Since receiving these mics a few months ago, I've used them on every recording session to date. And I've got a large collection of mics, with some of them costing many times what the most expensive of these GT mics retail at. If there's a sonic trait that all these mics share, it's the smooth, extended high end. I'm not a fan of overhyped condenser mics with high end that can be too sibilant to my ears. The GT mics seem to have tons of usable high end and vast amounts of "air" without sounding harsh. Coupled with my Hamptone Silverbox tube preamp (which has the cleanest high end of any of my mic preamps), the high-frequency detail I get using all of these GT mics is incredible. Plus, these mics respond really well to EQ. Even extreme boosts or cuts seem more musical on these mics than many of the condenser mics in my collection.
The GT60 features Class A tube electronics and a 3 micron, 1'' diaphragm. It comes with an external power supply. The GT50 is the Class A FET sister to the GT60. With both mics, I noticed a distinct lack of tubbiness when used at close proximity. The low mids don't ramp up as quickly as with other mics, even when using my vintage Neve and API preamps. I tried them on just about every source I could, from vocal to kick drum and everything in between. I liked the GT60's top end better, especially on extreme transients (e.g., for capturing the "bite" of piano and acoustic guitar), but for most sources, the two mics were voiced very much alike. When I mentioned to Sam Austin of Groove Tubes at TapeOpCon2006 that I was very much impressed with the smooth proximity effect and extended top end, he pointed out that GT's large capsule mics have Disk Resonators mounted in the middle of their micro-thin diaphragms to achieve these very attributes that I love.
The GT40 and GT30 are medium-diaphragm condensers (6 micron, 3/4''). The former has Class A tube (with the same external supply as the GT60) and the latter Class A FET electronics. I tried these mics on everything, and I couldn't find a source that they didn't seem to flatter. Again, lots of incredible top end without sounding sibilant, and a good wallup of controlled low end too. In fact, I ended up using these mics on guitar amps more often than not, and instead of reaching for the EQ to cut out some of the lower mids, all I had to do was move the mics back a few inches. Some mics get too thin when you pull them back, but these GT mics responded well to varying proximity without the low end disappearing. At closer proximity, the GT40 seems to have less lower mids than the GT30, and the top end is smoother, especially on loud amps. But the GT30's bottom is a bit clearer. My choice for a darker sound used to be a Gefell M300, but the GT30 is now my go-to. Also, both these mics seem to have ample amounts of upper mids to help guitars cut through the mix without stomping all over the vocals. And they're both fantastic at picking up the full sound of snares and toms.
My only complaint is that the multipin connectors on the tube mic cables are extremely delicate. You can't push while rotating to find the correct insertion angle like you can with 3- pin XLR cables. If you do, you'll bend the pins.
I recently opened a music production room in Dallas. I needed three mics for that room to cover all aspects of recording vocals and single instruments. I chose a Groove Tubes GT67 (the multi-pattern version of the GT60) for one of those mics, even though I'd never even used (or even seen) a GT67. But I was so impressed with the four GT mics I have in my Boston studio that I knew the GT67 would be great choice. (GT60 $699 MSRP, GT50 $399, GT40 $699, GT30 $399; www.groovetubes.com)