How to test KT33C with Knight 600 Tube Tester ?

Try doing a Google search for the tube tester you have. Narrow the search results by clicking on images. Sometimes people post documents, like owners manuals, in picture format. You may get lucky and find what you are looking for.

According to an older Sams Tube Substitution Handbook I have, the 25A6 can be substituted for the KT33. According to references on the web the KT33C is identical to the KT33 except that the KT33C provides a filament center-tap connection on pin 1, while pin 1 is unconnected on the KT33.

Therefore I suspect that settings for the 25A6 will work for the KT33C. I don't guarantee that, however :-)

Regards,
-- Al

Have you reached out to tube vendors Jim McShane, Brent Jesse or even Kevin at UpScale Audio? They may be able to help you with your request. When I saw KT33C I thought you made a typo for 6C33C tubes.