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May 2014
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8 Guitar-Amp Mods for Newbies

8 Guitar-Amp Mods for Newbies

It’s in a guitarist’s nature, I believe, that we can’t leave well enough alone. Most of us have an ideal sound (or sounds) in our heads, and we won’t rest until our vision is realized. We can have a perfectly fine guitar or amplifier, but we still have an inherent urge to tinker with it until it’s “just right” in feel or tone. On this premise—as well as the fact that many of us are on budgets that don’t allow us to buy every amp that strikes our fancy—the idea of modifying an amp we already own strikes a very appealing chord for many players.

Of course, before beginning any sort of amp modification, you’ve got to pinpoint exactly what you want to accomplish. And you have to keep in mind that an amp is full of many parts that interact with and affect one another, so even small changes to any of these parts can yield major differences in tone and performance. However, this exponential effect that small changes can have on tone means there are many relatively easy ways in which even inexperienced but adventurous DIYers can mod their amp.

Here we present eight short projects that pretty much anyone with rudimentary soldering skills can tackle. Even better, the mods we’re detailing here are all reversible. So if they don’t suit your fancy or you need to return your amp to its stock circuitry (for example, to sell it), you can do so without much trouble.

Essential Tools

No job can be done well without the proper tools—in fact, attempting to do so usually results in a nightmare of frustration. For the mods we’re exploring here, I recommend the following tools:

• Standard-size Phillips and/or flat screwdrivers (for re-moving and securing the chassis)
• Wire cutters/strippers
• 25–40-watt soldering iron
• Acid-free rosin core solder
• Safety goggles
• Needle-nosed pliers
• A copy of your amp’s circuitry schematic

Mod 1:
Swap Preamp Tubes to Adjust Headroom

One of the most common things guitarists request from us at our shop (schroederaudioinc. com) is the ability to get more or less headroom—either cleaner tones at higher volumes or more overdrive or distortion at lower volumes. Let’s begin by looking at some simple ways to alter your amp’s headroom.

Left: You can alter your amp’s headroom by swapping out the first preamp tube in its first gain stage—typically the small tube furthest from the power tubes. In this picture of a Fender Twin Reverb amp chassis, the power amp tubes are the two large glass bottle-like things at far left, which means the first preamp tube of the first gain stage is the small valve at far right. The phase inverter preamp tube is the third from left.
Right: A 12AX7 preamp tube (aka ECC83, left) typically has a gain rating of 100 and yields more distortion, while a 12AT7 (ECC81) has a cleaner gain rating of 70.

The first preamp tube (aka “valve”) in an amp’s circuit is used in its first gain stage(s) of an amp. It’s usually a 12AX7 (aka an ECC83 in Europe and abroad), and it’s the small tube located farthest from the larger power tubes. Typically, a 12AX7 has a gain rating of 100. One simply way to achieve more headroom in your amp is to replace this tube with a 12AT7 (aka ECC81), which has a gain rating of about 70 and will yield cleaner sounds than a 12AX7. Conversely, players who have an amp with a 12AT7 in the first gain stage can get more gain and overdrive from their amp by swapping it for a 12AX7.

Amp headroom can also be adjusted by swapping the resistor in a negative-feedback circuit for a different value. Here, the resistor ringed with gray, red, brown, and silver value marks is being desoldered, one lead at a time, to make way for another.

You can further alter your amp’s headroom by simply changing its phase inverter, which is the preamp tube located right next to the power tubes. It sends the signal from the preamp into the power amp, and swapping it with one that has a higher or lower gain rating (i.e., a 12AX7 vs. a 12AT7) will also adjust the amount of gain being sent to the amp’s power tubes.

Left: Common tubeamp capacitor types.
Middle: Before touching anything inside the chassis of a tube amp, bleed off any lingering fatal voltages being stored inside by attaching one end of a 100 kΩ resistor (inside the black shrink wrap in the middle of the green wire) to ground and touching the other end to the positive side of each electrolytic cap in the circuit (the blue ones) for a full minute each.
Right: To confi rm that voltage has been discharged, measure each cap with a voltmeter set to DC voltage and make sure none is detected. Touch the black lead to the chassis, and the red lead to the positive cap terminal.

All amplifiers contain lethal voltages. If after reading through this entire article you still feel unsure of your capabilities, please refrain from performing any modifi cation to your amp. If you decide to proceed, make certain the amp is unplugged and that all tubes have been removed before beginning. Next, remove the amp chassis from the box it is housed in and turn it upside down so the circuitry is exposed and easy to work on.

The most dangerous voltages in an amp are stored in electrolytic capacitors, even after the amp has been unplugged from the wall. It’s imperative that these capacitors are discharged before proceeding with any work on the amp. The best way to do this is with an alligator clip wire with a 100K resistor in series to ground. Clip one end of the wire to ground and the other end to the positive side of each electrolytic capacitor. This will bleed off any voltage that may be stored in the capacitor. To be certain all voltage is discharged, use a voltmeter set to DC voltage. After about a minute, the capacitors should be fully discharged. If you are unsure of this procedure, consult your local amp tech.

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