Rolled some 12ax7's recently...

Clipper- You'll have to turn your volume control up a bit to get the same SPLs with the 5751, as they have about 30% less gain than 12AX7s. Their "tone" is also a bit more laid back(richer/smoother/warmer-more 2nd order harmonics) than most 12AX7s, if that's what you're looking for.

I disagree that you'll need to compensate for the 5751's slightly less gain; because that difference doesn't really occur until past the normal operating range on the two tubes' curves. Most people can't hear any difference, and the military (who originally requested a 'ruggedized' 12AX7) wouldn't have traded that requirement for a loss in performance when they replaced their 12AX7's.

As for tone, that really depends on the specific tube and to a (lesser) degree on the make of amp. I specify 'amp' because that's where the tube produces the most gain and so the tonal qualities are most obvious. In a preamp, what you are looking for is low noise -- especially in tube phonostages where a 5751 might be a plus.

I don't have tube preamp/phonostages, but the majority of my friends with McIntosh tube preamps (with tube phonostages) seem to have settled with Telefunken AX's and AT's over the stock Chinese tubes, and haven't ventured much beyond that. People with hissy tube phonostages who can't find a quiet 12AX7 and/or 12AT7 should definitely try 3-mica 5751's and/or 6201's (and tell us what happens please ;-)
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Then you disagree, not with me, but the specifications of the tube itself: (http://www.boiaudioworks.com/5751) (http://www.tubedepot.com/5751.html) paragraph 5 of this page:(http://www.guitaramplifierblueprinting.com/files/Phaseinverter.pdf) (http://www.guitaramplifierblueprinting.com/Tube/5751.pdf) (http://www.guitaramplifierblueprinting.com/files/GE-12AX7.pdf) compare the plate dissipation/transconductance/plate resistance of the 12AX7 and 5751. They are different because the 5751 in not a 12AX7, and has less gain as a result of it's design.

I appreciate the input guys.

So it sounds like the 5751s wouldn't really be necessary for me then? I am just trying to figure out what tubes I should try in my Fisher. I am really curious to experiment and see if I can get a better sound than the JJs are making right now.

Thanks
-Clipper

I am using a Fisher 400 pre-amp now with Bugle Boy 12ax7's now and am very, very happy with them. This is the pre-amp I was using when I exchanged the JJ's for the BB's recently. With the JJ's I honestly thought I needed a sub-woofer, and when I changed to BB's I realized I didn't. The sound stayed crystal clear, the highs lost the edginess that I heard with the JJ's, the mid-range blossomed into sweetness, and the bass filled out the missing void.

I would recommend Bugle Boy's. The Orange Globe labeled Amperex tubes sound tempting to me. Telefunken's are definitely a step up from JJ's, but I prefer the BB's. The Tele's are very good at everything but miss the magic of the BB's.

For less money, I bet almost all NOS 12ax7's will beat out current production jobs. I have been satisfied with RCA and Sylvania NOS 12ax7's over the JJ's.

Give the tubes time to break-in before jumping to a conclusion. The JJ's became sweeter over time, but in my opinion could not stand up to the NOS tubes that I have.

Happy listening after your search!