🚀 go-pugleaf

I have an old (broken :') tube amp with a pair 6L6GC's for the output tubes.t
These tubes feed an output transformer which in turn drives the speakers.

My questions are these :

I have replaced all the defective parts that I can find. Even replaced
all of the tubes with known good ones. All voltages are correct. Still 
there is a loud screech at the speakers when the amp is powered up. Could
this mean that the output transformer is bad ? How can I test it ? Is there
something else that I am overlooking ?

Dang those output transformers are expensive ! Otherwise I would just 
replace it. Also is there a cheaper place to buy out. trans. ?

I know, alot of questions. How about these simple questions oh great
net wisdom......  :')

Thanks in advance,

Kurt Taylor