6L6 and KT66 can be interchangeable in certain situations (depending on the amplifier you are using). Our Horizon series also includes the 6L6WGC-AT.
The bias voltage requirements for KT66 and 6L6 are different. When interchanging them, it is necessary to readjust the bias voltage to ensure the tubes operate within the correct range. If the bias voltage is incorrect, it may cause the tubes to overheat or distort.