I'll say that my amp is nothing compared to a late 50's Plexi or a Metropolis remake but, it still cranks out the tones of a great tube amp when connected to a decent tube preamp pedal, not that the religious ones would bless this. 

The one thing that I've been hearing from a few tech friends of mine is that I can substitute the overly clean sound with a tube based pedal that will act, somehow, as a preamp of sorts.  The amp itself is amazingly loud, considering that it's a 1971 PA head, and I have been able to get a really great tone from the Big Muff that I've put in line.  My question is that considering that Univox was a bit sloppy in their amp construction, as far as leads and circuit soldering and attachment, is that should I invest in rebuilding this amp for use as a guitar enabled and tube driven goliath, or should I sell it and buy something that everyone has?

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.