12AX7 vs. 5751

5751s will work fine on a 12ax7 line stage.
OTOH, on a phono stage, you need all of the 12ax7 gain.
Good luck

Typically you can use 5751s where 12ax7s are called for. The 5751s having less gain just gives your volume control a little more flexibility, ie you can turn your volume control without a little adjustment giving you a big volume increase as 12ax7s give in some applications. With my preamp there is a noticable difference in gain between these two tubes. 5751s were speced for the military and are very rugged. They also sound very nice, IMHO. I too like their air and detail and I would add bass response. Just to make sure, send altavista an email, they are the experts on Counterpoint equipment.

There are no guarentees it will sound better with a 5751 set ...Each peice of equipment is a little different,so the only way to know is to try..I tried and like the RCA 5751 black plate ( circa 1962 ). Not much gain loss and gorgeous mids with good bass..Didn't like the Mullard ( to dark for my set up ) .I did however think the Tungsram 12ax7 ( Hungary ) is one of the best 12ax7 nos tubes I have ever tried...( available at Tubemonger.com )To me if I were going to try rolling I would suggest try either one of those tubes mentioned. Both great but slightly different..And as mentioned by Davt ,the Mullards work great for some also..Its a crapshoot!

I have used both A LOT and never noticed the gain difference in 5751s. But they sure sound different. The right tubes will depend on your tastes and equipment.

Arthur

I have an additional question.

Can a 12ax7 be used in place of a 5751?

the preamp is speced for a 5751 in the line stage.