The 6550 is the Audio Tube KT88's tube counterpart. Although the KT88 and 6550 are not identical, their similarity in terms of pin layout, physical size, and electrical properties makes them compatible.
The maximal anode dissipation of the two beam power tetrodes is about 35 watts. The KT88, on the other hand, frequently runs at a greater plate voltage and a lower screen voltage than the 6550, leading to slightly different electrical properties and performance.
Practically speaking, both tubes are compatible with the same kinds of amplifiers, and many guitar and high-end audio fans believe that the variations between the tubes are insignificant enough to allow for interchangeability. To be sure you are using the right tube for your particular situation, it is always a good idea to consult the amplifier's manual or a competent technician because there can be some subtle variances between individual tubes.