Among the many web sites I visited by fall 2019, here are some which I found the tube sections particularly serious:
The Valve Wizard/ is maintained by someone highly knowledgeable and technically professional; you find there excellent guidelines to bias a stage, to design your power supply, and a unique and must-have guide to correctly route and ground an amp, something critical for minimising hum and noises; additionaly, its sells cleverly designed PCBs and a full book “Designing Tube Preamps for Guitar and Bass”: I purchased it (Jan 2020) after the excellent comments on the web and I confirm it is a must for anyone interested in designing with tubes!
Lenard Audio Institute A very well documented place, covering topics far beyond tubes, and written by someone who does not mince words… Many concepts and advices sustained by a sufficient theoretical background; this site is very complete about sound topics.
Hosenlander triode common-cathode amplifier calculator provides an elaborated DC and AC calculator for triode gain stage. Only the ECC83 is modelised, but it covers all important aspects to simulate and design a gain stage.
Elliott Sound Products is actually a large book about the art of designing amplifier, be they with valves (tubes) or semiconductors. The author has accumulated an astonishing amount of practice and experiments.
Norman Koren Vacuum tube audio pages the father of the mathematical model used by my solvers.
Duncans’ Amp Pages a lot of stuff, including Spice models