Re: Picked up a Bogen Challenger 100
^ Thats one of the quintessenital "DIY" old PA tube amps for modification. There is a wealth of information on making those amps into guitars amps, in particular the CHB-100. They do have the expensive 7868 tubes though.
I had one that had all the tricks in the book done to it. A nice amp, to be sure, but nothing I'd call earthshattering, however needs a LOT of manhours of modifications (the parts cost shouldn't be a deterrent )- in that I couldn't disagree more with The Star- not to deter you, just so you brace yourself;
I don't have any links, sorry, mine was custom built for me, and I'm not a tech.