Well I actually tried a pair of 5751 in the 1st 12AX7 slots. It sounds great - this is an improvement over the Russian Tung-Sol gold pin 12AX7 I had in there, for sure. Sweeter midrange, a little more 3D - as is often the case going from Russian to good vintage tubes. Excellent clarity. It did NOT shift the tonal balance to darker. It’s just nicer, overall. It did lose a dB or 2 - that’s noticeable and easy to adjust for.
These are just RCA TM BP 5751, since I have very limited tubes available here where my "B" system is located. Also the "B" system is something I just recently put together, haha. So at some point I’ll bring over a bag of vintage 12AX7, 5751, and 12AU7 and go to town as I continue to familiarize myself with the B system and refine it (basically it needs a better turntable). We'll see if 5751 hangs with the better vintage 12AX7.
But 5751 in that first slot is good! No worries if you want to try it there. I guess I should also try the 2nd slot next.
The B system:
- Tannoy Glenair 10 speakers
- VAC Signature 200 iQ stereo tube amp
- Rogue Athena tube preamp
- Hagerman Trumpet MC
- Fluance RT85 w/ Ortofon 2M Blue - this will be replaced with Clearaudio Innovation Compact in a week - but it’s kinda neat how good it sounds through a downstream which far outclasses it.
Sure I get the concerns about 5751 subs affecting the RIAA EQ. That’s why I’ve shied away from using 5751 in phono stages in the past. But sometimes you just have to try things. There can be a lot of variance in measurements between 12AX7 tubes, too.