What Is A Tube Microphone And How Do Tube Mics Work?


If you’ve been involved in music or audio for any length of time, you’ve probably heard of tube equipment, including tube microphones.

What is a tube microphone and how do tube mics work? A tube mic refers to any active microphone (typically a large-diaphragm condenser) that utilizes a vacuum tube (typically a triode) as its impedance converter. Tube mics pass their capsule signals though a tube (and other components) before the signal is outputted.

In this article, we’ll talk about tube microphones, vacuum tubes, and tube mic examples in great detail.


What Is A Tube Microphone?

A tube microphone, as the name suggests, is any microphone that has a vacuum tube in its design. This is obviously a very simple answer to this question.

The role of the vacuum tube is to act as an impedance converter and amplifier for the mic signal. Tube condenser microphones have condenser capsules that output low amplitude audio signals with high impedance. The vacuum tube’s job is to effectively boost this signal while lowering the impedance so that the signal is able to travel within the rest of the mic’s circuitry and through the output connection.

Because vacuum tubes are active electronic devices (they require power to function), tube microphones are also active. The power requirements for vacuum tubes are relatively high so tube mics are designed with external power supply units that will supply the necessary power.

Tube microphones are most often large-diaphragm condensers. However, there are some small-diaphragm tube condensers and even some ribbon tube microphones on the market.

To learn more about large and small-diaphragm condensers and ribbon microphones, please consider reading my articles Large-Diaphragm Vs. Small-Diaphragm Condenser Microphones and Dynamic Ribbon Microphones: The In-Depth Guide.


What Is A Vacuum Tube?

A vacuum tube (also known as an electron tube or a valve) is an electronic device that controls current flow within a vacuum between electrodes when a voltage has been applied.

Microphone vacuum tubes are made of an outer container (made of glass or ceramic material). Inside this container is a vacuum where no air is present. It’s critical that no oxygen is present within the tube so that the device doesn’t burn up.

Within the tube are electrodes that cause the flow of electrons, hence producing an electrical current.

The simplest vacuum tube design is the diode, which was invented in 1904. It has the 2 most important electrodes in any vacuum tube: the cathode and the anode.

The cathode is the negative terminal/electrode of the tube and the anode is the positive terminal/electrode.

The cathode is heated either directly (the power supply heats up the cathode itself) or indirectly (the vacuum tube is designed with a separate, electrically isolated, heater).

When heat is applied to the tube, the cathode begins emitting electrons. Electrons are negatively charged and so the cathode repels them while, at the same time, the anode attracts them.

So by applying the correct power to hear the vacuum tube, we generate an electric current. Note that the tube needs to be a vacuum or else the oxygen within the tube would burn up the heater and the vacuum tube would not function.

With that basic explanation out of the way, let’s take a look at the triode tube.

Electro-Harmonix
6072A/12AY7EH Vacuum Tube

The basic vacuum tube used in a tube microphone is the triode. Triodes have a third electrode called the grid (otherwise known as the control grid) which is connected to the capsule of the microphone.

The signal that is connected to the grid essentially acts as a modulator of current flow between the cathode and anode. When the capsule’s AC signal hits its peak, so too does the flow between the cathode and anode. When the capsule’s AC signal hits its trough, the cathode to anode flow does, too.

With this functionality, a low-level high-impedance signal from the microphone capsule can control a relatively high-level low-impedance signal between the cathode and anode.

In this way, the triode vacuum tube acts as an amplifier and impedance converter within the microphone design.

Simple Triode Vacuum Tube Diagram
  • H: heater
  • K: cathode
  • A: anode
  • G: gate

How Do Tube Microphones Work?

Because tube microphones are generally large-diaphragm condensers, we’ll discuss how tube condenser microphones work in detail. At the end of this section, we’ll touch on the peculiar ribbon tube mics to compare. By the end of this section, you should understand how tube microphones work.

Tube Condenser Microphones

Tube condenser microphones are [generally] designed with the following:

  • Condenser capsule
  • Vacuum tube
  • Power supply unit
  • Internal circuitry
  • Output transformer

Let’s discuss each of these in short before putting them all together to explain how tube mics work holistically.

Condenser Capsule

A condenser capsule is the transducer component of the microphone. It consists of one (or two) diaphragms and is designed to convert sound pressure variation at the diaphragm into audio signals for the mic to output.

