Well, now that I'm looking at it, I wonder about the resistors directly after the pot. EDIT Nevermind, I finally re-read the very informative post by yourself explaining the optional parts and grid stop. You are very patient and helpful. I'm still wondering what happens if the resistor values are changed.
Thank you for your patience as I'm learning. I search all I can, but this has become a very, very long thread.
I'm not sure what you're asking when you say, "I'm still wondering what happens if the resistor values are changed."
Every volume pot is designed for zero volume at the bottom of the range, and zero-resistance/maximum volume at the end of the range. The only question becomes, "Where in that travel does your comfortable listening volume take place?" As mentioned many times previously, the stereo volume pot used in the Starving Student is not the best (low cost and the "starving student" theme applies). Cheaper volume pots, among other symptoms, exhibit poor channel matching, often at the extremes of the volume travel - both at the bottom of the range and the top of the range. If your comfortable listening volume happens to fall in either of those ranges where the channel matching is bad in the volume pot, then your listening experience is full of irritation.
What determines where the comfortable listening volume falls? Three variables apply: 1) the gain of the amp, 2) the efficiency (sensitivity) of the headphones, and 3) your personal listening preference. There is also the recorded gain setting of the music, but let's assume you've adjusted that through your media player, or that the differences are typically small enough that the change in volume pot setting is trivial. You can't change #2 unless you change the headphones. You can't change #3 unless you train yourself to listen at different volumes (could be ear damage as a result). So, you're left with doing something with the amp.
If the Starving Student were a typical solid-state amp with feedback, you could change the resistors forming the voltage divider ratio in the feedback loop. However, the Starving Student is not a typical solid-state amp. Neither does it have feedback. The basic gain is provided by the tubes and is not variable. Unfortunately, the basic gain is quite high with the tubes and circuit in the Starving Student, meaning that for most headphones and sources, the comfortable listening range for most people occurs within the first quarter turn of the volume knob. This is a bad place for comfortable listening. It does not give you much precision in adjusting the volume and the channel matching in that area with a cheap volume pot sucks.
In order to remedy that, we began the practice of applying resistors to the signal input of the circuit. Why? Well, the goal is to put the comfortable listening range further in the volume travel than sooner. In other words, we want to "push" the comfortable listening range of the volume pot from 9 o'clock to 12 o'clock. How do we do that? Well, continuing to turn the volume pot does what? It decreases the resistance. So, if we add resistors to the input, we increase the resistance in the circuit before it even gets to the volume pot. Done perfectly, the input resistors push the range to 12 o'clock, where once the comfortable listening range was at 9 o'clock, .
This is not rocket science, but the actual variable values are hard to determine. Every headphone is slightly different and so is your hearing. We supplied (or suggested) resistors in the 50K, 100K, and 150K range to take care of the differences. If you lower that input resistance or remove it altogether, it will have the effect of moving the comfortable listening range down to the bottom of the dial (with most headphones). However, if you happen to have an uncommonly high headphone impedance (affects how easily the amp drives the headphones) or low efficiency headphones, you may find yourself at the end of volume pot travel, yet still not receiving sufficient volume. In that case, lowering/removing the input resistance may mean that you end up in the 12 o'clock range, instead.*
* At some point, the power the amplifier can supply enters into the mix, but this has usually not been a problem with the Starving Student.
