I use a matched set of 5751 NOS RCA from 1959 in my Belles 21A preamp. I went to them from Mullard 12AX7. I found no audible difference in gain with my system. I found the RCA's to provide a little more air and detail between instruments, a little more depth of soundstage and very nice extention and tightening of bass. I got these from Vintage Tubes and have been very happy with the upgrade.
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