wire and more wire
Hrm. Well, I've only been building my own stuff for about 4 years now, but I have tried a few things. And, hey, it's a discussion forum so here goes:
Magnet Wire
I've used the Doc Bottlehead 20 ga magnet wire and various Belden gauges. Bottlehead wire seems to sound better and it is also a lot easier to remove the insulation - use a hot soldering iron to melt off the insulation and tin the end in the same pass using a '3rd hand' or other clamp. Magnet wire is nice because it is solid core and stays where ya put it, plus it doesn't need insulation (I still like to maintain an air gap between it and anything else for safety). Negatives - gotta get the insulation coating off before soldering; also it is copper and can break if bent repeatedly.
CAT5 Wire
Look for plenum grade (CAT5e and CAT6e) to get Teflon (FEP or PTFE) insulation - note that outer jacket is not Teflon except in the most expensive versions. For most cable manufacturers, getting PTFE or FEP to adhere to bare copper is a pain, hence the number of silver coatings you find in copper/teflon offerings - manufacturers really use the silver to improve the insulation application, not to improve the wire conductivity. But in the case of CAT5 we are talking about seriously large amounts of wire and consequent huge investments in quality/production controls. So with CAT5 the manufacturers can control the dedicated machinery well enough to get good consistency with the coatings. This is also why plenum CAT5 costs so much more than PVC CAT5, so they can amortize the equipment needed - the materials cost is not that different. I know several DIY tube guys that use CAT5e in their amps for all wiring and their stuff sounds great. I've used it for interconnect and speaker cable construction and have been very happy with the results. I think it's the most cost-effective quality wire out there, especially if you can scarf some cutoffs from your friendly neighborhood company network wiring guy (who is constantly rewiring our offices anyway). I ended up buying a 1000 foot spool of Belden CAT5e when a local electronics store went out of business so I have enough to last the rest of my life. Negatives - 26ga solid core only, and each pair has a different twist rate, and you need really sharp wire strippers to get the insulation off of it...
Solid Silver Wire
I've ordered silver wire from Michael Percy and also from Myron Toback (NY jewelry supplier), and it's a heckuva lot less money from Toback, but you have to convert gauge to inch diameter sizes and be sure to order 'Fine Silver' and not Sterling (an alloy). As WildWest said above, silver is just a better conductor, and I can hear more difference in solder joint quality and wire size than silver v. copper. That said, silver does have one giant advantage in wiring - silver oxide is a great conductor, while copper oxide is a semiconductor. This means that as our amps age and oxidation builds (rust never sleeps, etc.) even perfect copper joins start to exhibit small scale semiconductor effects and noise, while silver is... But there just isn't that much wire in a point-to-point tube amp anyway. Plus, silver is *pretty* and it impresses other DIY folks. Negatives - cost, but this is not as high with alternate (jewelry) sources, plus you have to use teflon or other sleeving for insulation in tight areas.
Wire Sizes
This seems to really matter. I use two different gauges of wire in my amps -
Signal wiring - 26 to 30ga wire. Smaller seems better, but it is more difficult to work (Dang! snagged it again). I bought a reel of 18ga teflon sleeving for general use and it is much stiffer than the 30 ga wire - starts to get unruly.
Power and Grounds wiring - 18 to 22ga wire. I have an unwarranted personal misconception that grounds are more important than signal in an amplifier, so I run star grounds, I shield grounds, I run large diameter grounds, and I run silver grounds. So sue me, it seems to work. Power wires should be sized based on the current requirements (hint, even with 845 filaments 18 ga is plenty big).
Hope this might help you, there's a lot more but I need to go work on a project for Blithe now... I'll check back later to see if anyone has questions or flames for me!
Thanks/pRC
Oh yeah, one more thing - the Radio Shack PVC insulated 18 ga copper wire sounds pretty good in power supplies. And the high quality solid-core copper FEP foam video coax works well for power and ground lines if you can find it.