Here's a quick guide on how to modify almost any amplifier or effect pedal. The part of the circuit we are going to examine is the input stage. The input stage is the first amplification of audio filtering stage of any amplifier or effect, even digital ones!
This stage has a Zin input impedance that your instrument "sees". Ever wondered why most instrument amplifiers or preamps/interfaces have a "Hi-Z" indication on them? It's mainly because, the larger the Zin of the amp/preamp/pedal your guitar is connected to, the less pickup output voltage and treble will be lost due to impedance mismatches. In some amplifiers, there's even a hi/lo switch or two (hi/lo) inputs. The "Lo" input has a smaller value resistor connected to ground to reduce the output of your pickups. We'll explain all that in a moment.
Inside every amplifier, there is usually a set of resistors and capacitors at the very first stage of the input circuit.
Here is an example of such a circuit:
The R1 and R2 resistors form a fixed voltage divider. Only a fraction of your instrument's output voltage passes through this stage. If R1 was a short circuit and R2 was an open circuit, 100% of the input signal would pass through to the next stage of the circuit, C1 and C2. All these components can be labeled otherwise on the PCB or the service manual/schematic diagram of your amp. For instance, R1 could be labeled "R549" or something. Also, there may be more or less components in the input stage of your amp. For example, R1 or R2 and C1 or C2 could be missing. That's something common but you can definitely spot these components inside your amp if you examine its schematic diagram or you trace it by yourself. It's easy to spot a set of resistors, one connected to the input terminal and one connected to ground. Tracing an input circuit is not hard at all as long as you have a simple DMM (digital multimeter) with built-in buzzer (continuity tester), a magnifying glass, a torch light, a guitar cable and these useful R/C value calculators:
http://www.muzique.com/schem/caps.htm
http://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-resistor-color-code-4-band
Tip: most input jacks connect the "hot"/positive input signal terminal to ground when a guitar cable is not plugged in. So, you have to plug a guitar cable in the input jack to read the right resistor values on your DMM.
Measuring the input dc resistance to ground could be done on the other side of the guitar cable. Just touch the positive wire of your DMM on the tip of the lead jack and connect the negative wire of the DMM to ground (sleeve of the cable jack). Measure the resistance to ground. It should be a couple hundred kOhms or about 1MOhm. Increasing this resistance value will increase gain and volume a bit, depending on the circuit.
Note: 1kOhm (kilo-Ohm) = 1000 Ohm = 0.001 MOhm (Mega-Ohm).
Let's now see how the R1/R2 voltage divider circuit works:
If U1 is the output voltage of your guitar, U2 is the voltage that passes through for amplification. U2 is always lower than U1.
C1 and C2 also form a voltage divider but it's not fixed. The total Z of this circuit depends on the input signal frequency. Some signal frequencies are filtered (cut off) while others pass through. Therefore, this C1/C2 divider is called "input filter".
http://en.wikipedia.org/wiki/Voltage_divider
Let's see how the value of these components could alter the tone of your amp:
R1/R2 divider:
The typical resistance value range of R1 is usually 1k-100kOhm.
Note: 1kOhm (kilo-Ohm) = 1000 Ohm = 0.001 MOhm (Mega-Ohm).
The lower the value of the R1 resistor, the higher the value of U2 voltage that passes through. Nevertheless, R1 does not divide U1 all by itself. Let's talk about the R2:
The typical resistance value range of R2 is usually 68k-1000kOhm (1000kOhm=1MOhm). The lower the value of this resistor, the lower the value of U2 voltage that passes through.
As you can see, there are two ways to change the amount of voltage that passes through this circuit. If you think your amp lacks a bit of gain and input sensitivity, check the R1 and R2 resistors located inside your amp. For high-gain amps, R1 is usually around 1k-10kOhm while R2 is 1M-10MOhm.
The "Lo" input jack of a typical amplifier might have these values R1=R2=68Kohm.
So, check the input resistors of your amp and replace them accordingly.
Safety first!
Be aware that replacing any components of your amp will void the warranty and must be done carefully. Do not try to modify an amplifier if you're not at all familiar with electronics, for your own safety! Start with something small and cheap such as a distortion/OD pedal that shares the same circuit principles but its 9V stored voltages are harmless. Tube/valve circuits on the other hand, store around 400V in their power supply capacitors. These voltages are lethal even if the equipment is not plugged in.
C1/C2 filter:
The C1 capacitor is blocking the dc voltage of your signal which is usually a good thing as the guitar signal is an AC signal. So, you really don't want to remove C1 and replace it with a jumper or something. However, C1 also reduced bass/low frequencies. It's a "high-pass" filter! It's like an EQ pedal set to roll-off bass. The lower the capacitance value of C1, the less bass frequencies of your input signal pass through. Typical values of this capacitor are 10nF-100nF. If you need a bit less bass, change the C1 capacitor for a lower value one (in the nano-Farad range). If you need a bit more bass, you could replace it with a larger value capacitor such as a 1uF (1 micro-Farad) non-polarized capacitor. uF range capacitors are usually polarized electrolytics, but you need a non-polarized capacitor for AC signals.
http://en.wikipedia.org/wiki/High-pass_filter
The C2 capacitor is responsible for cutting off the higher frequencies of your signal. It's a "Low-pass" filter! Sometimes, this is a good thing for amplifiers/pedals that tend to produce feedback and noise. Removing C2 could make the amp unstable (feedback issues). However, C2 might be a bit large in value (e.g. 100nF) and this could cut off much of the signal's treble. If you feel the tone of your amp is a bit muddy, C2 could be one of the reasons why this happens.
