Phase Inverters
How to split a single-ended signal into two balanced, anti-phase outputs for push-pull amplification. The phase inverter is the brain of every push-pull amplifier — and the most misunderstood stage.
Why Phase Inversion?
Push-pull amplification requires two signals: identical in amplitude, opposite in phase.
A single-ended amplifier has one output tube. A push-pull amplifier has two (or more), alternately conducting on opposite halves of the signal cycle. One tube "pushes" current through the output transformer while the other "pulls" — canceling even harmonics and doubling available power.
But your signal source — a preamp, a guitar pickup, a DAC — produces a single signal. To drive a push-pull output stage, you need two copies of that signal, 180° apart. That's what a phase inverter does.
The quality of your phase inverter determines the quality of your push-pull stage. An imbalanced phase inverter means imbalanced drive, which means incomplete harmonic cancellation, asymmetric clipping, and compromised performance. Every watt of output power passes through this stage.
The Cathodyne (Split-Load)
The simplest phase inverter: one triode, two equal loads, unity gain.
The cathodyne uses a single triode with equal plate and cathode resistors (Ra = Rk). The plate output is inverted (standard amplifier behavior), while the cathode output follows the input in phase.
The gain from each output is slightly less than unity:
The critical insight: both outputs have identical amplitude but different output impedances. The plate output has an impedance of Ra ∥ rp, while the cathode output has a much lower impedance of Rk ∥ (rp/μ). This asymmetry matters when driving capacitive loads (long cables) or output tubes with grid current.
The Long-Tailed Pair
Two triodes, a shared tail resistor, and the most versatile phase inverter ever designed.
The long-tailed pair (LTP) is a differential amplifier: two triodes sharing a common cathode resistor — the "tail." The signal enters one grid; the other grid is held at a fixed bias point. The tail resistor acts as a crude constant-current source, forcing any increase in one tube's current to produce an equal decrease in the other.
Unlike the cathodyne, the LTP provides real voltage gain:
With a 12AX7 and 100kΩ plate resistors: gain ≈ 34 per phase. That's 34× voltage amplification plus phase splitting in one stage.
The tail resistor value determines balance: the higher Rtail, the better the balance (approaching a true constant-current source). Typical values: 10kΩ–47kΩ, with a negative supply voltage to maintain proper bias headroom.
The Paraphase
Two amplifiers in series: one produces the signal, the other inverts a fraction of it.
The paraphase uses two separate gain stages. The first triode amplifies the input normally. A voltage divider taps a fraction of this amplified signal and feeds it to the second triode, which inverts it. The divider ratio is chosen so that the second stage's output exactly matches the first stage's amplitude.
The beauty of the paraphase is that each output has its own independent gain stage with identical plate loads. Both outputs have the same impedance and the same drive capability.
The weakness: balance depends on the divider ratio being precisely calibrated to the tube's gain. If the tube ages, or you swap in a different tube, the balance drifts. Some designs add a trimmer pot in the divider for adjustment.
Waveform Visualization
See how each topology splits the signal. Increase drive to observe clipping behavior.
At low drive levels, all three topologies produce nearly identical balanced outputs. The differences emerge under overdrive: the cathodyne clips asymmetrically (one side compresses before the other), while the LTP clips symmetrically. The paraphase's behavior depends on how well the divider is trimmed.
Topology Comparison
Choosing the right phase inverter for your design.
| Parameter | Cathodyne | Long-Tailed Pair | Paraphase |
|---|---|---|---|
| Tube count | 1 (½ dual) | 2 (1 dual) | 2 (1 dual) |
| Voltage gain | < 1 (unity) | ≈ μ·Ra/(ra+Ra) | ≈ μ·Ra/(ra+Ra) |
| Output balance | Excellent | Very good | Good (trimmed) |
| Output impedance | Asymmetric | Symmetric | Symmetric |
| Drive capability | Limited | Strong | Strong |
| CMRR | N/A | High | Moderate |
| Overdrive character | Soft, asymmetric | Symmetric clip | Complex |
| Typical use | Low-power hi-fi | Most amplifiers | Vintage designs |
The practical decision: For most push-pull amplifiers, the long-tailed pair is the correct choice. It provides gain, good balance, symmetric output impedances, and predictable overdrive behavior. The cathodyne is elegant for low-power designs where a preceding high-gain stage supplies the voltage swing. The paraphase is historically interesting and tonally distinctive, but its balance sensitivity makes it less reliable in production.
Designing an LTP
Step-by-step: component values for a 12AX7 long-tailed pair driving EL34 push-pull.
1. Choose the tube. A 12AX7 (μ=100, rp=62.5kΩ) is the standard. For lower gain and better linearity, consider a 12AT7 (μ=60, rp=37kΩ) or 12AU7 (μ=17, rp=7.7kΩ).
2. Set the plate resistors. Equal values, typically 82kΩ–100kΩ for a 12AX7. Higher values = more gain but lower headroom. With Ra = 100kΩ:
3. Set the tail resistor. The tail sets the operating point and determines balance. A higher tail resistor improves CMRR (common-mode rejection ratio) but requires a higher B+ or negative supply to maintain bias. Typical: Rtail = 10kΩ–47kΩ.
4. Set the bias. For the LTP, the non-driven grid is returned to ground through a resistor (Rg = 1MΩ). The tail resistor develops the bias voltage: Vk = Ia(total) × Rtail. With two sections of 12AX7 at 0.5mA each, and Rtail = 22kΩ: Vk = 1mA × 22kΩ = 22V. This means each grid is at −22V relative to its cathode.
5. Coupling capacitors. Typically 0.022μF–0.1μF to the output tube grids. The value determines the low-frequency −3dB point: f = 1 / (2π · C · Rg). With 0.022μF and 220kΩ grid resistors: f = 33Hz — adequate for full-range audio.
6. Check the voltage swing. EL34 output tubes need about 40Vpk grid-to-grid drive for full power. With gain of 61 per phase, you need only 40 / (2 × 61) = 0.33Vpk at the LTP input. In practice, much of this gain is available for feedback — which is exactly why the LTP is preferred in feedback amplifiers.
Phase Inverters & Overdrive
Why the phase inverter shapes your amplifier's clipping character.
In guitar amplifiers, the phase inverter is often the first stage to clip when driven hard. The way it clips defines the amplifier's overdrive character.
Clips asymmetrically. The plate output compresses before the cathode output, creating an imbalance that produces even harmonics (2nd, 4th). Sounds warm, round, slightly compressed. This is the Vox AC15 character.
Clips symmetrically. Both phases limit together, producing predominantly odd harmonics (3rd, 5th). Sounds aggressive, tight, articulate. This is the Marshall crunch — the phase inverter contributing as much to the tone as the output tubes.
The Marshall JCM800's legendary crunch comes largely from its LTP phase inverter being pushed into clipping. Jim Marshall didn't design it this way intentionally — it was a happy accident of the circuit's gain structure. But it's the reason a cranked Marshall sounds different from a cranked Fender, even with identical output tubes.
Key Equations
The math behind phase inverter design.
Test Your Knowledge
Validate your understanding of phase inverter topologies before moving on.
Why does a push-pull amplifier need a phase inverter?