Here's how the component values change the sound of a guitar tube amplifier.
Most amps are an AB push pull type of amplifier, so is the schematic here. Most of the description of this amp can be used on other amps of the same type.
The Tone Stack
I highly suggest downloading the Duncan amps tone stack calculator. It can show the frequency response of a tone stack. This is a very good area to tweak the sound of a tube amplifier. They also have other programs that are very useful. R4 is a 100K Slope resistor. Higher values equal more bass.
C2 is the treble capacitor. Higher values equal more upper mids.
C3 is the bass frequency capacitor. Higher values equal more bass and more lower mids.
C4 is the middle frequency capacitor. Higher values equal less lower mids.
P1, P2 and P3 are the treble bass in mid pots. Decreasing the values can remove some of their frequencies and scoop out the mids a little.
The Preamp
V1-A is the first triode in the first tube, V1-B is the second triode in the first tube. R1 is a one mega ohm resistor, which is the input sensitivity resistor. Lower values equal less sensitivity, higher values equal more sensitivity. R2 is V1-A’s plate resistor, lower values clean up the distortion, higher values make it more distorted. R3 is the cathode resistor, higher values equal less distortion, lower values equal more distortion. The same rules can be applied to V1-B and V2-A. In this example the second triode of the second tube is not being used.
C5 and C6 bypass the volume pot. These silver Mica capacitors can be switched in to make the overall sound brighter. C8 is a coupling capacitor, lower values equal less bass, higher values equals more bass. R7 is a grid leak resistor, lower values equals less volume, higher values equals more volume. R7 attenuates the signal level into the third gain stage.
The Phase Inverter
If you wanted this amp to have a master volume, you could change the resistor R10 in this schematic to a 1Meg audio taper potentiometer. C9 can smooth out the sound a bit. Larger values equals smoother sound, lower values equal a more raspy sound. R11 and R12 can be raised to make the sound thicker or fatter. You could try replacing R11 with a 33k and R12 with a 1K. Raising R12 to a 1K resistor will clean up the phase inverter significantly. C11 is a high frequency roll off capacitor. Higher values cut more high frequencies. A potentiometer and a capacitor can be connected in series here, which acts as a variable high frequency roll off. R14 and R15 are the plate resistors of the phase inverter tube. These values are purposely off set to smooth out the sound. R24 is the negative feedback resistor, or NFB resistor, for short. Lowering this value can really clean up the sound and makes the amp quieter.
Power Tubes and Transformer
R18, R19, R22 and R23 are the grid or screen resistors for the power tubes. These values don't change that much. R20 and R21 are the cathode resistors. The diodes connected between the power tube plates are for protection against power tube failure. Some transformers will only have one 8 ohm output for the speaker cabinet, but some will have three. This is why on some amps there will be an impedance selector.
The Power Transformer
The power transformer allows an alternating current source (wall outlet) to be passed from one side of the transformer (primary side) to another (secondary side or sides), and in the process can increase or decrease or match the voltage that goes into it. This is achieved by wrapping a wire around anything that will conduct electricity (the core), with wire that is coated or "insulated" so that the wire won't actually touch the core (insulated wire), but creates a magnetic field in the core (which conducts the electricity). This magnetic field is passed on to any other wire that's wrapped around the same core. How many times the primary wire is wrapped around the core determines the primary/secondary relationship. Tube amps require plate voltages around 480 volts or so to operate, so the power transformer takes the 120 volt source (US standard wall outlet) and converts it to 480 volts. For example 1,000 turns on the primary side and 4,000 turns on the secondary side equals 480 volts. But tube amps also require 6.3 volts to run the heaters, so another set of windings is wrapped around the core, independent of the others, about 50 times. Another set of windings produces the 60 volts needed for the bias supply, 500 turns on that winding.
The Output Transformer
The output transformer steps the voltage down to a manageable level that the speaker can handle. The plate voltage, which is around 480 volts, is too much for the speakers. Another thing about transformers is what they don't do; they don't pass DC (direct current), only AC since there is no physical connection between the windings. The primary side of the output transformer has the full 480 volts running through it and the tubes modulate that voltage (play loud or play soft).
The Power Supply
The solid-state rectifier converts AC voltage from the wall outlet to the DC current that the amp needs. The power supply circuit consists of voltage dropping resistors and filtering capacitors. The power supplies main function is to smooth out the DC ripple in the signal. It also drops the voltage in steps to feed the rest of the preamp circuit.
R30, this resistor can change the bias range. If you change your power tubes you may end up with a situation that you cannot bias correctly. You have two options; you can either change the bias pot to increase the range/scope or change the bias range with R30.