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Author Topic: Are Tube Microphones Less Reliable?  (Read 150 times)

Offline klaus

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Are Tube Microphones Less Reliable?
« on: June 08, 2018, 01:20:07 pm »
Are Tube Microphones Less Reliable?

If my life depended on the answer to this question, then, yes, tube mics are significantly less reliable than solid-state mics.

Start up a tube mic 100 times, and at least one time it will fail. A 1% failure rate is frustratingly unacceptable in a commercial recording environment, especially when you cannot easily predict and prevent when that moment will come.

After observing tube mic failures for more than three decades, I am still baffled by their utter unpredictability, which complicates prevention.

Here are some of the reasons why tube mics fail:

An example of how and why tubes fail can be found here: (http://repforums.prosoundweb.com/index.php/topic,1150.0.html). Other microphone tubes are not far off from these observations, where a good 10% of failures are beyond anyone’s ability to predict or prevent, even with extensive pre-testing and lengthy burn-ins.

Tubes fail most often, not because a filament wire breaks, or vacuum is lost, but because at impurities swirling around from electrode to electrode become audible- the tube develops discharge sounds that will never go away.

Why does that occur in some tubes after 10 years, and in others after 10 minutes? I have no satisfying explanation.
My best theory why and when tubes fail resembles my theory why we fall off the ladder and break a leg: instability during transition. Rarely does anything happen when things are stationary, but when stepping on or off the ladder, instability of position and movement change conditions under ones feet.

The same goes for tubes: every time I turn on the power supply, it stresses the system, even if only slightly.  That moment of instability can push a fault that’s been building to the surface.

Contacts. Contacts. Contacts.
Contact surfaces of electricity-carrying metals are inherently in a stressed state: electro-chemical reactions cause surface deterioration through current transfer, even across silver or gold plated surfaces.

Deterioration accelerates when mating with dissimilar types of plating. That’s why you try to avoid mating silver-plated XLR pins with a gold-plated ones.
But even across the same type of plating, oxidation continues until its resistive effect blocks the contact and shows up as noise or catastrophic failure. You get it on plug-in mic heads, tube sockets, output connectors, and, especially insidious, because hard to detect, on badly soldered connections od components and traces.

Because heat and heat cycling are an added accelerator of thermal and component instability, tube mics are more affected. Heat and heat cycling (mic on, then off, then on...) of metal surfaces speed up material transformation and contact deterioration through repeated expansion and contraction.

In sum:  when the properties of electricity-carrying components change enough through temperature effects, they become unstable.

An average mono tube mic with variable pattern has more than 40 soldered cable connections between mic and mic pre, and quite a bit more in a stereo mic.

Unless each of these wire connections was perfectly executed, using best soldering and cable routing practices, chances are one of these wire connections will fail, especially once physical stress from abuse is added. The first to fail are usually shield terminations, due to their unwieldy stranding that is often just mechanically clamped down under the connector’s stain relief.

Power Supply.
They work the hardest: AC rectifying, voltage filtering, dropping and dividing... most of these processes are stressed by high currents, voltages and heat - components dry out, resulting in voltage increases with addition

The instability caused by heat is exacerbated by a lack of periodic maintenance- probably the single most significant contributor to tube mic failures.

To minimize chances of sudden failures, here are three preventative measures every tube mic owner should undertake:

1. Have a qualified technician check supply voltages at least once a year, and immediately when tubes are changed. Have the voltages measured INSIDE the mic, not inside the power supply (voltage drop in cable). and adjusted to no more than the median values recommended by the mic (not tube!) manufacturer. Burn in a new tube for at least three days continuous operation. Have the voltages of new tubes re-checked after 24hr continuous operation, and rechecked periodically thereafter.

2. Use a plastic bag over the mic before and after every session. Keep the bag on the mic when stored (http://repforums.prosoundweb.com/index.php/topic,1146.0.html)

3. Operate the tube mic system properly:

Before the session: Connect the mic cable to the power supply and microphone BEFORE you plug in the AC cable, only then turn on power supply.

After the session: Turn off the power supply, THEN disconnect the AC cable from the power supply, then the cable from the mic

« Last Edit: June 08, 2018, 01:25:28 pm by klaus »
Klaus Heyne
German Masterworks®