My collection of ubiquitous 6SL7s. I use this tubes ostly at 1st stage of my phono preamplifier. Also I have power amplifier with driver stage natively on 12ax7 as SRPP but since I built tube preamplifier I need less amplification from it and that’s why I tried 6sl7s there. Finally changed it to 6sn7 to get less gain from the driver.
It is not a surprise that all these tubes are sounding different, we all know this. But the difference is critical when we speak about the 1st stage of Phono preamplifier.
Left to right: RCA VT-229. Military tube. Was the champion toll it began to hiss. Only one month served. A soldier is not indented to live long… 7F7 Sylvania, early 40s. 7f7s are generally the same as 6sl7s but the loctal base. This tube sounds extremely different, it delivers mighty bass. But it becames annoying soon. 7f7 GE. GE never surprised with a good sound. It sounds - that’s all. Quite boring. 7f7 Philco. A bit better than GE but still far from to be interesting. 6н9с. This is Soviet military one. With military rombs on it. 1988. Works great! Sounds great! But I have about 10 pieces of other 6н9с - they are nothing special. USSR made millions of them on different plants, you can never be sure what you get. They are too different. But they are absolutely champions in place of 12ax7 in driver stage if my power amp.
6sl7gt Sylvania, red print. This particular tube is quite weak and not equally balanced. 7f7 Sylvania. Plays as 6sl7gt Sylvania, no big difference. But noticeable more microphonic. I use adapter to plug in 7f7s into octal socket and discovered that most 7f7s are microphonic in comparison to 6sl7gt. But the reason may be in that adapter. Only 7f7 of RCA not tend to be microphonic from my experience. 7f7 RCA. Very beautiful sounding tube, no microphonic at all. Need to be compared directly to 6н9с - good chance for it to win! But I have to make test recording to be sure. Very promising tube and very cheap on comparison to these overpriced 6sl7s. 7f7 CBS. Not tested thoroughly yet. Just works. Vt-189 GE from 1961. Military 7F7s. Of GE. Nothing special, but less microphonic than 7f7s in general. 6sl7gt Sylvania, green print. This was my working horse for 3 years. Nice sounding but 6н9с and 7f7 of RCA are better now. May be this is due to it was worked out. Finally, my recommendation is to get various 7f7 of RCA and military 6н9с which are both cheap to find a better sounding if you want to use it in preamp. Or just close couple for power amp driver stage (SRPP only! Use of these tubes in other way as drivers will cause frequency roll off too early. But that is a long talk we leave for another post.)