"Triodes that are unbalanced sound just as good as long as one side tests strong." That’s true, IF the gain circuit is single-ended as opposed to balanced, and if the section with high transconductance is the one you are using. This is often how tubes become unbalanced; they have been used in a single-ended circuit which only requires one of the two sections where the other section (of a dual section triode) is not doing anything. Some canny manufacturers of SE preamps build their units so that one channel uses section 1 of a dual triode, and the other channel uses only section 2 of the same type dual triode. (Two tubes, one for each channel.) That way, when the tubes become worn, you can swap them from one side to the other of the stereo circuit and in essence you have new tubes. Balanced sections is much more important for a balanced circuit. Also, transconductance is not necessarily a correlate of "gain". Depends on the circuit and the tube. For example, a 12AX7 has highest gain among typical dual triodes, It is commonly used at the input of MM phono stages, because it has highest mu. Mu is equal to Gm*RP (transconductance X plate resistance), and the 12AX7 has very high plate resistance but relatively low Gm. Mu is a better correlate of gain. Gm is more a correlate of how the tube handles current.
Do vacuum tube test values reflect sound quality?
I recently ordered a pair of output tubes to try in my LTA MicroZOTL preamp. Two Sylvania 6SN7WGT tubes, Gold Brand. I’ve had good luck with Sylvania 12sn7gt bad boy tubes, so I thought I’d give them a try.
After I bought them, I saw a similar pair for more money that had slightly higher test values. Does that mean they might sound better or that they just have more life left? What properties do you look for in tubes?
Below are the values of the tubes that I bought.
Tubes are matched for dynamic gain and structure, testing at 3000-2750 and 3000-2750 where pass/fail is 1625 micromho. They have the same structure inside and out, having matching date codes of "922", the 22nd week of 1959
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@larryi They are ECC 801S. Clearly labeled with the Telefunken logo. I assume they are legit, but they aren't the $2000 pair that you mentioned. |
There are instances where someone gets tubes for well below market price, so it is entirely possible for you to have real Telefunkens. If they are not new in box, the price could be substantially lower than the price I mentioned. The good news is that these tubes have a very long life, so even old tubes can be good for a loong time. if you see the Telefunken diamond embossed in the glass on the bottom of the tube, that is a good sign; only the most sophisticated fakes have that embossed pattern. |
@lewm +1 |
@larryi You made me curious, so I pulled one. There is a diamond on the glass bottom. |
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