KT88 / 6550
The KT88 can be used anywhere a 6550 can. The KT88 has a higher screen voltage rating, meaning it is a heavier duty tube. The KT88 is also more rugged. These reasons make the KT88 an excellent choice for use in bass amps in place of 6550s, where the tubes are exposed to lots of vibration and plate voltages of 700 or more. We do not recommend using a 6550 in place of a KT88.