William Blair asks: The HP AC-4 counter modules (each can do a decade and they are occasionally re-wired to do other count sequences) came in several different speed grades, with different component values to accomodate the speed requirements. Different speeds had different DC biases and speedup capacitors and in some cases slightly different logic configs. (Yes, I believe that "speedup capacitor" is indeed the technical term). They use 5963's or 6211's which are computer/industrial-rated 12AU7's (constructed using materials to avoid cathode poisoning if biased off for a long time). They were designed to be true modules and have good interface descriptions. I have used 12AU7 SPICE models floating around the net to play with component selection etc. and the HP schematics are pretty optimal in design. I've built them from scratch on breadboards and they really do work. The HP units are cool because they use neon bulbs as both display decoders and as display elements. Berkeley Nucleonics and others built very similar modules. Some schematics and really good technical manuals on the web for these counters: http://www.hparchive.com/Manuals/HP-AC-4A-4B-Manual.pdf http://www.cs.ubc.ca/~hilpert/e/edte/HP520/index.html http://www.nixiebunny.com/hpac4/index.html Tim.