I have a BPM TB-95 mic with an 12ax7 sized tube inside with no markings
on it at all. I'd like to try another tube in there, but not knowing
what it is, I don't know what's a safe replacement. Is there any way to
determine what kind it is?

apa wrote:
I have a BPM TB-95 mic with an 12ax7 sized tube inside with no markings
on it at all. I'd like to try another tube in there, but not knowing
what it is, I don't know what's a safe replacement. Is there any way to
determine what kind it is?

Does it have one section inside or two? If it has two, it is probably
either a 12AX7-type or a 6DJ8-type. Try both on the tube tester. If it
comes up happily with a 12AX7 configuration, measure the transconductance
on the tester. Compare it with the 12AX7, 12BH7, 12AT7, and 12AU7 types
on the tester roll... they are all the same pinout but with different
transconductance and gain. Knowing this you should be able to make a good
guess about which of the four it is.

If it's a single section inside, and it's 12AX7-sized, it is probably an
EF86 or an EF804.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

I have one of those also. since it was designed to be a clone of a 67 I
wouldn't be surprised if its an ef86. Let us know when you find out. wanna buy
mine...cheap? think of the glory of having a pair!
Paul

Truth be told I don't really like it that much. Almost never use it. My
studio mates do though and the PSU stopped working while they were
using it. While I was troubleshooting it, I took the mic body apart and
thought I'd see if a different tube made it any more usable. I know
they use it as an overhead. Maybe they'd like a pair. I'll ask them.
How cheap is cheap?