Typical classic A valve amplifier designed by HIFIFever (300B) was shown below. It is a relative simple and easy to build system. You may able to modify it to mono block system. Both power supply and amplifier was divided into 2 channel(L & R) in order to drive the speaker independently.
Compare the above circuit with the old design. The value of resistors and capacitors are changed due to the power supply (i.e. Dual power in old design). 6 Nos. of 5.1K Ohm resistors are in parallel in order to spread the loads. There is a good example to build a valve amplifier from parts.