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.

Name	Mode	Va max	P_diss	Ia max	gm	µ	rp	Heater	Base
EL509 / 6KG6A	pentode	900 V	35 / 42 W	500 mA	18	—	—	6.3 V / 1.9 A	Magnoval
6080 / 6080WA	triode ×2	275 V	13 W	125 mA	7	2	280	6.3 V / 2.5 A	Octal
6AS7G / 6AS7GA	triode ×2	275 V	13 W	125 mA	7	2	280	6.3 V / 2.5 A	Octal
6336A / 6336B	triode ×2	400 V	30 W	400 mA	18	2.7	150	6.3 V / 5 A	Octal
6C19P (6С19П)	triode	300 V	11 W	100 mA	6.4	2.5	390	6.3 V / 1 A	B9A
6C33C-B (6С33С)	triode	600 V	60 W	600 mA	42	2.7	80	12.6 V / 6.6 A	Octal-large
EL34 (triode-conn)	tri-conn	800 V	25 W	150 mA	11	10.5	950	6.3 V / 1.5 A	Octal
6L6GC (triode-conn)	tri-conn	500 V	30 W	175 mA	6	8	1300	6.3 V / 0.9 A	Octal
KT88 (triode-conn)	tri-conn	800 V	42 W	200 mA	11.5	8	700	6.3 V / 1.6 A	Octal
EL84 (triode-conn)	tri-conn	300 V	12 W	50 mA	11	19.5	1700	6.3 V / 0.76 A	B9A
5687	triode ×2	330 V	4.2 W	35 mA	12	17	1400	6.3 V / 0.9 A	B9A

Pass tubes

Output Transformers

Power Supply & Rectification

Single-Ended Triode (SET)