C2 could be around 10-220pF (pico-Farads) to reduce radio frequency (RF) interference but higher value caps such as 10-100nF ones could reduce the treble frequencies of your guitar signal. It's like turning down the tone control knob of your guitar. Replacing a 100nF C2 capacitor with a 10nF one will alter the tone of your amp a lot and it will increase treble. If your need less treble, you should replace C2 with a higher value one. Typical values for C2 are a couple hundred pico-Farads or a couple nano-Farads. Rarely do we see uF capacitors as C2 (too much treble-cut).
http://en.wikipedia.org/wiki/Low-pass_filter
So, can I spot this divider/filter R-C circuit in other places other than the input stage of my amp? The answer is Yes, of course!
This is a really common circuit used in many amp/fx stages. It's also used inside most electric guitars and basses! R1 and R2 are essentially replaced by the Volume pot of your guitar while C1/C2 are replaced by the Tone pot and tone capacitor of the guitar.
Guitar wiring mods:
The principles are the same: replacing a 250kOhm volume pot with a 500kOhm or 1Mohm pot will increase the output of your guitar pickups a bit. Replacing a 250kOhm tone pot with a 500kOhm one will increase treble. Also, replacing a 68nF tone control capacitor with a 22nF one will increase treble too! Lower value pots or higher value tone-capacitors will decrease treble and volume(output).
If you're usually dialing down the tone and volume controls of your guitar, look up the term "treble bleed mod". It's an easy mod for retaining the treble and "brightness" of your pickups while turning down the volume and tone controls.
http://www.guitar-mod.com/rg_diag_treble_bleed.html
If you have all controls set to max. all the time, the treble bleed mod won't make much of a difference but changing the pots will!
Bypass capacitors
Treble bleed circuits can also be spotted inside many tube and older solid state amps between gain stages. There's usually a capacitor (value range: a few pF to nF) bypassing the "series" R1 resistor of a voltage divider just like the one we mentioned before. This capacitor is there to "boost" treble and it's sometimes connected to a "bright" or "top boost" switch. Removing this cap will make the amp sound darker. Replacing it with a slightly higher value cap, will boost the "mids" of the amp. If there's no cap bypassing the R1 series resistor, why not try a 100pF. If you like the result, you can experiment with the value of the "bypass" cap using the following on-line calculator (very useful stuff by the way):
http://www.ampbooks.com/home/classic-circuits/vox-ac30-preamp/
http://www.ampbooks.com/home/amplifier-calculators/cathode-capacitor/
How about Trimmers (aka Variable resistors)?
The same R-C voltage dividing circuit could be spotted inside your amp between amplification stages. For instance there might be an R1/R2 voltage divider circuit located between two opamps (operational amplifiers) or two 12ax7 tube stages. Replacing R1 with a lower value resistor and R2 with a higher value resistor, will increase the gain and maybe the volume of your amp. Bypassing R1 with a small capacitor (e.g.100pF) could provide your amp with a "treble boost". If you would like to experiment with different component values, you could use a set of variable resistors instead of fixed ones.
These variable resistors are called trimmers and they got 2 or 3 legs/pins. You will only need to use the two legs of a trimmer, its central pin and only one of its 2 outer legs. The 3rd leg won't be connected to anything and it should be taped to ensure it will not touch any part of the circuit.
The maximum resistance value of a trimmer is usually labeled on it. For instance a 100kOhm trimmer will have a max. value of 100kOhms. The lowest resistance value of a trimmer is always zero Ohms (short-circuit).
http://en.wikipedia.org/wiki/Trimmer_(electronics)
http://www.resistorguide.com/trimpot/
For example, a 470kOhm fixed 1/4Watt resistor could be replaced with a 500K trimmer set for max. resistance.
Always pre-set and measure the trimmer's value with your DMM before soldering it in place! A non-preset trimmer could short-circuit part of your amp and destroy it!...
Adding trimmers to the circuit will help you adjust and fine tune the tone of your amp. I would NOT recommend on-the-fly adjustments, even for low-voltage circuits. Also, do not replace high wattage resistors with trimmers. A 1/16, 1/8 or 1/4W resistor could be easily replaced with the right trimmer but higher wattage resistors are there for a reason: to dissipate heat that would burn out a simple trimmer.
There are simple linear or logarithmic trimmers and fine-tune (multi-turn) trimmers on the market. I would recommend using linear fine-tune trimmers. http://www.resistorguide.com/trimpot/
Cheers,
Thanos
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