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Supro Montauk 15-watt 1 x 10-inch Tube Combo Amplifier - Blue Rhino Hide Tolex with Silver Grille
This month’s mod Dan’s uses a 500k linear pot, a 1.5H inductor (L) with a 0.039 µF (39nF) cap (C), and a 220k resistor (R) in parallel.
Don’t let the name fool you, this is a purely passive device—nothing is going to be boosted. In general, you can’t increase anything with passive electronics that isn’t already there. Period. But you can reshape the tone by deemphasizing certain frequencies and making others more prominent (so … “boost” in guitar marketing language). Removing highs makes lows more apparent, and vice versa. In addition, the use of inductors (which create the magnetic field in a guitar circuit) and capacitors will create resonant peaks and valleys (bandpasses and notches), further coloring the overall tone. This type of bandpass filter only allows certain frequencies to pass through, while others are blocked, and it all works at unity gain.
“You can’t increase anything with passive electronics that isn’t already there … but you can reshape the tone by deemphasizing certain frequencies and making others more prominent.”
All the systems I mentioned above are doing more or less the same thing, using different approaches and slightly different component values. They are all meant to be updated tone controls. Our common tone circuit is usually a variable low-pass filter (aka treble-cut filter), which only allows the low frequencies to pass through, while the high frequencies get sent to ground via the tone cap. Most of these systems are LCR networks, which means that there is not only a capacitor (C), like on our standard tone controls, but also an inductor (L) and a resistor (R).
In general, all these systems are meant to control the midrange in order to scoop the mids, creating a mid-cut. This can be a cool sounding option, e.g. on a Strat for that mid-scooped neck and middle tone.
Dan Torres offered his “midrange kit” via an internet shop that is no longer online, same with his business website. The Torres design is a typical LCR network and looks like the illustration at the top of this column.
Dan’s design uses a 500k linear pot, a 1.5H inductor (L) with a 0.039 µF (39nF) cap (C), and a 220k resistor (R) in parallel. Let’s break down the parts piece by piece:
Any 500k linear pot will do the trick, in one of the rare scenarios where a linear pot works better in a passive guitar system than an audio pot.
(C) 0.039µF cap: This is kind of an odd value. Keeping production tolerances of up to 20 percent in mind, any value that is close will do, so you can use any small cap you want for this. I would prefer a small metallized film cap, and any voltage rating will do. If you want to stay as close as possible to the original design, use any 0.039 µF low-tolerance film cap.
(L) 1.5H inductor: The original design uses a Xicon 42TL021 inductor, which is easy to find and fairly priced. This one is also used in the Bill Lawrence Q-Filter design, the Gibson standard Varitone, and many other systems like this. It’s very small, so it will fit in virtually every electronic compartment of a guitar. It has a frequency range of 300 Hz up to 3.4 kHz, with a primary impedance of 4k ohms (that’s the one we want to use) and a secondary impedance of 600 ohms. Snip off the three secondary leads and the center tap of the primary side and use the two remaining outer primary leads; the primary side is marked with a “P.” On the pic, you can see the two leads you need marked in red, all other leads can be snipped off. You can connect the two remaining leads to the pot either way; it doesn’t matter which of them is going to ground when using it this way.
Drawing courtesy of singlecoil.com
(R) 220k: use a small axial metal film resistor (0.25 W), which is easy to find and is the quasi-standard.
Other designs use slightly different part values—the Bill Lawrence Q-filter has a 1.8H L, 0.02 µF C and 8k R, while the old RA Gresco Tone Qube from the ’80s has a 1.5H L, 0.0033 µF C, and a 180k R, so this is a wide field for experimentation to tweak it for your personal tone.
This mid-cut system can be put into any electric guitar not only as a master tone, but also together with a regular tone control or something like the Fender Greasebucket, or it can be assigned only to a certain pickup. It can be a great way to enhance your sonic palette, so give it a try.
That’s it! Next month, we’ll take a deeper look into how to fight feedback on a Telecaster. It’s a common issue, so stay tuned!
Until then ... keep on modding!