1. What are the different functions each resistor in the circuit?
2. What are the functions of each of the caps?
3. Why the two different types of caps?
4. Why the one set of carbon comp resistors?
5. Why the silver plated, teflon coated wire just for inputs?
6. How to measure DC offeset?
The forum is a great place to discuss the theory!
1. There are three resistors in the circuit (I forgot about the carbon comps when I wrote this, but answered that separately later). The 4k resistor that goes from the power switch to the plates of the 3S4's is the plate load resistor. The 1k resistor that is bypassed with a capacitor is the cathode resistor. Lastly the 470k (not 100% sure this is the correct value) resistors could just be referred to as output resistors, or maybe grid leak resistors (for the following stage).
The plate load resistor serves a few purposes. The value of this resistor combined with the operating points of the circuit gives the designer a way to know much voltage the circuit can swing (mU and the 0V line), and a sense of how the circuit might sound (is the loadline "cut" into equal length pieces by grid-cathode voltage curves?).
The cathode resistor is there to set that grid-cathode voltage.
The resistors at the output are more-or-less an arbitrary value (150k-750k), and they keep one end of each of those brown coupling caps at ground potential. If not for that, you might get quite a spark if you hot swapped the cables from the output.
2/3 The blue colored caps bypass the 1k cathode resistors. When using these capacitors, you increase the gain of the circuit and affecting the output impedance. The value of these caps is determined by the cathode resistor and how much low frequency information you want to preserve. Since this value ends up being so large (1,000uf), electrolytic caps are the sensible way to go.
The brown caps are the coupling caps. They block the DC voltage that is on the plate from being present on the output RCA jacks. These are sized according to the input impedance of the equipment that the preamp feeds (15,000 ohms instead of 1,000 ohms). Since these caps aren't so large in value, we are able to use high quality film caps for better sound.
4. Ah, I missed a pair of resistors! The little carbon composition resistors are configured as grid stoppers. They made their way into the Bottlehead kits when the Seduction came out, as the grid and plate stoppers were a great way to keep 6DJ8's from oscillating. While it may not be super likely that a 3S4 will be prone to high frequency oscillation, it is a very inexpensive preventative measure to include these resistors.
5. That wire is almost exclusively chosen because it is solid core and Teflon covered. With other insulation types, when the wire becomes too hot, the insulation will melt and curl off the wire, making a mess. Solid core wire is important also so that the wire can be inserted into small switch holes without shorting to adjacent switch holes.
6. In my understanding, there are two DC offset measurements that are evaluated in audio. One has to do with opamps problematically amplifying small amounts of DC, and the other has to do with DC being present at the output of a piece of equipment. I will assume that you are interested in the latter measurement. All you have to do is measure the DC voltage from ground to the center pin on each output RCA jack. Since there is a blocking cap and a resistor to ground the output end of that blocking cap, this voltage should always be extremely close to zero.
-Paul
