Useful Links.
AUDIO tutorials, questions and documentation on sound recording, studio technology, recording technology and sound engineering by the late Eberhard Sengpiel. The site is managed by his son Alexander Sengpiel. HUNDREDS of VERY USEFUL Audio and other calculators with explanations in German & English. Lots of Vox amplifier, guitar and organ information. Very useful.
Classic Audio Products, Inc (capi-gear). API info, clone parts etc. If you are interested in API gear, take a look.

3D Audio Bulletin Board: All about caps, not just for audio:

TECBRIDGE CIRCUITS. QUALITY PCB 's at the lowest possible cost - Single Sided - PTH - Multilayer.
SUGRU. Wicked glue that sets like rubber. Many different uses and colours. Great for fixing and making things.
THERMOMORPH. Mouldable plastic pellets. Heat to 60 deg C in water to use. Sets very hard.

LIBRE OFFICE. FREE Alternative to Microsoft Office BUT MUCH BETTER.
VISUAL ANALYSER. FREE software analyser for PC.
Oscilloscope, Spectrum Analyzer, Wave-form generator, Frequency meter, Volt meter, Filtering
ELECTRONICS ASSISTANT. Free Software & many useful calculators etc.
EVERY CIRCUIT. Create, simulate, share, and explore electronic circuits! For Apple, Android & Chrome.