Condenser capsules convert energy based on electrostatic principles. We can think of a condenser capsule as being a parallel-plate capacitor with one stationary backplate and one moveable front plate (this is the diaphragm).

This “capacitor” (known as a condenser in England, hence the name) requires a charge to function properly. With tube mics, the capsules are nearly always externally polarized and this polarizing voltage typically comes from the power supply unit.

Once the capsule/capacitor holds its quasi-permanent charge, it will work properly.

As the diaphragm moves (according to sound pressure variation), the distance between the parallel plates changes and a coinciding change in capacitance happens in the capsule.

Because the capsule holds a constant electrical charge, any variation in capacitance causes an inversely-proportionate change in voltage across the capsule. This voltage is taken from the capsule and is effectively the capsule’s audio signal.

The “audio signal” from the capsule is a reproduction of sound as an AC voltage. However, this signal is of poor quality: it has low amplitude and extremely high impedance.

For everything you need to know about microphone capsules, check out my article What Is A Microphone Capsule? (Plus Top 3 Most Popular Capsules).

Vacuum Tube

The vacuum tube of a tube microphone needs to be “at least” a triode, meaning it has to have at least 3 electrodes. This allows it to act effectively as an impedance converter and amplifier.

The grid of the vacuum tube is capable of taking in the extremely high-impedance signal from the capsule. This signal at the grid is then able to control a higher-quality “signal” between the cathode and anode of the triode.

In this way, we can think of the grid as being the “input” of the tube and the cathode-anode as being the “output” of the tube. With this simplified thinking, we see that the tube acts well as an impedance converter and amplifier and that it’s necessary for proper microphone function.

Without an impedance converter immediately after the condenser capsule, the capsule’s signal would degrade to an unusable level before it ever travelled through the mic cable to a mic preamp.

Power Supply Unit

Vacuum tubes require a lot of electrical energy to function. The power requirements are more than phantom power or other “audio cable powering methods” can provide.

Tube microphones, then, require external power supply units to supply the necessary electrical juice for the active components.

On top of that, the condenser capsules of tube mics are generally externally-polarized (as mentioned earlier). The power supply units also act to polarize these capsules.

Finally, there may or may not be any active circuits elsewhere in the microphone circuitry. PSUs would be designed to effectively power these components as well.

To learn more about powering microphones, check out my article Do Microphones Need Power To Function Properly?

Internal Circuitry

Besides the capsule, tube, and transformer, tube microphones will have other electrical components. The other internal circuitry of the microphone is made of passive, and sometimes active, components that allow for proper electrical flow and protection within the mic. This internal circuitry is also responsible for any special signal processing required within the microphone.

Output Transformer

Finally, an output transformer is often included in the tube condenser design.

Step-down transformers help to further reduce the impedance of the mic’s signal before it is sent through the mic’s output. More importantly, though, is that output transformers balance the mic signal and protect the microphone from DC voltage that may be sent to the output connector of the microphone.

To learn more about microphone transformers, check out my article What Are Microphone Transformers And What Is Their Role?

The Holistic Look At Tube Condenser Microphones

Let’s look at a simple diagram of a tube condenser microphone to help us visualize how they work:

A simple diagram of a tube condenser microphone.

As we see above, sound waves hit the condenser diaphragm which causes the diaphragm to move.

So long as the condenser capsule is polarized, this diaphragm movement will cause a coinciding audio signal at the capsule’s output.

The power supply unit (DC power source) effectively polarizes the condenser capsule. It also powers the vacuum tube. This power supply unit is often attached to the output of the tube microphone. These connections are often 5-pin XLRs (or XLRs with even more pins).

The low-level high-impedance signal from the capsule is used by the tube to modulate a higher-level lower-impedance signal.

This higher-quality signal passed through some more internal circuity within the microphone before reaching the output transformer. The output transformer effectively balances the signal and sends it to the microphone’s output.

For the microphone output, the mic signal is carried to a mic preamp (or to the power supply unit) via a mic cable.

A Quick Look At Ribbon Tube Microphones

Because there are ribbon tube mics on the market, they should be discussed here. Let’s a take a look at a simple diagram of a ribbon tube microphone:

A simple diagram of a ribbon tube microphone.

As we see, the sound waves hit the ribbon diaphragm and the ribbon diaphragm/element produces a coinciding audio signal.

This signal is generally very low in level. A step-up transformer with a high turns ratio will effectively boost the voltage of the signal at the expense of increasing the signal impedance to very high levels.

