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.

Adding a VR reference

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Chapter 4 / 85 min

Adding a VR reference

The physics of the cold-cathode VR tube + the cardinal cap rule.

A cold-cathode VR tube is a gas-filled diode with two terminals and one job: clamp the voltage across itself to V_ref. Add it to a single-tube CF regulator and the grid stops drifting with mains — line regulation improves by an order of magnitude.

ConceptStrike, run, hold

When V_raw across the tube reaches V_strike(typically V_ref + 30 V), the gas ionises and a glow discharge appears. The anode-cathode voltage collapses to V_ref and stays there over the operating current band [I_min, I_max]. If current drops below the hold threshold the glow extinguishes and you need to re-strike — which means re-reaching V_strike, not V_ref.

ConceptDynamic resistance r_d

V_ref isn't perfectly flat — it drifts ~2–4 V across the full current band. The slope ΔV / ΔI is the dynamic resistance r_d, typically 80–300 Ω for service VR tubes, 1.5 kΩ for the 5651 precision reference. Stack two VR tubes and r_d adds.

The series resistor R₁ sets I_tube. Size it so the tube stays inside [I_min, I_max] across the worst case of mains and load: low-line + high-load gives the lowest I_tube, high-line + low-load gives the highest.

Cardinal rule

Never put a capacitor > 10 nF directly across a VR tube. At ignition the cap dumps current into the gas, triggering relaxation oscillation (visible flicker, audible buzz, eventual destruction). Filter after the VR tube through a series resistor — never across it.

Calc · r1-ballast

Open →

R1 Ballast Calculator

The two-constraint geometry: R₁,max from low-line + high-load, R₁,min from high-line + low-load. If they cross, no R₁ works — you need to tighten the spec or pick a wider-envelope tube.

Lab · glow-discharge

Run →

Glow discharge — strike, run, hold

Sweep V_raw and watch the VR tube strike at V_strike, run at V_ref, then extinguish below the hold threshold. The interactive I–V curve highlights the three regions.

Check yourself

Your 0A2 (V_ref = 150, V_strike = 185 V) feeds a single-tube CF. V_raw at cold start is 175 V. What happens at power-on?

Adding a VR reference

Adding a VR reference

Output Transformers

Power Supply & Rectification

Single-Ended Triode (SET)