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What is a tube microphone?

Apr 8, 2022 5 min read

Mike Metlay (Guest author)

tube microphone

Microphone manufacturers make a big deal about tube mics – especially the ones that make tube mics, which shouldn’t surprise anyone. You hear language like ‘vintage character’, ‘authentic tone’, and especially ‘tube warmth’.

Does any of that mean anything at all? Yes, it does –  less than the mic makers would have you believe, but not much less. Let’s break it down.

How does a tube microphone work?

The central element of a condenser microphone is a condenser (duh!), the obsolete British term for a capacitor: a pair of charged plates with a varying electric field in between them, which changes as moving air (sound) moves one of the plates (the diaphragm) back and forth.

The electrical signals produced by the capacitor in a mic capsule are tiny, and they need to be amplified before they can be sent down a cable to the rest of your gear. This onboard amplification inside a mic’s body, which must be kept close to the capsule so it suffers the least signal loss, is called a head amplifer or head amp. Often just generically referred to as ‘the electronics’.

The primary component of any head amp is the amplifier itself, which can be based on a vacuum tube or a transistor. They do the same job – converting the very high impedance of the capsule’s output into a low-impedance signal while amplifying it – but in very different ways.

A typical tube for audio use is a triode – it has three conductors inside. When the tube heats up, electrons start to flow through the vacuum in the tube, from the cathode to the anode, while the grid sits in between them, watching the tiny signal coming from the capsule.

As electrons flow through the grid on their way from the cathode to the anode, the grid modulates the flow, turning it into a greatly amplified low-impedance version of the grid signal that’s ready to send down the cable (after it goes through the output transformer, which we’ll talk about another time).

A field-effect transistor (FET) in a solid-state mic’s head amp does pretty much the same thing – but it does it via the properties of the FET itself, which are slightly (but critically) different than the tube’s. That difference is the source of the tube’s reputation.

Click here, if you want to learn more about how a microphone works.

What does a tube add to the sound in a microphone?

That magic difference between tube and FET can be heard when you push the head amp harder than it’s designed for, by putting the mic up close to a loud sound source: a belting singer, a thundering drum kit, a blasting guitar amp cabinet.

When overloaded, a tube distorts and creates extra harmonics – even harmonics that the human ear perceives as euphonious (a fancy word meaning ‘nice-sounding’). The signal gently compresses, too, which sounds awesome.

Even harmonics created by a 100 Hz sine wave run through a tube compressor.
Above is an example of even harmonics created by sending a 100 Hz sine wave generator through a tube compressor. 

On the other hand, when a FET distorts, it generates odd harmonics, which also sound awesome – if you’re playing guitar through a fuzz pedal.

If an overdriven FET mic sounds harsh, why does anyone use them? Simple: because they’re not supposed to be overdriven – that’s why they often have built-in pads to lower their sensitivity to loud sounds. While it’s possible to build a tube mic that’s very clean and quiet, FET condenser mics do it naturally, so they’re chosen for how they sound when they’re not pushed beyond their design limits. Different sounds for different applications, right?

Are tube microphones better?

So where does all this ‘tubes rule’ stuff come from? Here’s the thing: for decades at the dawn of the recording industry as we know it today, tube microphones were the only condenser mics there were, with viable designs going back to the 1930s.

Solid-state condenser mics didn’t come along until the 1960s, and they’ve coexisted with tube mics ever since. That gap of 30 years gave musicians a lot of time to listen to and fall in love with thousands of hits made with tube mics.

Unfortunately, a fair number of them then decided that the great sound of these recordings had nothing to do with the talent of the artist, the skill of the engineer, or any of the other equipment in the studio: it was all because the mic (and maybe some of the other gear) used tubes.

That’s a pretty unlikely scenario – but it does sell a lot of tube mics.

Pros and cons of a tube microphone

All hype aside, a well-made tube mic really does sound fantastic on the right sources. However, like all microphones, there is no hard and fast rule that certain sources always sound best through a tube.

Everyone talks about how awesome tube mics are at capturing vocals, but some of the best singers in history sang their best through solid-state mics, ribbons, or even cheap dynamics. On the other hand, tube mics can bring surprisingly sweet tonality to places where they’re rarely used (at least these days), with drum overheads being a classic example.

There are ups and downs to using tubes. We’ve spent a lot of time on the upside, namely the tube’s characteristic sound when overdriven. The downsides come from a practical perspective: in casual use, tube mics can be a real pain!

Unlike a FET mic, which happily runs on whatever phantom power your mixer or interface provides, a tube mic needs a lot of heat to operate. That means it requires its own power supply, and a special (and sometimes really expensive) cable to connect the two.

A tube mic can’t just be turned on and put to use. It has to warm up, and once it does, it has to be handled like anything else that has a white-hot fragile glass tube inside – with great care. Tubes are fragile and even if you baby them, they’ll eventually die and need to be replaced – and the wrong tube can ruin the sound of the mic.

In sort, a tube mic needs a lot of love, but if you treat it well, it’ll treat you (and your ears) well in return.

Conclusion

Tube mics can be great in certain applications, but beware of elevating them to the status of icons or assuming they’re all hype and no substance. As always, you need to know the sound of your mics, and be able to match them to an application for the best results, rather than rely on what ‘everybody knows. Remember: if it sounds good, it is good.

If you're looking for a high-end tube microphone for your studio, you can check out the LEWITT LCT 1040. Its circuit is designed to make the tube actually audible and provides you with four distinct tube characteristics to cover all the notions one associates with tube sound in general. And, it also features a blendable FET circuit. 


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