The TELEFUNKEN GZ34-TK is an excellent replacement for any amplifier using GZ34 or 5AR4 rectifier tubes.
The GZ34 isn't a very talked about tube, but it is one of the most common rectifier tubes found in audio, especially in guitar amps. Rectifier tubes were some of the very first tubes to be invented since their purpose is one of the first requirements in electronic and audio design.
A rectifier is an electronic device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The rectifier is indeed an essential block that helps create the sound of an amplifier.