especially apparent when used close to a source, although the overall frequency response seems unchanged. As with any valve mic, it takes a few minutes for the tube to reach its optimum temperature, and there is no output at all for the first 30 seconds or so. The valve appears to be quite well isolated from mechanical shock, and I was unable to provoke any microphony in normal use. The elasticated shockmount supplied with the valve models also proved far more effective at minimising mechanical noise induction through the stand, compared with the 'hard mount' option.

The supercardioid and cardioid polar patterns on the GT67 were interesting to play with. Both seemed to have a significant rear tail at high frequencies, and the cardioid setting was very much wider than the supercardioid option at middle and low frequencies. The provision of this extra switch increased the flexibility of the mic, and I found I tended to use the supercardioid mode when close miking, to maximise separation. More distant sources — and especially those that tended to move — benefited from the cardioid setting.

Both valve and FET versions of these mics are very similar in general character, and performed to a very creditable standard. Nothing seemed to faze these mics, although they wouldn't necessarily have been my first choice in all situations. Having said that, when I felt the need for a different mic, the replacement was always substantially more expensive and frequently a small-diaphragm model — which says a lot for the quality of these Groove Tubes models!

These mics all produced fine results and would make good 'all-rounders', as they coped with pretty much every sound source well. Percussive transients were handled cleanly and with a crisp attack, and voices were portrayed with a full rounded tone, especially when close-miked. Moving up from the FET to the valve versions brought an extra richness to the sound — a subtle thickening and increased density in the character — while also managing to somehow sound more expensive in some subconscious way.

The bottom line is that these are good mics. They look and sound professional yet are priced within reach of the keen amateur. And with solid-state and tube models, offering fixed or variable polar patterns, there's a model to suit all requirements and budgets. Ignoring the minor niggles over some aspects of design, these mics provide high-quality audio in attractive, flexible and above all cost-effective packages.

infop.gif GT55, £349; GT57, £399; GT66, £599, GT67, £749. Prices include VAT.
infot.gif M-Audio +44 (0)871 717 7100.
Published April 2003