How Does a Cathode Follower in a Tube Audio Circuit Work?

It is essentially a common-plate amplifier (also called a common-cathode amplifier with 100% negative feedback) that provides a high input impedance, low output impedance, and unity (or a small amount of) voltage gain.

In the world of vacuum tube amplification, the cathode follower circuit occupies a unique and vital role. Unlike typical gain stages designed to amplify voltage, the cathode follower is configured to provide current gain and act as a buffer between circuit stages. This topology features a tube with its output taken from the cathode rather than the plate, resulting in a low output impedance and unity voltage gain (or slightly less). Such characteristics make cathode followers exceptionally useful for driving capacitive loads, interconnect cables, or the following stages without introducing significant signal loss or distortion. In high-fidelity audio applications, this configuration helps maintain signal integrity, reduce loading effects, and improve overall bandwidth and transient response.

Cathode followers are commonly used in tube preamplifiers and power amplifier input stages, where their buffering ability preserves the delicate audio signal. Their low output impedance allows them to effectively drive tone control circuits, long cable runs, and power tube grids, preventing high-frequency roll-off and minimizing interactions between stages. Beyond technical advantages, cathode followers also contribute to a more transparent and open sound, a trait highly valued among audiophiles. Their simplicity and effectiveness have made them an enduring choice in both classic and modern designs, reinforcing the importance of careful stage-to-stage interfacing in achieving superior sonic performance. By ensuring that each part of the amplifier receives a clean, unaltered signal, cathode follower circuits serve as silent custodians of musical nuance, helping to deliver the clarity and realism listeners seek from tube-based audio equipment.

How It Works

A cathode follower operates with the following key characteristics:

● Basic Configuration

1. The input signal is applied to the grid of the vacuum tube.
2. The plate (anode) is connected to a high-voltage power supply through a plate resistor or choke.
3. The cathode is connected to the output and also has a cathode resistor to ground.

● Voltage Follower Behavior

1. The voltage gain is close to unity (~0.9 to 1).
2. The output signal is taken from the cathode, which follows the input voltage but at a slightly lower amplitude due to the internal characteristics of the tube.

● Impedance Characteristics

1. High Input Impedance: This makes it an excellent buffer stage, as it does not load the preceding circuit.
2. Low Output Impedance: This allows it to drive lower impedance loads (such as the next stage in an amplifier) with minimal signal degradation.

● Negative Feedback & Distortion Reduction

1. The cathode follower inherently provides negative feedback, as the output signal (at the cathode) follows the input signal (at the grid). This feedback reduces distortion and stabilizes the circuit.

Applications in Audio Circuits

Example Circuit

Here is a simple cathode follower circuit using a 12AX7 tube: (Image^© https://forum.fractalaudio.com/threads/cathode-follower-again-no-rocket-science.74247/)

Pros and Cons

Pros

Cons

What is the Miller Effect?

The Miller Effect is a phenomenon that occurs in amplifier circuits (especially those using vacuum tubes or transistors) where the effective input capacitance of a stage is increased due to the amplification of the circuit. This can significantly affect the high- frequency response of an amplifier, often causing high-frequency roll-off or loss of treble.

How the Miller Effect Works

In a typical common-cathode amplifier, there is an inherent capacitance between the tube’s grid and plate (denoted as Cgp). Because the plate signal is inverted and amplified, any small capacitance between the grid and plate is multiplied by the stage’s gain, increasing the effective input capacitance seen by the signal source.

The Miller capacitance (CM) is calculated as:

CM=Cgp×(1+Av)

Where:

Since the voltage gain (Av) of a tube amplifier stage can be quite high (e.g., 50–100), the Miller capacitance can be significantly larger than the actual interelectrode capacitance of the tube, effectively slowing down the amplifier’s response to high- frequency signals.

Effects in Tube Audio Circuits

● High-Frequency Roll-Off

1. The increased input capacitance due to the Miller Effect forms a low-pass filter with the source impedance.
2. This can attenuate high frequencies, leading to a loss of treble in audio circuits.

● Reduced Bandwidth

1. The increased capacitance limits the amplifier’s ability to handle high-frequency transients, which is critical in hi-fi and guitar amplifier designs.

● Impact on Guitar Amps

1. In guitar amplifiers, the Miller Effect can warm up the tone by slightly rolling off highs, but excessive capacitance can make the amp sound “muddy.”

● Parasitic Oscillations

1. In high-gain circuits (such as in some high-gain guitar preamps), the Miller Effect can contribute to unwanted feedback and oscillations.

How to Minimize the Miller Effect

● Lower the Gain of the Stage

1. Since Miller capacitance is proportional to 1+Av, reducing gain (e.g., using a lower-mu tube or adding negative feedback) will reduce the Miller Effect.

● Use a Cathode Follower or Buffer

1. A cathode follower stage does not have voltage gain (A_v ≈ 1), so it does not exhibit significant Miller capacitance.

● Use a Grid Stopper Resistor

1. A grid stopper resistor (e.g., 10 kΩ – 100 kΩ) can limit high-frequency gain and reduce unwanted oscillations caused by excessive Miller capacitance.

● Reduce Plate-to-Grid Capacitance

1. Choosing tubes with lower intrinsic capacitance helps reduce the Miller Effect.

● Use a Low-Resistance Signal Source

1. The Miller capacitance forms a low-pass filter with the source impedance. Using a low-impedance source helps minimize high-frequency loss.

