"Feel the Gain" Part 1: Guitar Input

Let’s start at the beginning. Genesis 1:1, the Big Bang, the giant chicken egg. Your instrument’s

signal will need to enter the amp circuit. The most popular method is the 1/4” jack. One simply acquires or makes a 1/4” male to 1/4” male instrument cable, plugs one end into their guitar jack and the other end into the jack on our amplifier. “Ahem, excuse me, Author. Duh, I know that! Now what?”

Let’s take a look at a Fender Deluxe Reverb input. The guitar signal enters the first preamp stage through jack 1 or jack 2. On its way, the R3 1M resistor that connects from the guitar signal to ground (those tiny triangles) sets the impedance. The signal passes through R1 or R2 68K resistor (more on this in a moment) on its way to the control grid of the preamp tube. This resistor acts as a low pass filter to stop noise such as radio interference from entering the circuit. They also mix the 2 input signals together.

My ninja, are you ready for GAIN? Not yet! Before we talk about the gain stage, let’s take a step back and look at those input jacks. Most amp circuits should have “switching jacks.” This type of jack has an extra lug that when no plug is inserted, connects to the tip of the jack. (Lugs are the little tabs that you can solder your wires or components to.) Looking at the schematic above, the switched lug is the bottom most. In this case, and in most guitar amp circuits, that extra switched lug on the first input connects the tip to ground when nothing is plugged in. If you’ve ever played an old Silvertone, you’ll find that when you don’t plug in a guitar and turn up the volume, it gets as noisy as a skunk in heat! That’s because Sears didn’t use a 3 lug switched jack. Take a look at the second input, the switched lug is connected to the tip of the first input jack. Why not connect that to ground? On-account-a we want options! When using jack 1, the guitar signal will now travel through both 68K resistors on its way to the tube control grid. Two 68K resistors in parallel equals 34K of resistance. What does that mean? A larger signal passes through input 1! In most cases, this means a louder guitar or a higher gain signal.

To be continued in part 2...