Very tidy design, Eugene

I want to write a few words about the bypass capacitors C3 and C7. Perhaps, I misspoke when I said they eliminate noise, but what I actually meant was the following. The incoming wires from the power supply, being, for example, 20 cm long, have an inductance of say 0.2 µH and a small resistance. With a current at the plate ∆I equal to, for instance, 10 mA, we get a voltage drop at the power supply at frequencies above the audible range. At the same time, high-frequency noise gets mixed into ∆I.

In the circuit, we have an input stage, which they also reach the plate of. Meanwhile, there's a divider created by the resistance across the plate and the transconductance, through which the internal noises again reach the grid of the output stage. They seem to be supposed to be dampened, but due to unintended feedback through the parasitic capacitances combined with a phase shift, the signal reaches the grid out of phase at a certain moment, encouraging the tube to start oscillating.

In the audible range, this is not noticeable. However, in the high-frequency range, it leads to a) the tube operating in an abnormal mode, b) the heating of the tube, c) increased channel crosstalk, d) increased non-linearity, and e) further transmission to the power amplifier (which in our case will be transistor-based, and the consequences for it will be unpleasant).

C7 is in parallel to an existing capacitor and you really think there is gonna be a measurable voltage drop in an 18ga wire as the plate current varies a couple of ma? GL with your PC board version of this.

The following modifications have been made:

1. A decoupling capacitor has been added to the input to remove the DC component of the input signal. While it's possible to omit this capacitor, but I’ve opted to include it, as we already had a pair in our path.

2. A high-pass filter has been implemented at the input with a cutoff around 15 kHz to mitigate radio frequency and cellular noise.

3. Two decoupling capacitors and a resistor have been added to the power rail to filter any feedback from the plate of the output tube to the power supply and the input tube, with a cutoff set at approximately 15 Hz.

If these circuits were not present, the varying anode current of tube RO2 would create a voltage drop across the resistance and inductance of the wires leading to the power supply. Therefore, the voltage at the anode of RO2 would slightly fluctuate. The introduction of C7 prevents the alternating current from reaching the power supply unit, as the current is shunted through the capacitor instead. The R7C3 filter does not allow oscillations (should they occur) to reach RO1 and infiltrate its signal. This filter cuts off both the oscillations that may arise in the power due to the operation of RO2, and any potential residual hum (60Hz) in the power circuits, preventing them from reaching RO1.

And also R3 of 1M doesn't make sence after introducing R2.

R3 is only there in case the wiper of the pot fails, and prevent the input tube from running away. Many designs don't even include a resistor there There is no reason to increase it to 510k.

If there is DC on the input, you have other problems further up the chain! Adding another coupling cap in the signal path as a protection for bad equipment upsteam doesn't seem nessasary to me. And as you said, we already have two out of necessity.

C7 is in parallel with the existing 47uf cap on the output of the power supply.

R7/C3 isn't going to hurt anything, but the current drawn by the "output" cathode follower is so low, I can't imagine it introducing a signal into the resistance of the 18ga solid copper wire I use. Obviously if we were looking at a beam power tube pulling 60ma, that's a different story.