Clearing up some misconceptions about tube mics and preamps.
OK, here we go about tubes. As far as most tube circuits in mics and preamps are concerned, they're usually a single tube operating as a "Class A" device. Here's a picture of how a Class A circuit works:
See the signal coming in, at the bottom of the drawing? It goes into the tube (which is represented by that "S" shaped curve), hits the transfer function and can come out amplified (as in the right side of the drawing), or simply come out equally, depending on the circuit designer's intent.
It's important to realize that the output of a condenser capsule is very low, while the capsule's impedance is VERY, VERY high. Very high impedance sources don't travel well over long distances, so it's important to convert that high impedance to a lower impedance as soon as possible.
You have two choices: a Field Effect Transistor (commonly called an FET), or a tube (some tubes love seeing very high impedance sources). If you use an FET, it won't do much in the way of giving you any more signal, so you'll need to add some more transistors to boost the signal a little bit. And most transistor circuits tend to distort very easily (and in a nasty way) if pushed too hard.
Tubes, on the other hand, tend to simply round off the signal if they approach the top and bottom of the "S" shaped "Transfer Curve", resulting in more musical distortion components, i.e., more 2nd and 4th harmonic overtones, which are musically correct, creating a "warmer, fatter tone. That's what makes tube distortion so desirable in guitar amplifiers.
Some tube preamp designs add more distortion by using a very small plate voltage to effectively shrink the length of that straight line part of the "Transfer Function" so that the tube saturates quicker and distorts faster. To me, it sounds a little fake and un-natural, but a lot of people seem to like it.
So the main advantages to using tubes in mics are: natural impedance convertor, which also works as a gain stage, limiter, and as an even order
distortion generator, when pushed hard. One lessor known aspect of using a tube inside the body of a micrphone is that the heat from the tube helps drive out any moisture in the capsule when used in humid environments.
Since the tube must have heater and plate voltages supplied from an outboard power supply, it also makes sense to generate the 48 volt phantom power voltage from the power supply as well. This brings up another possibility when using dual membrame capsules for multiple polar patterns: continuously variable remote polar pattern selection from the power supply.
Remember earlier in this thread, we discussed how the different pressure gradient polar patterns are created by mixing the sound from two polar patterns; omni and figure 8? We can take that one step further since a dual membrane condenser mic is made into an omni by having both sapsules charged. Flip the polarity of the back capsule's signal and you have a figure 8.
As you continuously adjust the level and polarity of the back capsule, the mic will slowly change the polar patterns starting with omni, passing thru wide cardioid, sub cardioid, hyper-cardioid, and super-cardioid on it's way to figure 8.
If you are using one of these continuously variable polar pattern mics, it can be used to remotely change the tone and the amount of proximity effect of the mic as well. As you move from omni to figure 8, the proximity effect goes from almost none to maximum.
Many engineers will use the pattern selector switch as a tone control and ignore the different polar pattern choices for a particular singer, since the mic is used in a pretty absorbent situation; an iso booth, or a very dead room. for example.
Often, the decision to use a tube mic is mistakenly made to increase distortion, resulting in what some people describe as "tube warmth". In most modern mic designs, tubes are used for the performance reasons (listed above), not to add distortion, but to eliminate the often unpleasant distortions caused by poorly designed transistor mic circuits, which can often be described as harsh, edgy, brittle, etc.
One last point about LD mic design: a 1" wavelength corresponds to a frequency around 5 to 7 kHz. Ever wonder why all 1" capsules have a peak right around that frequency range? Now you know. Explaining what to do about it is a whole 'nother subject, which we'll get into at another time.
I hope some of this has proved helpful to at least somebody out there.
