What is a vacuum tube?

A vacuum tube is an electronic component that controls electron flow inside an evacuated glass envelope. A heated cathode emits electrons (thermionic emission), an anode collects them, and one or more grids modulate the current. Tubes are still used in audio amplifiers for their low-order harmonic distortion and soft clipping behaviour.

What is the difference between a triode and a pentode?

A triode has three electrodes (cathode, grid, anode) and produces low gain (μ ≈ 20 to 100) with predominantly second-harmonic distortion. A pentode adds a screen grid and a suppressor grid, raising plate impedance and gain (μ > 1000) but introducing more high-order harmonics. Triodes are preferred for single-ended output stages, pentodes for high-power push-pull.

How do I bias a vacuum tube?

Two common methods: cathode bias (a resistor on the cathode self-regulates grid voltage and is forgiving of tube variation) and fixed bias (a negative supply on the grid, used in high-power push-pull stages). The operating point should typically sit at 70 percent of the maximum plate dissipation for class-A audio operation. Use a loadline calculator to verify.

A loadline is a straight line drawn on a tube's plate-current vs plate-voltage characteristic curves. Its slope is -1/RL where RL is the load resistance, and it shows every operating point the tube can reach. The intersection with the bias curve is the quiescent point. Loadlines are essential for choosing operating point, predicting distortion, and matching output transformers.

Are vacuum tubes still made today?

Yes. Current production tubes come mainly from JJ Electronic (Slovakia), Electro-Harmonix and Sovtek (Russia), Shuguang and PSVANE (China), Western Electric (USA), and Linlai. They cover most audio receiving types — 12AX7, EL34, KT88, 300B, 6L6, EL84, 6V6 — and several high-end recreations of NOS classics. Quality varies; testing on arrival is recommended.

Is working with vacuum tube amplifiers dangerous?

Yes. Tube amplifiers run on plate voltages of 250 to 600 V DC, and reservoir capacitors can hold a lethal charge for minutes after power-off. Always discharge filter capacitors through a bleeder resistor before working inside, keep one hand behind your back when probing live circuits, and never work alone. Read the High Voltage Safety guide before opening any chassis.

By Jean-Michel Falligan Devergne

Understand vacuum tubes, all the way to designing your own.

55 interactive guides — live calculators, schematics you can pull apart, and circuits that hold up to measurement. Start from first principles, all the way to the loadline, biasing, and the output transformer.

Find your starting point Browse all guides

55interactive guides

3guided paths

2design studios

55 guides

What you'll be able to do

Read any datasheet

Plate curves, bias points, the limits that matter.

Draw a loadline

Place a Q-point, predict gain and distortion.

Design a stage

From triode to push-pull, formulas in hand.

Simulate in SPICE

Koren models — check it before you solder.

Day Zero▼

No diploma needed — just curiosity. This interactive guide teaches you the 5 fundamental laws (Ohm, power, Kirchhoff, series/parallel, AC/DC) with visual analogies, live calculators, and zero jargon.

Start Day Zero →1How Vacuum Tubes Work 2Passive Components 3Biasing Methods

Studios

STUDIO

Stage Designer

Step-by-step design wizard for every classic tube stage — RDH4 / Reich / Terman formulas with E24 snapping.

~45 MIN · STUDIO · STEP-BY-STEP

STUDIO

Voltage Regulation Studio

Full tube voltage regulator design studio — Autodesigner, calculators, SPICE export, and a guided course.

~60 MIN · STUDIO · 7 LABS

Curated learning paths

Three sequences that actually lead somewhere.

Starting with tubes

From thermionic emission to reading a datasheet, seven modules that give you the base vocabulary across triode, pentode and rectifier.

0/7

1How vacuum tubes work 2Reading a datasheet, guided 3Passive components 4Biasing methods 5Understanding loadlines 6Distortion analysis 7Measurement units

Start

Design an SET class A

Single-ended triode output stage — from loadlines to transformer matching, in sequence.

0/7

1How vacuum tubes work 2Understanding loadlines 3Biasing methods 4Output transformers 5Single-ended triode (SET)6Power supply & rectification 7Speakers & loads

Start

Restore a vintage amp

Safety first, then construction, then how to tell a good tube from a bad one.

0/5

1High-voltage safety 2Construction techniques 3NOS vs new production 4Tube testing 5Troubleshooting

Start

Jump toBeginner Toolkit7 Fundamentals6 Circuit Topologies7 Power & Output7 Applications5 Practical & Reference8 Advanced Techniques15

Beginner Toolkit

Beginner8 min

Reading a datasheet — guided

You'll decode a datasheet: pinout, curves, limits.

Three datasheets in one : 12AX7 twin triode, EL34 power pentode, GZ34 rectifier. Eight zones on the 12AX7, five on the others. Each opens a tailored interactive widget — pinout, limits slider, Av calculator, Miller, ultra-linear, sag.

→

Beginner3 min