Photo by Tom Moberly.
- NOS electrical specs (RCA Receiving Tube Manual)
- Classification by construction: “beam power tube”
- Filament volts: 6.3 V
- Filament current: 1.6 A
- Max plate volts and watts: 600 V, 35 W
- Max screen volts and watts: 400 V, 6 W
Introduced in the mid 1950s and made in the United States, the 6550 met the demand in hi-fi audio for greater power and less distortion. Ampeg used six 6550s in their SVT (“Super Vacuum Tube”) amp in the early 1970s, creating an extremely clean, round, and warm sound that would become a classic for bass players.
The 6550 is not commonly used in guitar amps, but in the 1970s the US distributor for Marshall sold many JMP guitar amps with 6550s instead of EL34s. (Supposedly, many EL34s were faulty by the time they arrived in the US, and the distributor concluded that the rugged 6550 had a better chance of surviving the duration of the warranty.)
Ampeg used six 6550s in their SVT amp in the early 1970s, creating an extremely clean, round, and warm sound that would become a classic for bass players. Photo courtesy of Chicago Music Exchange.
The KT88 is a common substitute for the 6550. There’s not a dramatic difference in tone between the two types. Used with guitar, the two tube types sound tight, bright, and powerful, with a slight overdrive crunch and relatively little compression compared to other tubes. They could be useful for maintaining clean sounds at high volume, or keeping the emphasis on preamp distortion instead of power-amp distortion.