Tubes Asylum

It's the gain tube as I understand it

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This Post Has Been Edited by the Author

In Reply to: RE: What does the middle 6NS7 do? posted by Rod M on April 02, 2023 at 19:41:34

Here is what VTA says on their website about their "classic" ST-70 driver board:

"The upgraded VTA driver board uses three new 12AU7 or 12BH7 triodes in place of the two mediocre sounding 7199 pentode/triode tubes. Dynaco saved money using a two tube 7199 board design but made some serious compromises in sound quality." Chris who built my amp says much the same thing in his owner's manual.

Since then, VTA started offering the octal driver board as an upgrade option.

And here is what inmate airtime said about that driver board when I first posted about buying the amp in February on the Amp Asylum:

"Because of its lack of linearity the 12AU7 should never be used as a gain tube. It sounds awful in that position and no amount of tube rolling is going to fix it. Designers are only using it because it quickly fixes a number of gain issues. The 6SN7 is the tube to use on that driver board."

As you can see on the linked page, VTA (tubes4hifi) still sell both as replacement parts and still seem to show the 12au7 board as being the standard approach, but Roy was clear when I emailed him that replacing the center tube with a 6NS7 and adapter should be an improvement.

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http://www.tubes4hifi.com/ST70.htm (Open in New Window)

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