Tube subtitutions 12AT7 for 12AX7

You might want to look into 5751 tubes instead which offer less gain than the 12ax7. Never heard of subbing a 12at7. Run the 5751 by AR.

TJ: ECC88/6dj8 is not a sub for a 12ax7. What model/version of the SP6 do you own and is it in stock form? Earlier versions used 12ax7's and later ones may have used both ECC88's and 12ax7s (can't remember for sure, but think that ECC88's might have been used in the phono section on some later versions).

Anyway, first start by using the proper tubes that it was designed for and/or if you still require less gain for a 12ax7 position then check with AR to see if a 5751 is OK.

There is no direct sub for the ECC88 family except perhaps a PCC88/7dj8 (you would not want this unless the amp runs the tubes very hard).

The ECC88/6dj8 family consists of ECC88/6dj8, E88CC/6922, 6h23 (Russian E88CC/6922 type) and E188CC/7308 tubes. Again check with AR in regard to using the three latter types, just in case that there is a design quirk that prefers ECC88/6dj8 tubes (can't imagine this to be the case as they are all so close in spec, but better safe than sorry).

TJ: ECC83 is the same as 12ax7, so again this leads to checking if 5751's will work. This should lower the gain, and hopefully you will prefer their sound.

Who knows, perhaps the gain will end up being user adjustable when you check with AR?

I would also run this through the Audioasylum forums as someone there may have already gone through this issue with an SP6.