These are called single-ended amplifiers and usually consist of a single preamplifier tube and a single output tube. The most common design is to use 12AX7 for the preamplifier and EL84 for the mains.
Most often, the basic circuitry used is that of the Finnish Cream Sound, Bad Cat, Gibson Skylark or many other circuits that abound in the world. Naturally, the couplings are very similar, as the 2-tube design doesn't leave much room for variations. Cream Sound is very much like Fender Champ, but implemented on EL84.
