WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
Single ended really isn't the best choice for OTL. You have to make compromises which really outweigh any benefit the SE would otherwise give you. At least consider a class A push pull. You could get 5wrms out of a pair of 6c33c's class A