Internal Wire Gauge?

The wiring should be UL1015 listed

That's really specific and unnecessary. UL1015 is 600V  PVC shielded appliance wiring material. It's in an enclosure, there is often no need for shielding at all, and it would be detrimental to the build in many locations.  Where shielding is required, there are any number of UL style wires that would be suitable depending on where used, how supported, etc.

I'm not saying you should not use premium wiring, however, for price consideration:

If/when you take the bottoms off vintage equipment or backs off vintage speakers, you will be amazed how small and common the wires were/are.

Keep in mind, this is what made those vintage audio companies famous.

@greg7 - I have designed and tested many DIY cables and the following are the wires I have found to excel when used for the signal/live wires in all my cables

One of the things that can hinder the performance of wire is the insulation

• insulation acts like the dielectric in a capacitor and causes noise to be generated in the signal
• The Dielectric Constant (Dk) of the insulation is a guide to the "level of impact"
• the Higher the number the more noise is created
• Dk of a vacuum is 1.0 and of Air is around 1.1

Neotech UP-OCC copper with Teflon insulation (Dk=2.2) is an extremely good wire
VH Audio UP-OCC copper with AirLok insulation (Dk=1.45), is also extremely good, but for some reason I preferred the Neotech, while others preferred VH Audio
VH Audio also has a UP-OCC solid silver wire with AirLok insulation (Dk=1.45) that is about as good as you can get

Solid wire is better than a stranded wire

I avoid the use of silver or gold plated wire for the signal wires - plating seems to impacts sound quality

Mundorf has a Silver with 1% gold wire that lots of people like

• I preferred the Neotech and VH Audio wires.
• They were more dynamic and provided better imaging and clarity

Duelund Tinned Copper in Cotton/oil insulation (DK is around 1.3-1.4) is often referenced on the forum,

• and some people prefer it to other wires,
• but I found is was not as dynamic as the other wires above, but it is a good wire

I hasten to add when you get into this level of wire quality - ALL the wires I have mentioned above provide exceptional sound quality - it could be a matter of personal preference

As a previous posts above mentioned, the best insulation solution is to use Bare wire in a slightly larger Teflon tube (Dk is very close to 1.1) ,

• I now use this approach on all my cables
• it is about as good as you can achieve on planet earth :-)

WRT the neutral wires in my cables - I use a stranded silver plated copper Mil-Spec in Teflon wire

• I have tried several better wires for this duty,
• but strangely, I am yet to find a wire that outperforms the Mil-spec for the Neutral

So does using premium wire really make a difference?

• I believe that depends on YOUR ears and the rest of your system components
• I can hear the difference in my cables between all of the wires mentioned above regardless of whether they are Interconnect, speaker of power cables, so I would have to say YES!

As for which gauge - that depends on the power requirements of the circuit, but it seems from the posts above 18 gauge will suffice and bearing in mind you are dealing with high voltage inside a tube amp, then the current in play would probably be quite low. But I would recommend you check this out thoroughly before proceeding

Regards - Steve

@williewonka Great read, thanks for the detailed post. 

And glad to see we're hearing similarly.  Always good to find another person who hears what you do so that you can pay more attention to their comments and recommendations.

And yet $100,000 CD players (and amps) are built on PCBs using generic copper, typically FR-4 substrates, though I have seen the odd PTFE, though their is little justification for it.

PTFE (Teflon like) insulated wire does not need to cost an arm and a leg, it is common for industrial and military uses and costs a fraction of "audio" wire..

Then again, using PTFE for hookup wire is pretty silly unless you are running twisted pairs (or similar).  Individual wire runs don't benefit for a low dielectric constant. You need two conductors to have a capacitor, so if one wire is 10mm away from another wire, and you have 0.1mm Teflon insulation, the air is still by far the dominant dielectric.

If you are selling things and want to impress people who don't know any better, sure use expensive wire that will make no difference. However, if you are building your own stuff, every dollar you don't waste on wire can be used in places where it will actually make a difference.