Tech's Files: Mysteries of Old Tube Mics

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

PEELING THE LAYERS OFF THE GLASS ONION

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A Neumann U67 with EF86, a pentode wired as a triode

A Neumann U67 with EF86, a pentode wired as a triode

Wines have been to known to improve with age, but what happens to a microphone after 50 years or so? A vintage mic with that special “something” is unlikely to sound the same as it did a half-century earlier. The sound will be similar, yes, but whether the user feels it sounds better or worse with age may be highly subjective and may depend on the application.

You might be too love-blind about a mic that rocks your world to notice its decline until it needs specialized care. Then the question will be, “What are my options and to what degree can the options be manipulated so that the mic comes back to life the way I'd like?” Notice that there are options; some are under our control, some are more spendy than we'd like.

In vintage tube mics, the signal path is very direct. Between the vibrations in the air and the output connector are six components: grille, capsule, vacuum tube, capacitor, output transformer and cable. There's also a power supply, a handful of resistors and other capacitors, but two of these sonically critical components — tubes and capsules — are subject to aging factors and availability. Each imparts a sonic signature so recognizable that the technician's challenge gives new meaning to the phrase “First do no harm.”

If you suspect the problem is with the tube (generally a user-serviceable part) or have already made a swap with no improvement, then before proceeding consider the plight of the capsule. Its life begins subject to the manufacturing tweaks and tolerances of the production line. Then add the sonic equivalent of Hurricane Katrina and Mt. St. Helens — rock star breath and explosive spit, two aging accelerators that gradually migrate the capsule's character outside of the “factory tolerance window.” From any perspective, vocal mics and kick drum mics have a tough existence.

All condenser mics will inevitably need a tune-up; the most typical problems are breath sensitivity (humidity) and random noises. When both of these issues are related to capsule condition, the three options for repair/restoration require a specialist's expertise. In order from least to most dramatic possible change and/or improvement, the options are cleaning, reskinning (new diaphragms) or replacement.

Cleaning usually reveals the “hidden damage.” Scarring of the gold coating may not be pretty, but it's not cause for alarm like missing gold or a puncture wound might be. In such cases, reskinning is your best shot at preserving character. The predetermined magic stems from how the brass backplate was drilled, combined with the spacer thickness between the diaphragm and the backplate. Variables include diaphragm thickness and tension, and when the initial work is done, your warm, dark-ish, tastefully airy vintage mic may brighten up and tighten up. Tubes can have a similar effect. Then the tweaking begins.

Every page of Mix could be filled with mind-numbing details on the subject, but the short story is that a mic capsule is a Helmholtz resonator at the microscopic level. Imagine the challenges the original designers faced when the three primary ingredients were knowledge, a slide rule and time to experiment. No modeling software — just trial, error and word-of-mouth.

Vacuum-Style Variables

When microphone noise is not capsule-related, the next wear-item in the chain is the vacuum tube. If you didn't know, New Old Stock (NOS) is the phrase that refers to tubes that are made rare and spendy by collectors and manufacturers who have scarfed up the best (or large quantities). The remaining NOS has either been picked through or deemed less likely to yield a winner, so truly exceptional tubes are increasingly rare. If you have the resources and a reliable NOS source, that's great, However, I wouldn't pay top dollar for an untested tube, no matter how many stars are on it.






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