That's right Dave, plywood top and enough internal bracing to support a motorway fly-over doesn't do a lot for the sound projection but you can beat the crap out of them for thirty years and it will sound and play the same as the day it was made!
I always used mine with a clip on pick-up but the quality of Seventies technology left a lot to be desired and I found I had to tape over a couple of the pick-ups that were too sensitive and add a few extra lashings of tape to keep the thing in place!
It's testament to the manufacturers that so many of them are still in regular use.