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Groove Tubes GT55 and GT66 Microphones

The Groove Tubes line of microphones is exclusively distributed worldwide by M-Audio (formerly Midiman), with the only exception being the Model 55 and Model 66 that are sold exclusively through Guitar Center stores in the USA market. Outside of the USA, M-Audio also distributes the Model 55 and Model 56.

Groove Tubes began in the late 1970s when Aspen Pittman, the company’s founder, hired some technicians to investigate why various tubes sound and perform so differently. With a personal collection of vintage tube amps and mics in excess of 250 pieces, Pittman had always been fascinated with tubes, tone and amps. Driven by curiosity, as much as an itch to start a new business, Aspen and these engineers discovered entirely new tube operating characteristics to measure and new ways of interpreting these test data results.
Product PointsApplications: Studio

Features: 1.1-inch 3-micron gold-eavporated diaphragms; Disk resonator technology; Class A; 10 dB pad; low-frequency rolloff filter

Price: GT55: $499 GT66: $899

Contact: Groove Tubes at 818-361-4500, Web Site.
This research became the foundation for the Groove Tubes performance testing and matching process they still use today. Since its beginning, Groove Tubes has grown from a modest garage workshop to an international business with over 1,200 U.S. retail outlets and distributors in more than 40 countries worldwide. The company is still run by music lovers, and while the organization has long since moved out of the garage, Groove Tubes takes pride in continuing to maintain their tinkering workshop mind-set.

The Groove Tubes line of microphones is exclusively distributed worldwide by M-Audio (formerly Midiman), with the only exception being the Model 55 and Model 66 that are sold exclusively through Guitar Center stores in the USA market. Outside of the USA, M-Audio also distributes the Model 55 and Model 56.


The beautiful flat-silver Groove Tubes GT66 is a no-compromise tube mic delivering better performance and sound than the many mics costing much more. A Groove Tubes GT6205 tube powers this microphone’s Class A electronics. Groove Tubes philosophy regarding tube microphones has always been that great tube mics start with great tubes. The Groove Tubes’ tube microphone design is different from other companies in that they use triode wiring on specially selected miniature pentode tubes to yield low noise and incredibly realistic dynamic response. This provides a smooth vintage tone combined with modern studio high-fidelity specs. Accurate tube selection is also a high-priority at Groove Tubes. The Groove Tubes quality control department is so discriminating that they surprisingly reject more tubes than they accept. The GT66 power supply is designed with a load-balancing circuitry that allows cable runs of more than 200 feet with no deviation from optimal performance. Included with the GT66 is the PSM1 power supply, a multipin 25-foot soft audio cable, a hard mount, a shockmount, and a zipper mic case all for $899.
Groove Tubes Microphone ComparatorGroove Tubes originally developed the Microphone Comparator to give trade show attendees the opportunity to accurately compare the performance of various microphones. Engineers and producers began to realize that this device would be right at home in the studio providing a quick, easy and accurate way to shootout up to four microphones. Volume can be very deceptive when comparing audio devices and having the ability to precisely match the gain of the microphone prevents falsely selecting the microphone with the highest gain as sounding the best. A phantom power switch provides 48V phantom power to the fourth microphone input. Four Microphone Select switches determine which microphone will be auditioned. Output is provided via a front panel 1/4-inch TRS jack labeled Record Out. The box is also equipped with four headphone out jacks on the rear panel and four headphone volume controls on the front panel.
The flat black GT55, the solid state version of the GT66, is a true Class A condenser microphone equipped with a 1.1-inch, hand-assembled and tested large-diameter capsule with an ultrathin 3-micron gold evaporated diaphragm. The GT55 has a cardioid polar pattern and warm Class A FET electronics. Included with the GT55 is a hard mount and a padded zipper mic case all for $499.

At the heart of the GT55 and the GT66 is a hand-assembled, hand-tested 1.1-inch diameter capsule. This 3-micron evaporated gold diaphragm with Disk Resonator technology provides amazing sensitivity that delivers a sparkling, transparent sound across the full frequency spectrum. The Groove Tubes’ Disk Resonator technology gives the GT55 and GT66 an extended frequency response ideal for accurately capturing the realism of just about any sound source. Large-diaphragm microphones often get bogged down resonating at low frequencies preventing them from efficiently reproducing frequencies above 14 kHz -15 kHz. Groove Tubes’ Disk Resonator interacts at high frequencies with part of the diaphragm, extending the range of the capsule well beyond 20 kHz.

Both the GT55 and GT66 feature a low-frequency rolloff filter (75Hz) and a 10 dB pad. The rolloff filter is useful in reducing or eliminating low frequency noise such as floor rumbles, mic stand noise, passing trucks, etc… It can also be used to compensate for the microphone’s proximity effect. The pad is useful to prevent the microphone and/or preamp from overloading when recording loud sound sources such as a close-miked guitar amp or kick drum.

In Use

Although the GT55 and the GT66 both work exceptionally well on vocals, I always preferred the sound of the 66 over the 55. I am nearly always partial to a valve microphone on vocals and this is no exception. While working with a vocalist whose voice had a tendency to get a bit edgy and piercing in the higher frequencies, I found that the GT66 would actually correct this problem by warming up and smoothing out the edge without losing any sparkle or air.

I had exceptional results using both the GT55 and the GT66 to record a wide variety of acoustic string instruments including acoustic guitar, mandolin, violin, viola and national slide guitar. I generally preferred the sound of the GT66 but there were a couple of instances (mandolin and national slide guitar) where I favored the sound of the GT55. I was able to get a fantastic acoustic guitar recording by using the GT66 on the neck of a Duncan guitar and the GT55 on its body.

I used the GT55 to capture room ambience during a string quartet recording which provided nice results. This was one of only a few instances that I wish the microphone had selectable pick-up patterns. My results were wonderful but I believe they would have been even better if I would have had the option of using the mic in an omni position.

The microphones also worked well on drums and percussion. I substituted the GT55 for my standard hi-hat microphone (Neumann KM 86i) and was extremely pleased. This instance alone was enough of a reason to purchase the GT55. I used the GT66 to capture drum kit room ambience also yielding wonderful results. Both microphones worked well capturing the sound of tambourine, shaker and triangle.


The GT55 and the GT66 are outstanding microphones with a similar sound yet still unique. With a combined price that is less than I would expect to pay for either one they are a bargain as well.