I have owned many and still own gear that uses both types of tubes. Some are very good and some are very bad. I think it is more in the execution and circuit design than the actual tube itself...IMHO.
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I agree with Mofimadness. I own the EAR 88PB and have owned a few preamps with 12ax7s. I also had a custom made preamp that had both 12ax7 and 6922/7308 tubes. Execution is a key factor in determining the quality of sound as are the tastes/preferences of the preamp's buyer. Incidentally, the 88PB actually uses 7DJ8s, however, 6DJ8s,6922s or 7308s may be substituted.
You're right, I was reading your thread title and didn't pay attention to the fact that all the units mentioned in your text are phono stages; my comments were regarding line stages. The Atma-Sphere preamps actually use 12AT7s in the phono section and 6SN7s for output buffering. There are some DIY octal phono stages:
which probably sound great, but no commercial 6SN7 phono stages that I know of. It's tough to get enough gain out of 6SN7s for phono duty.
Ummm.... this is all baloney. The designer chooses, or should choose, the tube best suited to the job. The 12AX7 is a very high mu (gain) tube that is best suited as all or part of the input voltage amplifier in a phono stage. IMO, one never ever needs that much gain for a linestage. However, a linestage does need to have a very low output impedance and the capacity to deliver current. (The two go hand in hand, actually.) Thus you can say that the 6922 and its congeners are best suited to use in a linestage. However, they can also be used to make a very nice phono input stage, by using a pair in cascode, for one example. The 12AX7 has a very high plate resistance and tolerates only a very small amount of current. The high Rp gives it a high output impedance and very little current is delivered, which makes it a lousy choice for a linestage. If 12AX7 is used in a linestage (as is often the case in commercial products due to its cheapness and availability), then one almost certainly needs an additional tube to be used as a cathode follower, to achieve a useably low output Z. The 12AU7, which has the same electrical parameters as a 6SN7, is frequently used for this purpose. (Worst of all is when a 12AX7 is used as CF, which I have seen in some products.) However, IMO, the 6SN7 sounds way way better than any 12AU7 (I have done the listening tests, FWIW). Among medium mu, high transconductance tubes (like the 6922), there are a few other choices, like the 5687 and its relatives, that I personally like better. Then also there are low mu triodes, like the type 27, etc, that sound great in a linestage. Once the proper tube type is chosen for the job, then sound quality will vary depending upon other things, like the linearity of the tube at operating points determined by plate voltage and current, etc, the nature of the plate resistor, coupling caps, etc. SO, it is folly to talk about how a tube sounds without considering these many other factors.
Thank you for the link, Dave...I will digest that in the course of the week.
Lewm...if I understand you correctly, for a phono stage such as the EAR 834p with three 12AX7's and a pair of output transformers, is that a reasonable compromise to use 12AX7'S in that context? When modified, it is quite impressive at its price point to my ears. I haven't auditioned phono preamps and preamps with phono stages above $5K, both of which it seems to compete against. I would like to avoid using a preamp, and create a phono only system. Thank you for such a detailed and knowledgeable response.
Dear Tom, Yes, IMO the 12AX7 is fine in that application. If I understand you correctly, then the 834P is using the output transformers to reduce the output impedance that would pertain if the output was taken directly from the 12AX7. So the transformer substitutes for a cathode follower. In no way did I intend to question the work of Tim de Paravicini. He's one of the best.
The 12AX7 is great for noise and low distortion, but is bad at bandwidth, making it a poor choice if you want passive EQ. The 6922/6DJ8 has good bandwidth, but the design of the tube is not for audio: while very good linearity, the design is prone to excessive microphonics. IMO anything designed with them will exhibit a sonic signature that has a lot to do with microphonics.
12AT7s are also designed for audio and have less microphonics. They are also higher mu yet allow for wide bandwidth and so seem to me a good compromise of gain vs bandwidth vs coloration compared to the other two types, assuming that you want passive EQ.
So in this case we see that the design of the circuit says a lot about what the tube choice is, for example if you are running feedback for the phono EQ the 12AX7 can do quite well.
I don't see how the choice of cables will have much effect in ameliorating noise due to the tubes. The presence or absence of shielding on a cable will help to block (or not block if no shield) RF, but that is an external problem, and noise due to RF is of a different character compared to tube noise. Just spend your money on low noise selected tubes from a reputable source, e.g., Upscale Audio, to name one I know of. Ralph likes to remind us that sometimes very expensive NOS tubes are not "low noise". You may be better off with Russian or Chinese tubes of recent vintage, if noise is paramount.
The gain required in a Phono pre is greater than what a 6SN7 can deliver. If you want to use all octal then I have been told and the shematic link above agrees that a 6SL7's higher mu will be needed. I am sure there are other more obscure octals than can be subbed.
The 12AX7 gain can certainly be excessive in a line stage. My ARC SP6B has the legendary gain from hell.
6SL7s have similar gain to 12AX7s. IME they are often prone to microphonic issues and so have to be very carefully hand-picked. Of course in a LOMC phono section *all* tubes must be picked for low noise. It is this reason why truly quiet NOS tubes are so hard to find; there have been LOMC cartridges around since the late 1970s. So most of the really quiet tubes were sold off the shelves decades ago.
Back then, if a tube did not measure up you could easily return it, so in time, given that the tube production stopped, the shelves would eventually have a greater percentage of noisy tubes.