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Old 16th October 2013
  #3
Lives for gear
 
If you get a filament voltage up to the rated value (the AC701 used a 4V heater) there are quite a number of tubes that will work and sound good in a condenser microphone if you use the proper circuitry around them. I use a 6AH6 mini which isn't really designed for microphone use but they have a sound that I like. If I need a subminiature tube then I have several in my stash but I usually use a 5840 as it is quite common and works well. The Russian tube looks like that's what they were using in Lomo mics so it probably is just fine. By 'modern equivalent' that might be a bit of a stretch but the tube still probably sounds good. Using one's ears is especially true of a U67-based circuit since the U67 never used the AC701 - it doesn't much matter if the 6S6B is a lot like an AC701 unless you are servicing a KM54 or M49 or something like that. If you have to replace the AC701 then it matters. In a mic circuit you do need to worry about grid leakage but a lot of tubes work just fine if the envelope is cleaned up enough.