Question 1

What is a vacuum tube?

Accepted Answer

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.

Question 2

What is the difference between a triode and a pentode?

Accepted Answer

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.

Question 3

How do I bias a vacuum tube?

Accepted Answer

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.

Question 4

What is a loadline?

Accepted Answer

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.

Question 5

Are vacuum tubes still made today?

Accepted Answer

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.

Question 6

Is working with vacuum tube amplifiers dangerous?

Accepted Answer

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.

Nom	Famille	V_ref	V_strike	I_min	I_max	r_d	Culot
0A2 (6626)	Service	150 V	185 V	5 mA	30 mA	150 Ω	B7G
0B2 (6074)	Service	108 V	133 V	5 mA	30 mA	100 Ω	B7G
0A3 / VR75	Service	75 V	105 V	5 mA	40 mA	100 Ω	Octal
0B3 / VR90	Service	90 V	125 V	10 mA	30 mA	270 Ω	Octal
0C3 / VR105	Service	105 V	133 V	5 mA	40 mA	100 Ω	Octal
0D3 / VR150	Service	150 V	185 V	5 mA	40 mA	100 Ω	Octal
85A2 / 0G3 (Philips)	Précision	85 V	125 V	1 mA	10 mA	300 Ω	B7G
5651 / 5651A	Précision	87 V	115 V	1.5 mA	3.5 mA	1500 Ω	B7G
5783WA	Précision	86 V	120 V	1.5 mA	3.5 mA	1400 Ω	Wire-lead
NE-2 / NE-2H	Indicateur	65 V	90 V	0.5 mA	2 mA	8000 Ω	Wire
СГ-1П (SG-1P)	Service	150 V	180 V	5 mA	30 mA	250 Ω	B7G
СГ-2П (SG-2P)	Service	108 V	140 V	5 mA	30 mA	200 Ω	B7G
СГ-3С (SG-3S)	Service	105 V	130 V	5 mA	40 mA	200 Ω	Octal
СГ-4С (SG-4S)	Service	150 V	185 V	5 mA	35 mA	250 Ω	Octal

Tubes VR

Output Transformers

Power Supply & Rectification

Single-Ended Triode (SET)