Preamp stage of 6D30 simulated

I've been doing a lot of wiring on the amp and realizing some of the subtleties of circuit design, as well as the practical aspects of fitting all of this stuff into a combo cabinet. The final sound is really where the proof lies, but elegance can be recognized anywhere. In the spirit of learning about each stage of the amp, I will try to simulate as many of them as possible. Below is the schematic for the first preamp stage of the 6D30. I have employed here, the generic triode model from Duncan Amps.

In case you are interested, the file for the schematic is here.

The above circuit seems innocent enough: two inputs running into the circuit via the ubiquitous 68k resistor and separate 12AX7 stages mixed at the output with a gain of ~30 (15 dB). Thus, given a 300 mV sinusoid, the voltage across the 500kOhm resistor will be ~8.7V sinusoid. This can be seen below:

The image plots the input signal and the voltage across the 500k resistor on the output. 300 mV is a reasonable approximation to a guitar signal. As you can see, this is a pretty big amplification from a small signal. This is why the wiring from the input jack should be both SHORT and SHIELDED. Due to the low signal level at the input, a smaller amount of noise can drive your signal to noise ratio (SNR) very low fast. Then every stage downstream amplifies your noisy signal resulting in .. loud noise mucking up your sweet guitar tone! :(

The other thing worth noting in this circuit is the high-pass filter that couples this stage to the next stage. This consists of C2 and R6 and is a typical RC circuit. The -3 dB (half-amplitude) point for this circuit is ~600 Hz (found by using 1/(2*pi*R*C)). Why am I mentioning this? The fundamental frequency range for guitar is from 80 Hz to 1 kHz. Since 600 Hz and below are attenuated by more than half in the first stage of this circuit, the guitar will certainly be affected. Frequencies above 1 kHz account for the "timbre" of a guitar, so none of that is lost. If channel 1 has too many highs, this might not be a bad place to throw in some more low frequencies by increasing the value of the C2, and dropping the -3 dB to more like 400 Hz.

As a note to anyone building this amp, the resistor on the layout says 1M, but the package contains a 500K resistor, and 500K is what the schematics say. I have noticed one should trust the schematic more than anything with the kit. The wiring layout is really confusing to me, since it seems to be mirrored and upside down. Maybe I'm just not thinking about it right.