Some manual bias amps have enough range on the bias pot that they can use KT88 and KT66. A good example is an amp like the Quicksilver Mid Mono it can use EL34, KT - 66 , 77, 88, 6l6 and a few other EL 34 / 6L6 variants.
Always check with manufacturer! Early versions can only use KT88. New versions can use KT120 & KT 150 , running changes over the years allow for this.
The newer version has more robust transformers, and other slight running changes Perfect example of how you can damage your amp if you don't use the tube it shipped with or was compatible with at time of build.
