How do I find the right output transformer for my tube amplifier?

First, please clarify whether it is a single-ended or push-pull output stage. After that, the primary impedance, power rating or the power tube type must be known in order to make the correct selection:

German category	Category
Output transformers for push-pull tube amplifiers PP	Audio PP transformers
Output transformers for single-ended Class-A tube amplifiers SE	Audio SE transformers
Output transformers for tube guitar and bass amps	Guitar amp output transformers