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.

Why regulate, and what to measure

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Chapter 1 / 84 min

Why regulate, and what to measure

Five figures of merit + a decision tree for picking an architecture.

A regulated B+ rail isn't a sound quality cult — it's a specification. Five numbers tell you whether your supply does the job. The right topology depends on which numbers matter for the load downstream.

ConceptFive figures of merit

Load regulation: ΔV_out / ΔI_load — output stiffness when the load draws current.
Line regulation: ΔV_out / ΔV_mains — how much the rail follows mains drift.
Ripple attenuation: 800 mV pk-pk raw → how many µV residual at 100 / 120 Hz.
Output impedance Z_out(f): flat across 20 Hz–20 kHz is the audio goal.
Drop-out: minimum V_raw − V_out before the regulator loses control (≈ 25 V for tubes).

These five aren't independent. A bigger error amp µ lowers Z_outand improves ripple atten by the same factor (1 + T). A higher V_ref/ V_out ratio gives more loop gain. A choke costs dropout but kills ripple.

ConceptThe decision tree, in one paragraph

Guitar power: no regulation, sag is part of the sound. Stable mains and modest needs: 1 tube CF (Module 03). Want ±10 % mains immunity: add a VR reference (Module 04). Want Z_out < 2 Ω and ripple < 1 mV: add an error amp (Module 05). Phono / mic / lab supply at measurement floor: multi-tube cascade or feed-forward(Module 06). Low current with possible current-sinking: shunt(Module 07).

Note

Most tube regulators in DIY literature optimise the wrong number. People chase ripple atten while accepting awful Z_out(f) above 1 kHz. The studio's bench measures all five so the trade-off is visible.

Calc · autodesigner

Open →

Autodesigner

Pick a use case + V_out / I_out / ripple budget, and the autodesigner proposes the simplest topology that meets the spec.

Check yourself

Your phono stage needs 250 V at 30 mA with ripple below 100 µV pk-pk. Which topology fits the bill at minimum complexity?

Why regulate, and what to measure

Why regulate, and what to measure

Output Transformers

Power Supply & Rectification

Single-Ended Triode (SET)