Tech's Files: Mysteries of Old Tube Mics

Jul 1, 2009 12:00 PM, By Eddie Ciletti



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When you don't have the NOS resources, the alternate philosophy is to take advantage of current production tubes — not simply because some can do the job, but also because the expertise of vacuum tube manufacturing will not live and prosper if we don't support it.

Not all vintage gear is necessarily classic, nor do all NOS tubes have low noise, low microphonics (mechanical sensitivity) or great tone. Production and material variations yield a bell curve, and nowhere is this more obvious than with vacuum tubes. Finding an exceptional tube (NOS or modern) requires buying sizeable quantities, sorting for low-noise/microphonics and then burning-in the candidates for at least 24 hours — if not several days. From this subgroup, the winners are then regraded for noise and evaluated for sonics, and then burned in again.

Quite often, just finding a low-noise tube is acceptable, especially for AKG C-24 owners who need both sections of a 6072 dual-triode to be quiet. (The AKG C-12 and ELA M-251 only use half of the 6072.) Untested NOS tubes are at least four times the price of current production versions, which means you can buy 10 new tubes for the price of two NOS tubes. Any tube that isn't microphone-grade can be used in less critical applications, such as in mic preamps, EQs or guitar amps.

End Game

While we needn't be fearful of changing components, we should be prepared when the need arises because the resurrection results can range from better, same, different or worse than “the way it sounded before it stopped working.” Your go-to mic may have evolved to its current state through some combination of aging and previous repairs.

If your classic mics are essential to your sound, consider embarking on a sample library expedition that highlights what makes each mic special before they bite the dust or become unrecognizable. When it gives up the ghost, your ability to communicate — and provide samples (recording history) — can make the job easier for those entrusted with “maintaining the magic.” For that very special microphone, the phrase “good as new” takes on an entirely new meaning.

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Tube Terms, Types and Technology

One feature common to vintage tube mic designs is circuit simplicity. In the schematic shown, the principle tube components are the heater/filament, cathode, grid and plate, while the latter three (or more) components add up to define a tube's type, whether it is triode (three), tetrode (four) or pentode (five). The plate resistor is a critical passive component and is one of the most commonly replaced pieces in vintage tube gear repair.

The two most common tubes used in microphones are the 6072 (12AY7) dual-triode and the EF86 (6267), a pentode wired as a triode. Others in the 12A?7 Series are also used. Tubes often have two or more “names” as shown, either to denote country of manufacture (USA or Euro) or class/quality (consumer or military/commercial); both of these are 9-pin “miniature” tubes. The EF86 is used in the “export” version of a Neumann U67, while the Euro/broadcast version of that mic, the M269, uses the AC701, a subminiature triode (with wires instead of pins). Other vacuum tubes found in microphones include 6AK5/5654, 6AU6/6136, 6CW4/13CW4, 7586, EF-12, EF-14, VF-14, 6C315-P and the 9002/6s1p (6c1n).

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