Tube rolling 12AX7As

Newbee and Lowrider are correct. The C2300 is a wonderful unit out of the box and you'll want to know how it matches with everything else in your system before fooling around with changing its sound. Took me about 6 months with mine until I had a good handle on it all.

The 12AX7 and 12AX7A is essentially the same tube. In the 12AX7, there are variants used for their quietness, ruggedness and even broadcast and military versions. They even have balanced versions and general upgraded variants.

You'll want to familiarize yourself a bit before shelling out serious money for a NOS Valvo or Telefunken tube. The label often doesn't indicate the tube's origins when dealing with the NOS stuff. I've seen Valvo tubes that were actually Mullards and Amperex tubes that were out of Blackburn as well. There is a way to tell, but don't trust anyone but the most respected sellers until you figure it out if you get the itch to try expensive NOS stuff. North American tubes have similar problems, but they don't usually bring such a premium as the Tele's and Valvos.

As for the burn-in time, I've found that tubes need very little to reach their optimum sound...like less than 20 hours. They will also sound better after being heated for 30 minutes or so.

For what it's worth, if you don't need all the gain of the 12AX7, the lower gain 5751 can really impress and all that I have ever seen are very rugged with closely balanced sections. You wouldn't want to use it in the phono section, but the preamp might like it. There is just way too much gear that uses far too much gain in their designs. Whether or not your Mac falls in that catagory, I don't know.

Congrats on the Mac, btw.

Telefunken 12ax7 ribbed plates. Deep pocket, Telefunken ecc803s.

The Giant Killing Psvane 12AX7T-II tubes are best bought from a retailer (Canada?) as direct China dealers can be annoying. A tip.

I like the JJ 803S