> what signal is even feeding the grid of the first half of the tube...
Which tube? There are nearly a dozen tubes on that plan.
We know the 7025 is just a 12AX7, but for some reason the only one marked "12AX7" on this plan happens to be the Tremulo tube. I'll pretend you mean that one.
The left-side grid is fed the signal from its own plate. Hold your hollow-body gitar too close to its amp: even if you don't touch the strings, you get feedback and a loud sound. That's an Oscillator. Works about the same as an organ pipe: blow air through, feedback at the throat encourages a wobble in the air, changing the pipe size changes the pitch. Blow electrons through the power circuit of an electronic oscillator, it amplifies its own output, even random noise, into a full-power wiggle. Instead of a pipe, we use R-C networks to set and change the pitch. Plate signal passes through the 0.02, 0.01, 0.01 chain, loaded by several Meg resistors, one user controlled. For Tremulo use, we pitch it so slow you can see it wiggle. We know (or should know) that 0.01uFd and 1 meg gives a 17Hz roll-off, which is far below guitar pitch and on the edge of flicker frequencies. We might suspect that the cascade of three C-R networks works a little slower(?), and we see one network is 0.02uFd and 100K-3.1Meg so it works slower when turned to the limit. Probably tunes from "flicker" to "throb" frequencies.
Don't try too hard to analyze this LFO. It is incredibly funky. It violates all the design rules for a nice phase-shift oscillator. You could do a far better job with a chip, except most guitar-amp tremulo schemes need high voltage, and the Classic Designs were laid in stone before chips appeared. Just learn about Oscillators: an amplifier plus something to set the frequency.
