Valve high-gain stereo preamp

Posted on Mar 25, 2013

A cathode follower based on the 6BL7 or 6BX7 is ideal for output. I used a mu-follower circuit with no feedback, a 12SN7 as the gain stage, and a 6BL7 as the cathode follower/current source. (I have many old GEs I wanted to use, and converting to 6SN7 means simply changing the hookup to the 24V filament transformer). You might argue that the current source should be a pentode of highest possible gain, but pentodes can be noisy, and one of my design goals was minimalism. The line stage (Fig. 1) is a straightforward mu follower, with medium-mu triode 12SN7GT below and low-mu triode 6BL7GT above. I paralleled both sides of each tube for maximum lifetime and for minimum output impedance. I chose resistors from my junk box, so there is nothing sacred about the values shown.

Valve high-gain stereo preamp
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Valve high-gain stereo preamp - image 1
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I have many opportunities to experiment with other equipment. I have tried innumerable preamps based on 12AX7 and 6DJ8 types, including Dynaco PAS-3s with every modification imaginable; McIntosh C11, C20, and C22; Scott 130; various Audio Research units; old NYAL Minuets; Lafayette KT-600 (probably the best of the vintage units); and some home-brews. Not one of them had the real magic. In my opinion, most 12AX7s have a "lumpy" sound and do not maintain similar frequency response between channels, because negative feedback tends to "amplify" response aberrations, especially when tubes age and drift. The 6DJ8s have a wiry, irritating sound to me. I believe that some listeners confuse this kind of distortion with extreme sonic detail. Although probably the best of the manufactured preamps, the old MFA Luminescence (currently out of production) isn't perfect. It is too complex, the phono stage is a bit of a monstrosity, and the line stage uses unnecessary negative feedback. It uses all octal-based tubes, which tend to produce better sound than similar miniature types. Maybe octals have a less microphonic character, but of a more pleasing type. Two of my acquaintances, who each have a Luminescence, both complain about the unit running too hot to touch. And the line stages drift, so they often must look for closely matched 6DN7s. Also, the 7591 in the power supply and all the 5691s in the phono stage add to the expense...

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