4x2n3055 not a problem. I have made circuit and pcb for 3 years only with permanent marker and paint, I made pcb for microcontrollers circuit (for diy soldering station) without any softwares, not difficult but very simple and save money.
OK Nikolai, I will investigate 20W and 40W class A amplifiers using 2N3055s.
The most critical area with class A semiconductor amplifiers is dissipating the power in the output transistors, the 40W version in particular. With the 20W version, you could build a stereo pair plus power supply in one case, but for the 40W version a separate case for the power supply and each amp would be best.
Here are some points:
(1) Constraints
(1.1) The junction temperature of the output power transistors (2N3055) must not exceed 200 deg C.
(1.2) Every output power transistors must be kept within its data sheet Safe Operating Area (SOA)
(1.3) The connections from the driver circuit to the output power transistors must be short. Ideally, the transistor leads should solder straight on to the PCB. This may seem at odds with the other requirements covered here but it is not, and can be done: more on this further down the line
(2) Implementation
(2.1) Use an insulator between the transistor case and heatsink with the lowest possible thermal resistance. Without going to esoteric and expensive insulators, mica is the best. Also ensure that that each power transistor is bolted down firmly with the biggest bolts that will fit the holes in the transistor case. A very thin layer of the best thermal grease will also be required. The heatsink under the insulating washer must be perfectly flat. As a side note, a commercial UK audio amp had problems with output transistors blowing. It was discoverd that the heatsink machining stage had been missed out!
(2.2) Use a heatsink with the lowest possible thermal resistance, heatsink to air. This normally means big and expensive. But you can make a heatsink at minimal cost.
(2.3) Use a separate heatsink for each power transistor.
(2.4) Use the mono-block approach where each amplifier is in its own case.
(3) Making a heatsink
(3.1) Get some scrap copper or aluminium and fabricate a heatsink. The metal will have to be thick in the area where the transistor bolts down, say 10mm min. Scrap yards, in my area anyway, are full of suitable aluminium scrap. Also garages throw out aluminium castings and the like that would be ideal.
(3.2) Use fans to cool the heatsinks. These can be salvaged from scrap PCs. Fan cooling a heatsink can radically lower the thermal resistance, heatsink to air.
(3.3) Use liquid cooling, as is used on some high-power PCs to cool the processor and graphics chips. Liquid cooling may sound crazy but it isn't that hard to do and would provide the ultimate in thermal dissipation.
(4) Power Supply
The power supply will also be big, but should not be too critical to design. You will need some very high value reservoir capacitors; many in parallel would be fine. I imagine at least 100,000 uF will be needed. 4 x 20A upwards rectifiers, or bridge will also be needed. You have mentioned the mains transformer before, and plan to use one from a microwave. That should be fine. Almost certainly you will have to replace the secondary winding to suit the amplifier. The type of core etc is not relevant to the electrical performance or the quality of the audio amp, especially in the case of class A. When considering the last knocking in amplifier design the type of transformer may be significant, but it is a moot point. As long as the transformer provides enough current at the right volts, all will be well.
All this is for your considerations. The position will be much clearer when the amplifier designs are complete and I have done the thermal calculations.
One final point: you will need a thermometer to measure up to around 150 deg C. High accuracy is not needed but the sensing probe will need to be small enough to measure the temperature of the transistor base and various points on the heatsink.
I didn't know your skill level, but as you now say that you have made PCBs and built electronic units before, there is no reason why you shouldn't have a successful class A amplifier project, but think big amplifier and power supply!