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Left: You can alter your amp’s headroomby swapping out the first preamp tube in itsfirst gain stage—typically the small tube furthestfrom the power tubes. In this pictureof a Fender Twin Reverb amp chassis, thepower amp tubes are the two large glassbottle-like things at far left, which meansthe first preamp tube of the first gain stageis the small valve at far right. The phaseinverter preamp tube is the third from left.
Right: A 12AX7 preamp tube (aka ECC83,left) typically has a gain rating of 100and yields more distortion, while a 12AT7(ECC81) has a cleaner gain rating of 70.
Amp headroom can also be adjusted by swapping the resistor in anegative-feedback circuit for a different value. Here, the resistor ringedwith gray, red, brown, and silver value marks is being desoldered, onelead at a time, to make way for another.Warning!
Left: Common tubeampcapacitor types.
Middle: Before touching anythinginside the chassis of a tube amp,bleed off any lingering fatal voltagesbeing stored inside by attachingone end of a 100 kΩ resistor(inside the black shrink wrap in themiddle of the green wire) to groundand touching the other end to thepositive side of each electrolyticcap in the circuit (the blueones) for a full minute each.
Right: To confi rm thatvoltage has been discharged,measure each cap with a voltmeterset to DC voltage andmake sure none is detected.Touch the black lead to thechassis, and the red lead tothe positive cap terminal.
The phase inverter in a 5E7 is a cathodyne type, not the more common long-tail pair. It uses only one triode. It operates at unity gain, so there is no real advantage to be had by using a high-gain tube.
It’s in a guitarist’s nature, I believe,that we can’t leave well enough alone.Most of us have an ideal sound (orsounds) in our heads, and we won’t restuntil our vision is realized. We can havea perfectly fine guitar or amplifier, butwe still have an inherent urge to tinkerwith it until it’s “just right” in feel ortone. On this premise—as well as thefact that many of us are on budgets thatdon’t allow us to buy every amp thatstrikes our fancy—the idea of modifyingan amp we already own strikes avery appealing chord for many players.
Of course, before beginning anysort of amp modification, you’ve gotto pinpoint exactly what you wantto accomplish. And you have to keepin mind that an amp is full of manyparts that interact with and affectone another, so even small changesto any of these parts can yield majordifferences in tone and performance.However, this exponential effect thatsmall changes can have on tone meansthere are many relatively easy ways inwhich even inexperienced but adventurousDIYers can mod their amp.
Here we present eight short projectsthat pretty much anyone with rudimentarysoldering skills can tackle.Even better, the mods we’re detailinghere are all reversible. So if they don’tsuit your fancy or you need to returnyour amp to its stock circuitry (forexample, to sell it), you can do sowithout much trouble.
No job can be donewell without theproper tools—in fact,attempting to do sousually results in anightmare of frustration.For the modswe’re exploring here,I recommend thefollowing tools:
• Standard-sizePhillips and/or flatscrewdrivers (forre-moving and securingthe chassis)
• Wire cutters/strippers
• 25–40-wattsoldering iron
• Acid-free rosincore solder
• Safety goggles
• Needle-nosed pliers
• A copy of youramp’s circuitryschematic
• Wire cutters/strippers
• 25–40-wattsoldering iron
• Acid-free rosincore solder
• Safety goggles
• Needle-nosed pliers
• A copy of youramp’s circuitryschematic
Mod 1:
Swap Preamp Tubes to Adjust Headroom
Swap Preamp Tubes to Adjust Headroom
One of the most common things guitaristsrequest from us at our shop (schroederaudioinc.com) is the ability to get moreor less headroom—either cleaner tones athigher volumes or more overdrive or distortionat lower volumes. Let’s begin bylooking at some simple ways to alter youramp’s headroom.
Left: You can alter your amp’s headroomby swapping out the first preamp tube in itsfirst gain stage—typically the small tube furthestfrom the power tubes. In this pictureof a Fender Twin Reverb amp chassis, thepower amp tubes are the two large glassbottle-like things at far left, which meansthe first preamp tube of the first gain stageis the small valve at far right. The phaseinverter preamp tube is the third from left.
Right: A 12AX7 preamp tube (aka ECC83,left) typically has a gain rating of 100and 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 firstgain stage(s) of an amp. It’s usually a12AX7 (aka an ECC83 in Europe andabroad), and it’s the small tube locatedfarthest from the larger power tubes.Typically, a 12AX7 has a gain rating of100. One simply way to achieve moreheadroom in your amp is to replacethis tube with a 12AT7 (aka ECC81),which has a gain rating of about 70 andwill yield cleaner sounds than a 12AX7.Conversely, players who have an ampwith a 12AT7 in the first gain stage canget more gain and overdrive from theiramp by swapping it for a 12AX7.
Amp headroom can also be adjusted by swapping the resistor in anegative-feedback circuit for a different value. Here, the resistor ringedwith gray, red, brown, and silver value marks is being desoldered, onelead at a time, to make way for another.
You can further alter your amp’s headroomby simply changing its phase inverter,which is the preamp tube located right nextto the power tubes. It sends the signal fromthe preamp into the power amp, and swappingit with one that has a higher or lowergain rating (i.e., a 12AX7 vs. a 12AT7) willalso adjust the amount of gain being sent tothe amp’s power tubes.
Middle: Before touching anythinginside the chassis of a tube amp,bleed off any lingering fatal voltagesbeing stored inside by attachingone end of a 100 kΩ resistor(inside the black shrink wrap in themiddle of the green wire) to groundand touching the other end to thepositive side of each electrolyticcap in the circuit (the blueones) for a full minute each.
Right: To confi rm thatvoltage has been discharged,measure each cap with a voltmeterset to DC voltage andmake sure none is detected.Touch the black lead to thechassis, and the red lead tothe positive cap terminal.
All amplifiers contain lethal voltages.If after reading through thisentire article you still feel unsureof your capabilities, please refrainfrom performing any modifi cationto your amp. If you decide toproceed, make certain the amp isunplugged and that all tubes havebeen removed before beginning.Next, remove the amp chassisfrom the box it is housed in andturn it upside down so the circuitryis exposed and easy to work on.
The most dangerous voltagesin an amp are stored in electrolyticcapacitors, even after the amp hasbeen unplugged from the wall. It’simperative that these capacitorsare discharged before proceedingwith any work on the amp.The best way to do this is withan alligator clip wire with a 100Kresistor in series to ground. Clipone end of the wire to ground andthe other end to the positive sideof each electrolytic capacitor. Thiswill bleed off any voltage that maybe stored in the capacitor. To becertain all voltage is discharged,use a voltmeter set to DC voltage.After about a minute, the capacitorsshould be fully discharged. Ifyou are unsure of this procedure,consult your local amp tech.
Homebrews > • Phase inverters
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