What is the purpose of the 0.05µF cap and two 22MΩ resistors on V1 across the grid and plate (pins 6 and 7)? I haven't seen this before.
Looks like some kind of feedback loop to me.
I must not be as smart as Dumble; I don't see how they do anything.
1st-guess for the closed-loop gain of a negative feedback circuit is R
fb/R
g, where R
g is the series 220kΩ resistor in the Dumble and R
fb is 44MΩ (2x 22MΩ resistors in series with a cap for blocking d.c.).
But 44M/220k = 200, and we know the actual open-loop gain of a 12AX7 will only
approach its Mu of 100 (with a crafty active load, like a transistor instead of a resistor load). True open-loop gain will be closer to Merlin's example of 64.
So the feedback resistors are too high a value to do anything; they're too large to feed any signal back to the grid to cause any feedback.
Or maybe I'm too blind to see the "Crystal Lattice."
Can a lower value like 4.7MΩ be used?
Yes. Lower values equal more negative feedback. If you source the feedback after the plate's coupling cap, you can even do away with the cap (but then you lengthen a wire run for feedback, maybe not a good thing).
Keep in mind that simply omitting the cathode bypass cap for that stage will create local negative feedback, and cut stage gain by about half. That's a simpler plan if you want ~1/2 gain in the stage.
If you're certain you need something more than 1/2-gain but less than full-gain (or also want the tone-shaping of a cathode bypass cap of 1uF or less to shave bass), then you can jigger the feedback resistor vs the series grid resistor. But the small value of R
g with a reasonable value of R
fb will create a low input impedance and make the stage hard to drive; conversely, large values of R
fb increase resistor noise.
For these reasons, Merlin suggested pretty low values of closed-loop gain (like 10 or less) to balance the trade-offs. I'm guessing you don't really want to drop gain that much, especially since the original circuit didn't drop gain at all.
What If?What if the resistors are really 2.2MΩ (a pretty common value by comparison)? Now you'd have a 220kΩ working against 4.4MΩ for a gain of 20 (or a bit less when computed by Merlin's method). The 0.05uF cap blocks d.c. while placing the bass roll-off in sub-sub-sonic territory. The 500pF cap would work against the 220kΩ resistor to create a high-pass filter that's -3dB at about 1.5kHz. That would cause gain of the stage to rise from about 20 over most of the guitar's range up to full gain at a band of frequencies that only covers the guitar's harmonics (full-gain around 3kHz and up).
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