Microphone Re-Tube

[reshp1]

My Groove tubes GT66 is starting to sound scratchy and occasionally cut out. I'm thinking it's the tube starting to go bad, and since it's out of warranty anyway, I'd like to try putting a replacement, or upgrade tube, in it to see if it corrects the problem, before shelling out the $75/hour labor fee plus shipping for Groove Tubes to fix it. Any suggestions on where to get pro audio/microphone tubes? I'm an electrical engineer by profession but I have limited experience in re-tubing so any tips or things to watch out for would be greatly appreciated too. I'm guessing there's no rebiasing involved

 

[kidvybes]

...I've done tube upgrades on all of my tube microphones except my Mojave MA200...that particular microphone shares a characteristic with your Groove Tubes GT66 (not having any "hands-on" experience with the GT66, but researching it's design online)...both mics utilize miniature pentode type tubes (soldered in place) rather than socket-mounted triode tubes used in many of the common tube microphones, which are much more easily swapped out...
...if your soldering skills are up to par, replacing the tube shouldn't be too dificult...the 6205 pentode tube
http://www.tubecollector.org/cv2432.htm
is not hard to find...reputable sellers like Brent Jessee
http://www.audiotubes.com/bjtubes.htm
offer a selection of vintage NOS tubes...and of course, Groove Tubes also offer replacement tubes...the downside is that "auditioning" tubes is virtually impossible due to the need to solder the tube in place (socket type tubes allow you to easily give various tubes a "test run" before chosing the flavor you like best)...therefore, purchasing a pentode tube from a reputable seller who thoroughly tests the tube before shipping, would be critical...
...a popular substitute for the 6205 is the EF734...vendors like Tubestore
http://www.thetubestore.com/noslist-0000-9999types-6000-9999types.html
and TubeDepot
http://tubedepot.com/tcp26.html
offer substitutes for the 6205...
...you're correct in assuming that rebiasing is not neccessary...so, put your electrical engineering skills to work (heat up that soldering gun) and start researching some of the NOS vintage tubes available...keep an eye out for "military spec" tubes, as these tend to provide the lowest microphonic characteristics...hope this info was useful...Good Luck!

PS...the upside is that these tubes are in very low demand (unlike the 9-pin triode tubes) so prices are bargain-basement level...under $10. for vintage military spec...

 

[kid klash]

Here's another source for tubes... they're really nice folks, and I've never been stung by them. Many antique radio collectors use them because they have more variety of tubes than most :

http://www.tubesandmore.com/

 

[reshp1]

Cool, thanks man! Yeah, I noticed the pins were direct soldered and heatshrinked in the GT66. My soldering skills are pretty good though, except for the part where I can't seem to avoid burning myself.

 

[kidvybes]

My soldering skills are pretty good though, except for the part where I can't seem to avoid burning myself.

...I always thought that was part of the process...I wouldn't know I'm soldering if it weren't for the smell of burning flesh...