Swapping Tone-Stack Resistors
Another way to alter your amp’s frequency response is to swap the slope resistor for one of another value. In this picture of our Twin, it’s the one with brown, black, yellow, and silver bands being gripped by one lead with needle-nose pliers.
The part of an amp’s circuit that governs the ranges of its tone controls is known as the tone stack. This part of the circuit is most commonly a combination of three potentiometers (for bass, mid, and treble knobs), three capacitors, and a resistor called the slope resistor. One simple mod that will change the tonal character of your amp is to experiment with the value of the slope resistor, which controls how frequencies are divided over each tone control. Simply put, the slope resistor changes the slope of the midrange dip if it were charted on a frequency-response chart.
Typical slope-resistor values range from 33 kΩ to 100 kΩ. A larger value yields a sound with more of a midrange scoop (i.e., where treble and bass frequencies are louder than the mids). Smaller values accentuate midrange. In our Twin Reverb, the vibrato channel’s slope resistor is the 100 kΩ one (with brown, black, and yellow rings) attached to a 100 kΩ resistor on one end and two blue .1 μf coupling capacitors on the other. To change the value of the slope resistor, follow the previous instructions on how to replace a resistor.
Removing the Bright Cap to Tame Harsh Treble
To tame treble response in a Marshall head, simply clip or desolder the bright cap on the volume pot. In case you decide to reverse the mod in the future, make sure you leave as much of the capacitor’s leads intact (if you decide to clip it) to facilitate easy reinstallation.
If your amp has a treble response that feels too harsh to your ears—especially at lower volumes—you can tame it by removing the bright cap. In a Marshall amplifier such as a Super Lead, you simply remove the capacitor that lies across two legs of the volume pot. This cap allows the high frequencies in the guitar signal to bypass being attenuated by the taper of the volume pot, so removing this cap eliminates the amp’s severe-sounding highs at lower volumes.
To remove a bright cap, simply desolder the leads or clip them at a point near the lugs on the pot. Be sure to leave enough lead on the cap so that, if you later decide to reinstall it, there will be enough length left to be able to solder it back into place.
Adding Shielded Wire to Reduce Noise
If your amp has a lot of hiss and background noise, you may want to check and see if the wire connecting the input jack to the grid of the first preamp tube are made with unshielded wire. If so, replacing it with shielded wire should decrease noise. Here, we’re stripping the shielding from one lead prior to soldering the connection, then tinning the gathered shielding lead that we’ll solder to the input-jack side.
Our final project here is a mod that will subdue hiss or unwanted background noise in your amp. A lot of the time when an amp is plagued with this malady, it’s because it uses unshielded wiring in key sections of the circuit. Strategically replacing these lengths with shielded wire is a fast, easy way to improve the amp’s noise floor.
Perhaps the best place to start adding shielded wire is the section going from the amp’s 1/4" input jack to the grid of the first preamp tube. The grid in question for a 12AX7/ECC83 or 12AT7/ECC81 tube socket will be pin number 2. Any noise picked up in this part of the signal path is passed through each of the amp’s gain stages, getting amplified each time, so adding a shielded wire here should yield significant noise reduction.
To perform this mod on an amp like our
• Snip the lead or desolder the wire where it attaches to the input jack. (A standard soldering iron will work for desoldering, but a solder sucker/ desoldering pump will create a cleaner joint for the new connection by removing excess solder.)
• Snip or desolder the other lead where it attaches to the grid pin of the preamp tube. The grid on a 12AX7 will be pin 2 or 7
• Solder the two leads from a length of new shielded wire to the newly vacated spots.
Ground the new wire by soldering the shielding on the input-jack side to the ground on the input jack. On a vintage Fender-style amp, this is the lug that is making contact with the chassis. Only ground this shield on one end.
Go Forth and Mod
I hope you’ve found some modifications here that seem like projects worth pursuing on one of your amps. Although these projects yield pretty significant and impressive results considering how little work is involved, I know it can be pretty daunting to poke around inside a device with significant safety risks for the first time. The safety measures we’ve outlined should alleviate any danger, however if you have any doubts about your ability to pull these off, it’s always better to be safe than sorry. But even if you decide to have a qualified tech execute these mods for you, at least this information will give you a better understanding of some of the nuances and possibilities of guitar amp modifying.