• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

What's right, or wrong, with this line stage

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As drawn, gain is nearer 8.

And the BIG "feature" is that it can drive 600 Ohms, even 32 Ohms, with no stain. Is that in your design requirements?

If you only drive "normal hi-fi loads", you could save a BUNCH of bucks and pounds with a simple mod. If the chassis looks bare, get some "vintage transformers- as-is" off eBay and nail them on.

About the worst load I've seen is a few pro amps that have a 5K input impedance if driven unbalanced.

One little thing- I don't like having the pot wiper act as the ground reference for the grid on the input tube.

If it were me, I'd replace that (presumably log) pot with a linear pot and use a law faking resistor on it (which would also serve as the ground reference for the grid). Maybe I'm just unlucky, but I've never had good luck with 2-gang log pots tracking well, and I can't stand stepped attenuators.

My first tube project was something along these lines. I initially planned to use a 12BH7 in mine (better linearity), but wound up using 12AU7s because I had them on hand. Quite honestly, even with the famously non-linear 12AU7 in it I still have perfectly respectable distortion performance of somewhere around .05%. It's been a couple years since I built it so I don't really remember the details and I can't find the schematic, but I do remember it turned out very similar to the one this guy built (well, not nearly as pretty).
DIY 12AU7 Tube preamp

In retrospect, I should have bit the bullet and bought some Jensen transformers (and a chassis with enough space for them) when I built mine. It would have made it much more versatile. All the amps I build now have differential inputs, and I while I know it doesn't seem to be considered "fashionable" on HiFi equipment, I really prefer it.

It does need a muting circuit, otherwise the turn on/off transient will be a problem for solid-state amps. A simple RC circuit with an SCR (which drives a relay) works fine.
 
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