I have a Groove tubes model 6tm. It has a chinese K67 capsule and a cinamag transformer, I replaced the tube on this mic with a 5840 / 6225, Pentode, Sharp Cut-Off from antique supply and it sounds okay to me. I really took my time and labeled everything, but it is noisy. It needs like 50db of gain on a Great River nv-2 to produce a good vocal level, it sounds good and has some of what I think are 67 characteristics. I once asked Klaus Heyne about this particular mic and he said it was an entry level microphone. I think i bought it in 96 and don't recall it being quite as noisy back then. Thanks in advance for any suggestions.
the mic is at least 24 years old. some caps have probably drifted out of spec. it would most likely cost more to service the mic than it's worth. maybe time to up grade?
Thanks for the reply,
I ordered two microphones last night a Golden Age GA-47 and a Austrian 808.
I probably will still mess with the Groove tubes. I don't know a lot about microphone design, but like to tinker. I was told that the tube I installed may not be a real 5840. It was really cheap. I have more to examine thanks to Matt ant Mic-parts.
if you have the skills you could test the caps in the GT. perhaps they are fine and it's the tube as you suggested. if the GT is through the hole construction you could up grade the caps and even get one of the capsule replacements that Marco ( @pcrecord ) posted about in his last tutorial video.