I bought a Univox U-50 PA head a while back (at a steal price) and learned that it was a hybrid; solid state pre with a tube power section.  The amp has two 6L6's plus a 12AX7, which I recently replaced, and has some solid state rectifiers as well.  I was wondering if it was possible to change out the solid state rectifiers in favor of some 5U4-G's.  I've since hooked up the amp, which is 50 watts RMS (more like 40 or so) to a cab, added a Big Muff, and got a really great tone out of it.