Designing amplifiers is a complex task; while there are very well-documented circuit topologies in the literature, for each implementation there are voltages, gain factors, bias currents and resistances and capacitances to be taken into account.
Deciding on these factors requires an understanding of electrical principles and usually quite a lot of calculation.
In the golden age of valve gear – the 1950s and 60s, there was no software, the calculations needed to be done manually. Like the constipated accountant, circuit engineers needed to work it out with a pencil and paper.
Now we have it easy. Software which allows you to select your topology and simulate it, and giving recommendations for correcting anything that would cause a problem. Like this:
Do you think the program author might be a trekkie?
This is from Tube CAD by glass-ware software. Legacy software that you'll need a Windows XP machine (physical or virtual) to run it on.
There's also PSU Designer 2 for designing power supplies. With valve gear the power supply is usually a complicated mix of capacitors and inductors requiring careful balancing to get multiple different voltages to different parts of the circuit.
Finally there's Circuit Maker for drawing circuit diagrams. Bit like this which is a long-tail pair phase splitter idea I was playing with, something like this might make it into the final design, maybe...