The El-34 (6CS7) began life in the mid-fifties as being the highest power pentode. It was designed for use in Walt Disney "Fantasia" film. It trickled down to consumer use and was sold to sucker "hi-fi" enthusiasts in the late 50's who could then boast to own a 50 watt amplifier
(compared to other amps at the time which had only 25 watts). The problem with this pentode is that they need much higher voltages and more elaborate circuitry than other output tubes (6l6, 7591 at 325 volts). At over 450 volts this caused the chassis to get very hot and all the componets to change values and become introduce noise. I owned a Marantz 8B that had a dull and lifeless sound due to the 6SC7's, it was noisy and had a compressed sound, unsable with horns. I have a pair of pro Altec amps that also use these tubes and the sound is the same as the Marantz. I have yet to hear any El-34 amp that sounds good and enjoyable. The El-34 should be classified with the 6550 as being higher powered but "non-hi-fidelity"
