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6922 6DJ8 or 12 AX7 tubes better for preamp?

I have found conflicting information regarding these 2 tubes for preamps. One camp, including Roger Modjeski in this very detailed article with testing data http://store.electron-valve.com/suof6dforau.html
feels the 6922 is superior. Tim Paravicini (EAR) uses the 6922 in the 88PB Phono preamp and is used in the Manley Steelhead. My local retailer, whose opinion I value, strongly believes that 12AX7 tube is superior, which is used in the Aesthetix IO and in the less expensive EAR phono preamp, 834 P. The 12 AX7 was reportedly specifically made for audio. Detractors of the 6922 state it sounds sterile. The IO and its 12AX7's has been criticized for being noisy, unless one takes extraordinary care with cables, tube selection, and tube matching, which would appear to support Roger's thesis. Am I sinking into the depths of audiopilia nervosa in evaluating tubes as part of my selection criteria for a great phono preamp? It goes without saying that audition is essential in the final decision. I have heard a modified EAR 834P on Quads Unlimited 57's and liked everything about what I heard, except would have liked a somewhat larger soundstage, which was likely more related to the speaker. I hope to audition an IO soon.
tompoodie
atmasphere's avatar
atmasphere
7,068 posts
 
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.
rnadell
63 posts
 
I have an Aethetix Rhea that is noisy. Mentioned in the initial question is that extraordinary care needs to be taken with cables. Could someone expand on that requirement. I am going to retube the unit and perhaps I need to deal with cables also.
thanks
lewm
6,770 posts
 
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.
mechans
2,999 posts
 
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.
atmasphere's avatar
atmasphere
7,068 posts
 
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.
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