Push-Pull Amplifiers
The symmetrical output topology that enabled high-fidelity power amplification. From the Williamson to the Dynaco ST-70, push-pull defined an era of audio engineering.
Why Push-Pull?
The topology that made high-fidelity power amplification practical
The symmetrical topology cancels 2nd, 4th, and 6th harmonics. Only odd harmonics remain, giving a cleaner signal at high power.
Two tubes share the load. A pair of EL34s in push-pull can deliver 50W or more, versus 8-12W from a single-ended EL34.
Plate currents flow in opposite directions through the output transformer primary, cancelling DC magnetization and preventing core saturation.
No DC bias in the core means smaller transformers can handle more signal power. The full B-H curve is available for signal swing.
Power supply ripple is a common-mode signal that gets cancelled in the output transformer, significantly reducing residual hum.
Class AB push-pull can reach 60-78% efficiency versus the 25-50% ceiling of single-ended Class A.
Push-Pull Circuit
Long-tailed pair phase inverter driving matched output tubes through a center-tapped transformer
The LTP splits the input into two equal, anti-phase signals. Grid 2 is AC-grounded; the shared tail resistor sets common-mode rejection.
Each tube amplifies one half of the signal. When V3 plate current increases, V4 plate current decreases, and vice versa.
The center-tapped primary combines both halves. DC cancels (no core saturation), while the signal adds constructively.
Class A vs Class AB
Adjusting bias point determines conduction angle, efficiency, and distortion character
Class A (360°): Both tubes conduct the full cycle. Maximum linearity, lowest distortion, but limited efficiency. Idle current equals maximum signal current.
Class AB (180–360°): Tubes share duty at high levels. Small crossover region adds odd harmonics but dramatically increases power output.
Crossover distortion occurs when neither tube fully conducts during the transition. Proper bias minimizes this notch.
Push-Pull Power
Estimate output power and operating conditions for common PP output tubes
The Williamson Architecture
The 1947 circuit that defined high-fidelity: four stages with global negative feedback
High-gain common cathode. Sets overall sensitivity and provides voltage gain.
Long-tailed pair produces two equal anti-phase signals with low distortion.
Cathode-coupled drivers provide low impedance to swing the output grids.
Push-pull output with global negative feedback for low distortion and flat response.
Tube Matching
Push-pull demands symmetry. Mismatched tubes reintroduce the problems PP was designed to solve.
- • Idle current — within 5% at the same bias voltage
- • Transconductance (Gm) — within 10% ensures balanced gain
- • Plate resistance (rp) — affects load sharing
Fixed bias (adjustable): Individual bias pots per tube allow precise balancing. Preferred in high-power designs (Marshall, Fender). Requires periodic adjustment.
Shared cathode bias: Single shared cathode resistor auto-balances DC but limits the benefit of matching. Common in lower-power designs (Vox AC30).
Individual cathode bias: Separate cathode resistors for each tube. Good compromise; each tube self-biases but no adjustability.
Classic PP Designs
Landmark amplifiers that defined eras of audio and music
Dynaco ST-70
1959The evergreen audiophile amplifier. Warm midrange, smooth treble, exceptional value. Millions produced.
Williamson
1947The first true hi-fi amplifier. Set the standard for low-distortion design with global feedback.
Fender Twin Reverb
1965Clean headroom monster. The studio standard for clean guitar tone. Bright, percussive attack.
Marshall JTM45
1962Based on the Fender Bassman. The birth of British rock tone. Rich harmonic breakup.
McIntosh MC275
1961The reference power amplifier. Patented unity-coupled transformer. Authoritative bass, silky highs.
Key Equations
Essential formulas for push-pull amplifier design
Test Your Knowledge
Validate your understanding of push-pull amplifier design.
What is the main advantage of push-pull over single-ended topology?