The V1B gain stage is the second half of the first tube on the left (from rear view). Each of the pre-amp tubes are dual-triodes, which means that they contain two separate triodes in one tube. The V1B triode is used as the channel 1 treble channel gain stage. It amplifies the low-level signal from your guitar. The Rk/Ck values connected to the cathode determine the gain and frequency response of the treble channel. The classic values for lead amps are 2.7K and 0.68uF.
The cathode resistor Rk works in tandem with the plate resistor to bias the preamp tubes. If you change either of these too much in one direction or the other, you'll get asymmetrical clipping as one side of the waveform distorts before the other. This is not necessarily a bad thing. Now, add a capacitor, and you get even more gain. A small capacitor like Marshall's .68uF boosts highs and mids. A 25uF cap will boost all frequencies in the guitar range.
