The 6550 Tube is a powerful beam tetrode tube designed for audio and instrument amplification by the TungSol company in the mid 50’s. The KT88 is a newer, slightly more powerful “kinkless tetrode” version of the same tube. They are related to the 6L6 in design functionality. Great for applications where high headroom is beneficial. Provides maximum clarity and distortion free sound so they are perfect for bass guitar and HiFi amplifiers.