How To Simply Measure and Evaluate Microphone Tubes
I have arrived at a "semi-objective" test protocol which, over the years, has served me well to weed out marginal microphone tubes, including those with hidden defects. This article will use the VF14 as a prime example, but the method is equally helpful for other tubes used in microphones, including the decibel (dB) references I used.
I have several healthy, low-noise-select, VF14M at my disposal. I install one of them in an optimally-functioning U47. That mic is inserted into an acoustic isolation chamber which blocks external noise down to -120dB. (You can stuff the mic under a heavy pillow- same effect).
I maximize the monitor level of the headphone output of a special Nakamichi mic preamp I use which also has a white noise generator whose output is adjustable.
I switch back- and forth between mic noise and white noise, until I have calibrated, with my ears, the pre-amp’s white noise output to exactly match the level of the mic's white noise. (If you don't have a white noise generator, you can fake one by dialing an FM radio receiver between stations, and feed that signal into a channel of your board/preamp whose level you then adjust to be equal to the noise output of the reference tube.)
I then substitute the healthy reference/calibration tube in the mic with the unknown tube to be tested. I let the mic run for a day or two, after repeatedly re-calibrating the supply voltage to be within specs for the model. That means ca. 105VDC for a U47, or 210/6.3VDC for a U67, for example. Now I compare the tube's white noise level to the reference white noise level. As my pre-amp is equipped with an attenuator stepped in two dB increments, I can quickly assess the unknown tube's noise floor relative to that of the Neumann-spec reference VF14, represented by the calibrated white noise whose level I do not change in this test.
After all of that, I finally undertake an "in the field" test. From many years' experience I know at roughly what level tube noise will interfere with quiet passages of a recording session in a modern-day, state-of-the-art recording studio. I can extrapolate from that experience what amount of noise would be objectionable by simply talking at moderate levels into the mic at a vocalist's distance from the capsule. There is not much human tolerance for noise from a VF14 or any other tube-equipped condenser mic: even an untrained ear will instantly detect a 4dB elevated noise level in a tube (a trained ear can distinguish as little as 1/4 dB noise elevation, especially in direct comparison to another noise source).
My conclusion and yardstick for selection: Levels of white noise above +2dB from the norm established by a low-noise selected microphone tube will interfere with music in a recording session in two ways: tube noise at that level can be heard directly. It also produces masking noise around the audio to be recorded: audio, especially at higher frequencies and in percussive passages, will sound slightly muffled, matted, and opaque when masking noise interferes.
In sum, I have established a reasonable cut-off for usability of a VF14 at around +2dB, when compared to a known good, low-noise-selected VF14M. +2dB is also a noise threshold where generally most people, especially recording engineers producers and artists, will start to notice tube noise from a microphone.
If anyone thought that this is a "seat-of-the-pants" approach to objective noise analysis, it is. But then I remind you that a lot of supposedly objective quantification in audio is quite subjective: for example, the “decibel” was originally defined by Bell Labs by sitting a bunch of people in a room, and have them raise their hands when they start hearing a faint sound fed into the room.