20 Watt power tube amplifier diagram use the tube of EL34, this is mono amplifier, build similar circuit will make it stereo.
The EL34 was introduced in 1953 by Mullard and its parent company Philips and, although no longer made by them, is manufactured by JJ Electronic, Shuguang, Svetlana and Reflector (Sovtek, Electro-Harmonix, Tung-Sol and some brands), amongst others. Some firms make a related tube called an E34L which is rated to require a higher grid bias voltage, but which may be interchangeable in some equipment.
In the electrical sector, a schematic diagram is usually used to describe the design or model of equipment. Schematic diagrams are usually utilized for the maintenance and repair of electronic and electromechanical devices / units. Original schematics were made by hand, using standardized templates or pre-printed adhesive symbols, but nowadays Electrical CAD computer software is often used.
In electronic design automation, until the 1980s schematics were virtually the only formal representation for circuits. More lately, using the progress of computer system technology, other representations were introduced and specialized computer languages were developed, because with the explosive development of the complexity of electronic circuits, classic schematics are getting less practical. As an example, hardware description languages are indispensable for contemporary digital circuit design.