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The phono section of this preamp uses 12AU7s as buffer tubes and 12AX7s as driver tubes. I've put some nice NOS Telefunkens in for the 12AX7s based on a friend's recommendation and that was a great improvement. The stock buffer tubes are EH Russian. What is the purpose of the buffer tube and is it as critical? What 12AU7s do you like for these?
Edits: 07/26/14Follow Ups:
Depending on how the preamp is wired, the 12AU7 may be providing just a little bit of gain and more capability to drive the level control at the input of the linestage.
Alternatively, the 12AU7 could be setup as a cathode follower, but this would make no sense if this is a phono preamp inside a larger linestage, and it still leaves two extra triodes.
One 12AX7 per channel would provide enough gain for MM phono, so I believe the most reasonable guess as to the function of the two 12AU7 tubes would be one SRPP stage per channel.
Sorry, I can't figure that one out.
Either the new Tung-Sols or the Genalex B749 would be a nice tube for those positions.
Seems like opinions vary here. I'll proceed with swapping in some better tubes and see for myself. The EH tubes are working fine but where I have upgraded elsewhere it really was noticable.
I've had good luck with clear top RCA's in the buffer position.
"Buffer" sections are typically named that because they usually have no gain and "buffer" the output by lowering the output impedance. The most common way of doing this is using the tube as a cathode follower. Since there is no gain, you do not need to go nuts on the primo NOS tubes, a good functioning EH will work and sound just as well as anything, IMO.
With preamps where I have swapped tubes (not including the Cary 98), the cathode follower tube is as sensitive to tube rolling as gain tubes. With the Hagerman Cornet phono for instance many people on the Hagerman forum said the cathode follower was actually the most sensitive.
That is interesting. There is no telling how each design is running their cathode followers. Some designs run them pretty low current for extra life. In that situation they could be more sensitive as there are some tubes that are more linear than others at the extremes of the tube curves.
When I want to use a cathode follower, I just use a MOSFET source follower because it is better at it. Talk about being able to make current...
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