The 6550 Beam Power Tetrode: The Black Sheep Vacuum Tube?

Despite its impressive looks and ubiquity in the hi-fi and electric guitar world, is the 6550 beam power tetrode the “black sheep” of vacuum tubes in terms of tonality?

By: Ringo Bones

Ever since 1990s era vacuum tube high fidelity audio amplifier manufacturers began designing their amps that make the output tubes visible from the outside, the 6550 beam power tetrode vacuum tube gained fame to the curious and uninitiated due to its impressive size (they are bigger vacuum tubes but they became a lot scarcer when the 1990s came). But long time vacuum tube hi-fi enthusiasts have doubts about its tonality when compared side-by-side with its more “gorgeous-sounding” counterparts – i.e. the EL34 and KT66 just to name a few – and does this mean that the 6550 is the “black sheep” of the power output vacuum tubes? But first, here’s a story on how this now “iconic and venerable” tube came to as we now know it today.

The 6550 high power beam tetrode vacuum tube was originally developed by Tung-Sol in 1955 originally for use as a servo amplifier. Given the vacuum tube’s “high-tech” origins, its probably the reason why U.S. Navy researcher named Donald Leslie used the 6550 as a cost effective power output tube for the power amplifier section of his now venerable Leslie Model 145 Organ Speaker to make it more affordable for a lot of people after he developed it back in the 1940s. The late, great guitar god Jimi Hendrix used the Leslie Model 145 Organ Speaker when he recorded the studio version of his now classic Little Wing. Thanks to Donald Leslie, the 6550 beam power tetrode gained a second life as an audio amplifier output tube instead of just a part of a servo motor driver controlling those lifts that send planes from the bottom to the flight deck of aircraft carriers – or controlling the flaps of the XB-70 Valkyrie as it flies at three times the speed of sound at altitudes of 70,000 feet. 

Based on the 6L6 beam power tetrode, the 6550 does appear to be a bigger-sized version of the 6L6 only that the 6550 is designed to have more power and stability than its predecessor. The 6550 operates with a plate voltage of 600-VDC and a screen voltage 400-VDC and a plate dissipation of 35-watts. The KT88, KT90 and 6550 although not identical are often interchangeable, dependent on external circuit parameters of course. The 6550’s glass envelope was originally wider in the middle than at the top and bottom, but a straight-sided design was later introduced by General Electric and Philips. 

Despite its widespread availability via U.S. Department of Defense contractors, the 6550 has been remanufactured for quite awhile now by Electro-Harmonix. Used in the 1978 Marshall JTM 50-watt combo amp, its tonality resembles that of a well-designed solid-state guitar amp. In a Dynaco Stereo 70 type amplifier that accepts EL34, KT77, KT90 vacuum tubes – the 6550 vacuum tubes has a lively, articulate sound with a silvery overtone reminiscent of 1990s era low feedback solid-state hi-fi amps – i.e. those 3,000 US dollar French solid-state integrated amps that are raved by hi-fi audio gear reviewers to be sounding like a powerful push-pull triode connected amplifier. Hence the “black sheep” label for this “servo amplifier vacuum tube”.