We just got a very cool Webcore 4903 outboard amplified speaker in for repair and upgrade and lucky for us the folks at Webcore put a schematic inside the unit to make repairs a snap. this is a simple amp that has very little gain. It uses two el84 tubes and one 12ax7 tube.
I converted it to be used as a guitar amp, by swapping out the RCA input to a switched 1/4 inch jack and I put a 1m resistor to ground to help control oscillation, other than that I just replaced all the leaky caps…. and it sounds great, I played my Takamine acoustic through it and wow it sounded sweet. the only drawback is volume since there is only 1/2 of a 12ax7 for pre-amp gain it was not very loud.
here is the schematic (click to enlarge)