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Single-Stage Tube Amplifier

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Recipe #1 — a single tube voltage-amplifier stage

A triode can use a small change at its grid to control a larger change at its plate. This basic common-cathode stage appears in radios, preamplifiers, guitar amplifiers, oscillators, and many other tube circuits.

High-voltage warning: vacuum-tube plate supplies can remain dangerous even after power is removed because capacitors may retain charge. This schematic is for learning how the stage works, not a beginner construction project.
SINGLE-STAGE TRIODE VOLTAGE AMPLIFIER B+ R2 100 kΩ plate IN C1 0.022 µF grid R1 1 MΩ grid leak cathode R3 1.5 kΩ cathode bias C2 0.022 µF OUT small AC input larger AC output 12AX7 triode section

Illustrative parts

  • one 12AX7 triode section
  • R1 = 1 MΩ grid-leak resistor
  • R2 = 100 kΩ plate resistor
  • R3 = 1.5 kΩ cathode resistor
  • C1 = 0.022 µF input coupling capacitor
  • C2 = 0.022 µF output coupling capacitor
  • high-voltage B+ supply appropriate to the tube stage
Follow the signal: IN → C1 → grid → tube controls plate current → voltage changes across R2 → C2 → OUT.

What each part does

C1: lets the AC signal enter while blocking DC from the previous stage.

R1: gives the grid a DC reference path.

R3: develops cathode bias as tube current flows.

R2: turns changes in plate current into larger changes in plate voltage.

C2: passes the amplified AC signal onward while blocking the plate's high DC voltage.

The resistor does part of the amplifying work

The tube controls current. The plate resistor converts that changing current into a changing voltage. The useful output signal is the changing voltage at the plate.

The stage inverts

A positive-going change at the grid tends to increase plate current. More current causes a larger voltage drop across R2, so the plate voltage moves downward. The output therefore has the opposite polarity from the input.

Actual operating voltage, current, gain, distortion, and safe resistor wattage depend on the tube, B+, load, and chosen bias point. A tube datasheet is the next authority when moving from the concept to a real design.

Try these yourself

What is the main job of this triode stage? SINGLE-STAGE TRIODE VOLTAGE AMPLIFIER B+ R2 100 kΩ plate IN C1 0.022 µF grid R1 1 MΩ grid leak cathode R3 1.5 kΩ cathode bias C2 0.022 µF OUT small AC input larger AC output 12AX7 triode section
A common-cathode triode stage is a voltage amplifier.
What is the purpose of C1 at the input?
The coupling capacitor passes AC signal changes while preventing the DC bias of one stage from directly disturbing another.
What happens to signal polarity in a common-cathode stage?
As grid voltage rises, plate current tends to rise, increasing the voltage drop across the plate resistor and causing plate voltage to fall.
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