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.

Stability, compensation, real loads

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Chapter 8 / 84 min

Stability, compensation, real loads

Phase margin, the C_load pole, R_iso, and how to read a step response.

Every loop has poles. A regulator typically shows three: the dominant pole p₁ from the error-amp anode load × Miller capacitance, a parasitic p₂ from layout and tube interelectrode caps, and p₃ from whatever capacitance you hang on the output node. The sum of their phase lags at the crossover f_c sets the phase margin.

ConceptPhase margin reading

≥ 60° — clean step response, no overshoot, settles in 5 / f_c.
30°–60° — 5–25 % overshoot, visible ringing.
< 30° — heavy ringing, on the edge of oscillation.
≤ 0° — oscillates.

ConceptThe C_load pole is the killer

A 47 µF cap on V_out + an open-loop R_out of 70 Ω puts p₃ at ~48 Hz. Inside the loop band-edge. Phase margin collapses; the regulator rings or sings at startup.

ConceptR_iso to the rescue

Insert a small series resistor R_iso between R_out and C_load. The loop now sees C_eff = C_load × R_out / (R_out + R_iso), which pushes p₃ up out of the loop band. 22–47 Ω usually recovers 30–40° of margin at the cost of 1–2 V of dropout.

WarningDon't trust simulation alone

Phase margin sims ignore the parasitic that'll kill you: lead inductance, layout coupling, heater-cathode coupling capacitance. Always test with a real load step on the bench, scope on V_out, and confirm the overshoot matches the predicted figure within 50 %.

Calc · stability

Open →

Loop stability

Three sliders — A_ol, C_load, R_iso — read out phase margin in degrees, crossover f_c, and predicted step-response overshoot percentage.

Lab · stability-dashboard

Run →

Stability dashboard

Three synced plots: Bode (gain + phase), pole-zero in the complex plane, and step response. Move sliders, watch every view update together.

Check yourself

Your loop has 60° phase margin at 5 kHz crossover. You add a 100 µF output cap with no R_iso. What happens?

Stability, compensation, real loads

Stability, compensation, real loads

Output Transformers

Power Supply & Rectification

Single-Ended Triode (SET)