Hookup wire for tube amplifier?

Of course 7N copper exists. It is even a subject of international conferences like this one:

http://jp4.journaldephysique.org/index.php?option=com_article&access=standard&Itemid=129&url=/articles/jp4/abs/1995/07/jp4199505C747/jp4199505C747.html

In addition to limits of purity Continuous Casting process produces extremely long crystals (many feet long) by very slow cooling of the copper in hot forms (to prevent crystallization). In comparison common oxygen free copper has thousands of crystals per foot. Impurities resides between crystals. Copper Oxide is a semiconductor.

Kijanki, I don't think you will find 7Ns copper in any wires anytime soon. As soon as its extruded you are down to 99.99%. The wires as seen in the article don't seem to be commonly made.

Further, it might be interesting for audiophiles to understand that OFC was not invented for audio reasons. It was invented to improve wire flexibility, for things like brushes in generators and the like.

Dracule1, we have compared silver and copper many times over the last 30 years. So far no-one has been able to come up with a wire that shows a particular advantage for silver or copper- unless you include the break-in time. During the period of break-in silver seems to have an advantage. This is not to say that all wire is created equal though!

If you are soldering silver wire, the most basic form of solder to use is SN62 which is silver bearing. There are more exotic forms of audiophile solders that seem to work fine too. SN96 works too but you need an 800 degree tip which is a real pain, and despite the SN96 being eutectic, it can still crystallize, which isn't supposed to be good from a sonic point of view. The modern lead-free (RoHS) solders seem to work OK in a pinch. If you are working with copper then SN63 is the preferred solder. It has no silver but retains eutectic behaviours.

Atmasphere some great tips thanks.
Most audiophiles would get more benefit from going to solder school than fussing about copper or silver. I have even heard significant improvements in interconnects from well respected manufacturers that have been re-terminated by solderers who learnt their craft in aviation electronics, and the difference can be illuminating. My advice if you want to go and mod gear, start by learning how to solder properly. Do a basic electronics course at least.
My technician has a different outlook, he sees more dry solder joints than audiophile components in most gear, some of the best looking can have the worst soldering.

Ralph, I believe Cardas draws their ultra pure wire in inert atmosphere (nitrogen I think) to prevent oxidation. I don't know what process is used during drawing for OCC wire. But the oxidation will affect only the outermost layer of the wire, correct? Not the internal portion of the wire.

I agree about having good soldering skills. Some advocate tinning the two wires separately before soldering them together. Others advocate making a good mechanical contact (eg, twisting two wires together) before soldering. Any opinions on this? Any useful links would be appreciated.