The Miller Effect is an important consideration in vacuum tube amplifier design, particularly in high-gain preamps and audio circuits. It can limit high-frequency response, but can also be used deliberately to shape the sound. Engineers and amp designers must carefully balance gain, impedance, and circuit topology to control or reduce the Miller Effect for the desired sonic characteristics.

Solid-state Equivalent to the Tube Cathode Follower Circuit

In solid-state designs, the functional equivalent of a cathode follower is the emitter-follower in bipolar transistor circuits, or the source-follower in field-effect transistor (FET) circuits.

Like the cathode-follower, an emitter-follower (using a bipolar junction transistor, BJT) provides current gain, low output impedance, and unity (or slightly less) voltage gain, making it ideal for buffering and impedance matching. In this configuration, the input signal is applied to the base, and the output is taken from the emitter, with the collector connected to a positive supply (similar in concept to how the cathode output is derived in tube circuits). This allows the emitter follower to drive heavy or capacitive loads without significant signal degradation, just as the cathode follower does in tube amplifiers.

In FET designs, a source-follower performs the same role. The input is applied to the gate, and the output is taken from the source. Again, it delivers a low output impedance and isolates stages to prevent signal loss.

Both emitter followers and source followers are widely used in solid-state preamplifiers, tone control circuits, and output stages. They help preserve linearity, extend bandwidth, and improve the ability to drive subsequent stages or external loads. While tubes and transistors have different electrical characteristics, the basic principle of providing a buffer between stages remains the same, reflecting a universal design solution in both tube and solid-state audio circuits.

The cathode follower is an essential tool in tube-based audio design. It provides signal integrity, impedance matching, and buffering, making it a critical component in preamps, guitar amplifiers, and hi-fi audio circuits.

REFERENCES

Valley, G. E., & Wallman, H. (1948). Vacuum Tube Amplifiers. McGraw-Hill.

Covers fundamental tube amplifier design, including cathode followers and impedance matching.

Langford-Smith, F. (1953). Radiotron Designer’s Handbook (4th ed.). Wireless Press.

A definitive resource on vacuum tube technology, providing in-depth explanations of cathode followers and their applications in audio.

Mullard Technical Handbook (1960s). Application of Mullard Tubes in Audio and RF Circuits. Mullard Ltd.

Details on practical cathode follower implementations in high-fidelity and RF circuits.

Morgan Jones (2003). Valve Amplifiers (3rd ed.). Newnes.

A highly recommended modern book covering cathode follower theory, implementation, and real-world applications in hi-fi audio.

Merlin Blencowe (2010). Designing Tube Preamps for Guitar and Bass. Wizard Amps.

Discusses the cathode follower in the context of guitar amplifiers, including its effect on tone and signal buffering.

O’Connor, K. (1995). The Ultimate Tone. London Power Press.

Focuses on high-performance tube amplifier design, with sections on

MIT Radiation Laboratory Series (1947). Electron Tube Circuits. McGraw-Hill.

Provides an early but rigorous analysis of cathode followers in electronics applications.

National Semiconductor Application Notes (1970s). Understanding Cathode Followers and Buffer Stages.

Although primarily focused on general amplifier design, this includes insights into cathode follower behavior.

Aiken, R. (2000s). The Cathode Follower: Impedance, Distortion, and Applications. Aiken Amps

A detailed explanation of how cathode followers work, their advantages, and practical applications in tube circuits.

Elliott Sound Products (Rod Elliott). The Cathode Follower in Hi-Fi Audio. ESP Website

A practical guide discussing the use of cathode followers in preamps and audio equipment.

Valley, G. E., & Wallman, H. (1948). Vacuum Tube Amplifiers. McGraw-Hill.

A classic text on vacuum tube amplifier design, covering the Miller Effect and its implications.

Langford-Smith, F. (1953). Radiotron Designer’s Handbook (4th ed.). Wireless Press.

A comprehensive resource on vacuum tube theory and amplifier design, including the impact of Miller capacitance.

Mullard Technical Handbook (1960s). Application of Mullard Tubes in Audio and RF Circuits. Mullard Ltd.

Discusses interelectrode capacitance and its effect on high-frequency performance in audio circuits.

Morgan Jones (2003). Valve Amplifiers (3rd ed.). Newnes.

A modern and practical guide on tube amplifier design, with a detailed discussion on Miller capacitance and ways to mitigate it.

Merlin Blencowe (2010). Designing Tube Preamps for Guitar and Bass. Wizard Amps.

Covers how the Miller Effect impacts guitar preamp stages and techniques to control high-frequency response.

MIT Radiation Laboratory Series (1947). Electron Tube Circuits. McGraw-Hill.

Explores the mathematical derivation of Miller capacitance in amplifier circuits.

National Semiconductor Application Notes (1970s). Understanding the Miller Effect in Amplifier Design.

Though originally written for solid-state circuits, the principles apply directly to vacuum tube circuits.

Aiken, R. (2000s). The Miller Effect and Its Impact on Tube Amplifiers. Aiken Amps

A detailed breakdown of the Miller Effect in tube circuits, with practical design considerations.

Elliott Sound Products (Rod Elliott). The Miller Effect in Tube and Transistor Circuits.

Provides practical examples and calculations for understanding Miller capacitance.