• 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.

Why does no-one use the Anode Follower?

I just whipped this up inside a little mono phono preamp chassis I had handy. I'd been meaning to try this. Forgive me if I call it an 'anode follower'. Force of habit, I guess.

I figure it's delivering only about 7dB of gain. I've found that a phono stage with about 48dB gain gives me playback levels similar to what comes from my digital sources. I have a Hagerman Bugle set up for about 42dB of gain, and I had to pin the volume control pretty high up to get only moderately loud levels. Another 6 or 7dB of gain should get that closer to the right spot, gain wise.

I tried it, and it works great. It added just a bit of sparkle or something to the sound from the Bugle (probably just more gain). LTspice predicts very low THD at typical line levels, into a 100k load. With the 6DJ8 biased to only 2.3mA plate current, it won't drive any kind of difficult loads. I might go back and try to improve it.

The experiment was successful, though. This is a good sounding circuit. It's also very quiet, despite a primitive psu made from a vintage multi-section electrolytic capacitor (already in the little chassis) plus an extra stage of decoupling and a plain old AC heater. I guess the NFB keeps the noise down.

That was fun.
 

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I haven't tried to look at this anode follower/shunt feedback stage (which I'll call the "Little Monster") on a scope yet, but I gave it a listen in my headphones setup. Listening in headphones lends itself to noticing tiny differences, so I thought this would be a good test. That setup is tough for tube stages because it has a 10k ohm volume control at the input (it's fed by a Raspberry Pi with DAC, which is designed to be loaded by such things).

I put the Little Monster after the DAC and before the headphone amp, so the Little Monster sees a 10k load. I could hear the 6dB of gain, so that works. I could also hear a slight 'clouding' of the sound compared to the setup without the Little Monster. I think the high-ish output impedance (predicted to be about 4k ohms) and weenie 2.5mA of plate current isn't enough to drive the load + feedback loop cleanly. But it didn't sound bad either. Just the tiniest bit different.

Encouraging that it survived this torture test.