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.