History and uses
The 6550 was developed by Tung-Sol in 1955  originally for use as a servo amplifier. It was used in audio equipment before semiconductor power amplifiers came into use, and continues in production today. It is often found in valve-driven bass amplifiers such as Marshall and Ampeg guitar amplifiers and the Leslie speaker system.
Based on the 6L6 vacuum tube, the 6550 was designed to have higher output power and better stability. It operates with a plate voltage of 600VDC, a screen voltage of 400VDC, and plate dissipation of 35 watts. The KT88, KT90, and 6550, although not identical, are often interchangeable, dependent on external circuit parameters. 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 GE and Philips.
- http://vintagetubeservices.com/page16.html Tung Sol, Vintage Tube Service
- Jac Music History of KT88, KT66, and 6550
- Boi Audio Works
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