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RE: NOS 12AT7 at "first 2 position (Cascode?)" in MP3?

In your exposition, you say that you've removed the four 12AT7s that apparently constitute the phono stage. (I don't own an MP3, so this is a guess. I do own an MP1.) I am rather surprised that this did not cause a problem with the filament supply and maybe also the high voltage supply, because both supplies were designed to carry the load, both for filament current and plate current, presented by those four tubes you removed plus the two tubes you are running. On the other hand, it may be perfectly OK to do what you've done. I presume that either you asked Atma-sphere about doing this or that the owners manual addresses any issues related to doing it.

Anyway, you then ask whether all six 12AT7s have the same "amplification requirement". Actually, that term is rather meaningless. I can only think you mean to ask whether they all are set up to the same plate voltage and plate current parameters. The answer is almost certainly no. The linestage tubes (the front two, I presume) are probably identical to each other as to plate parameters, because they are respectively the left and right channels in the same circuit. Ralph can tell you what's up with the phono stage tubes.

Oddly enough, back when my own MP1 used 12AT7s, I experimented with NOS tubes. The GE 6201 gold pins were very good but just as good or even better were some select Chinese tubes; I am not sure you can get them any more. The Chinese tubes also had the problem of short life span, but they did sound the best. Mullard CV4024 were liked by many, but I found them to be too bright in the phono section of my MP1.

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