Morgan Jones authored 2 books, "Valve Amplifiers" and "Building Valve Amplifiers". I found them an invaluable source of information while rebuilding my ST70 (my first ever). In fact, after reading the books, I would say the ST70 is a "textbook example" of amplifier design. It's very straight forward in design and an excellent platform to learn from. I took my time, referred to the books and understood every step as I built my amp. For example, the books explain with examples and diagrams the right and wrong way to lay out heater wires. It was like taking a class in "Amplifier design and construction" with lab.
I used to wonder "why did the designer choose THAT value for THAT component", and now I understand. In fact, going step by step with the book as reference I understood the amp so well I didn't need to follow the kit instructions. I was able to check my work against the schematic. It all made sense to me.
That being said, to answer some of your questions, I chose the K40y's by reputation and price. You get a lot of PIO bang for your buck compared to other PIO caps. I always check and match values with a meter before installation, and the K40Y's are very consistent. Even so, I usually order twice what I need to make certain I'll get my matches and to have spares in case of accidents or for other projects. I prefer PIO's because I think they're "less noisy" than the other types. My ST70 may not be as detailed as my other amp (GTA SE40), but I can listen to it all day long, it's just syrupy smooth and seductive. The only problem I had with the PIO's are their size, they're big. They will never fit adjacent to each other on the board and allow you to install the front tube sockets or access the mounting screws.
I also used 600 volt wire for the build purchased from Antique Electronic Supply.
I chose cloth covered wires for that "retro" look and used different colors to simplify circuit tracing.
I chose to use tube rectification rather than solid state for that "tube sound". If you decide to use solid state rectification you will need a delay board to prevent the sudden inrush of current before the heaters have had time to warm up. Solid state may sound "tighter" in the bass than tube, but that will depend on your choice of tube rectifier and capacitance. It's a tube amp that came with tube rectification, so I decided to retain that quality. More explanation about that in the books.
I chose the twist lok cap for the look and simplicity. It's also "right there" at the "star ground".
I use the low gain VTA driver board with a 6189 driver tube and two RCA Cleartop 12au7A phase splitters.
Finally, I personally think the vintage tubes sound better than the new production tubes, especially for the rectifier. For this amp, I'm a Mullard man. Other favorite brand is RCA.
Below are some images of my amp.Take your time and enjoy the build!!
Wow, that is very nice work...I should have sent mine to you!! lol Thank you very much for the input and suggestions, I should have most of the parts and pieces sometime this week. My first order of business is getting the chassis cleaned up and trannies repainted. In my perfect world, I'd like the ST-70 to look mostly stock, with great sound and long lasting when it's powered up. Think COPO Camaro.....a sleeper at the light and you never knew what hit you.