Internal Wire Gauge?

2.0 welding cable should be fine. Make sure it's grounded well.
Pure silver multi strand, I figure about 25k for cable, 500.00 valve amp..
LOL

Reality,

20 -16 unless it's a PS. The small wire is a must for dynamics.. I tried a few times.. I like solid core copper and silver in certain places. I like cloth wire covers, cotton, silk, some had asbestos in the blend..

So you gonna build, or REbuild, or just want to know?

It's not big wire, BUT DIY usually go to big.. Great solder, short paths, twist the right wires for noise... Quiet as a mouse with good valves and quiet transformers..

Regards

Agreed with @oldhvymec on the use of solid core. 

Stranded blurs the sound in my experience.  I had a preamp and took Duelund's tinned-copper in oiled cotton, compared one input with the original 16ga wire to another with a single strand of the tinned-copper wire in an oversized teflon tube.  The single strand was superior in every way.  

That said, I personally prefer solid core silver.  I use VH Audio's 99.999% pure silver in cotton for the signal path of all my electronics.  28ga, so I'm even smaller than 'ol mec.  https://www.vhaudio.com/wire.html

When I'm going out to the speakers from the amplifier boards, I use 14ga solid silver from tempo electric (http://www.tempoelectric.com/cables_speaker-cables.htm).  You've got real current at that point, so larger gauge wire is better.

Based upon advice from either John Curl, of John Risch (forget which) I ended up using 18 or 20g solid core (SC) for the ground runs/busbar in a vintage pilot 240 (the 26g SC that I first tried was noisy).

I used 26g SC (47 Labs OTA) for the signal paths and it worked well.

I left the original stranded power wire intact, so don't have an opinion on that.

This said, there was guy advocating Huge gauge stranded wire off the OT's in DH/SET amps (maybe 8g-10g), so my guess is that it depends upon the specific amp (PP with gobs of feedback VS DH/SET with none).

So...

You have ground/power/signal paths in the mix to figure out what seems best (for each) for your specific amp.

DeKay

Addendum:

One of the John's is a Jon (sorry forget which, but both have given me sage advice in forums over the years - which was much appreciated).

DeKay

18 gauge for the power wiring to and from the mains xfmr and 20 gauge for the signal wiring to and from the output transformer. The wiring should be UL1015 listed (600 volts and 13 amps for #20 and 18 amps for #18 at 40 degrees C)