What is a tube microphone?

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 is called a head amplifier or head amp.  

The primary component of any head amp is the amplifier itself, which can be based on a vacuum tube or a Field Effect Transistor (FET). They do the same job, but in very different ways. 

A tube or triode 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. 

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

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. 

On the other hand, when a FET distorts, it generates odd harmonics, which can 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. 

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. 

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: 

Power use: 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. 

Set up time: 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.  

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

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 PURE TUBE.