I have a few NOS (new, old stock) 6AK5 tubes that I picked up from a radio repair shop when we got our group buy mics - they fit the TnC ACM-900 (tube ribbon). Most are GE. I picked out the ones I like best and I don't have a use for the others. Actually, I do have a use:
I bought extras specifically so that I could send them for free to other ACM-900 owners
but strangely, I've only been able to give one away so far. I'm not aware of any other mics that they would fit, but there may be others. My preference will be for ACM-900 owners (since it was my goal to improve the cool-ful-ness of these mics as a group, however slightly), but at this point, I'll consider others, if you're sure I won't be sending one of the little guys into useless-land. Remember, they might be older than both of us put together, and they've been waiting for action on a shelf that whole time!
One of these tubes may improve the sound of your ACM-900 (or maybe not!) - they're *free*, shipping included. Just PM me with a mailing address.
Some of them have a little hiss, but *all* of them have less hiss than the tube that came stock in my ACM-900. I don't have the golden ear, or maybe it's that I don't have the golden audio source, to characterize/verify an improvement to the sound, but I can say, um, yes, they made my mic sound better.