This high-level high-impedance signal is then fed to the grid of a vacuum tube and is used to modulate a higher-quality signal.

The resulting output signal from the tube is then sent through the output step-down transformer for proper balancing before the output.

In this case, the ribbon microphone is actually active, even though the transducer element is still passive.

Why would a manufacturer go through the trouble of putting a tube in a ribbon microphone? Well, having an internal tube amp in a notoriously low-level microphone type is interesting. The ultimate answer, though, is likely because the technology makes it possible to do so and ribbon tube microphones are just really cool!

For an article on how all microphones functions, check out my article How Do Microphones Work? (A Helpful Illustrated Guide).


Tube Microphone Examples

It’s always a great idea to look at examples when learning about any new subject, especially when that subject is a particular microphone type.

Let’s look at 5 tube microphone examples:

  • AKG C 12 VR
  • Telefunken ELA M 251 E
  • Rode NTK
  • Blue Bottle
  • Royer R-122V

AKG C 12 VR

The AKG C 12 VR (link to check the price on Amazon) is described as an enhanced version of the legendary AKF C 12 microphone (which was produced from 1953-1960).

AKG C 12 VR

The AKG C 12 VR, like its predecessor, utilizes a multi-pattern dual-diaphragm CK 12 condenser capsule and 6072A vacuum tube.

It has 9 selectable polar patterns to choose from which are remotely controllable from the power supply unit. The PSU connects via a large-sized 12-pin Tuchel connector.

Telefunken ELA M 251 E

The Telefunken ELA M 251 E (link to check the price at B&H Photo/Video) is the second clone in this section on tube microphone examples. In this case, it is the clone of the original and legendary microphone of the same name, the Telefunken ELA M 251 E.

Telefunken ELA M 251 E

Like the aforementioned AKG C 12, the ELA M 251 E is designed with a high-quality dual-diaphragm CK 12 capsule and 6072A vacuum tube.

This mic features 3 polar pattern options (cardioid, bidirectional and omnidirectional) which are controllable via a switch on the mic body below the headbasket.

The ELA M 251 and the AKG C 12 are both vintage microphones that have achieved legnedary status in the audio world. Both these mics are featured on My New Microphone’s Top 12 Best Vintage Microphones (And Their Best Clones).

Rode NTK

The Rode NTK (link to check the price on Amazon) is a more recently released tube microphone.

Rode NTK

This tube microphone is a Rode original and features Rode’s proprietary “HF2” edge-terminated cardioid capsule along with a Sovtek 6922 vacuum tube.

Unlike the other mic examples, the NTK is transformerless and rather uses a transistor-based balanced output circuit, which is strange for a tube microphone.

Blue Bottle

The Blue Bottle (link to check the price at B&H Photo/Video) is an example of a tube microphone with modular capsules. This microphone is loosely based on the first-ever produced tube microphone, the Neumann CMV3 (which was/is better known as “the Bottle”).

The Blue Bottle is designed with an EF86 pentode vacuum tube in triode mode and 9610 Tube Power Supply. It features 8 different modular capsules designed by Blue and will even work with some modular capsule designs by Neumann and Violet Designs.

Royer R-122V

The Royer R-122V (link to check the price at B&H Photo/Video) is an example of a ribbon tube microphone.

Royer R-122V

This microphone utilizes a JAN 5840W subminiature pentode vacuum tube (wired as a triode).

AKG, Rode, Blue and Royer are all featured in My New Microphone’s Top 11 Best Microphone Brands You Should Know And Use.


Related Questions

What is a FET microphone? A FET microphone is a solid-state active microphone that utilizes a field-effect transistor as its impedance converter rather than a vacuum tube. Most condenser mics on the market are FET mics though manufacturers really only specify a mic as having a FET if it is to distinguish it from a similar tube mic.

To learn more about solid-state FET microphone, check out the following My New Microphone articles:

What Are FETs & What Is Their Role In Microphone Design?
What Is A Solid-State Microphone? (With Mic Examples)

What is a ribbon microphone? A ribbon microphone is a type of dynamic mic (works on the principle of electromagnetic induction) that utilizes a thin, conductive and corrugated ribbon-like diaphragm. Ribbon mics generally capture audio smoothly and accurately but are relatively fragile.

To learn more about ribbon microphones, check out my article Dynamic Ribbon Microphones: The In-Depth Guide.

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