How our columnist’s risky purchase turned out to be a dusty pre-CBS jewel.
This month, I’d like to share the story of my 1964 Fender Vibrolux Reverb. It was a really risky purchase that had some big surprises.
In October 2011, a black-panel Vibrolux Reverb appeared on eBay with a short bid time. It was poorly described with miserable pictures and barely any details or description of condition and origin. Normally I walk away from such auctions, but there was something that caught my eye. First, some red on the speaker labels led me to believe they were perhaps OEM Jensens. And while the amp’s faceplate was unreadable, I thought I saw a long pattern of four words with a very short last word, as in “Fender Electrical Instruments Co.,” and not the more common “Fender Musical Instruments.” What if this was a 1964–65 pre-CBS amp and no one else recognized it? In the automated eBay watch-and-bid sniper tool I used back then, I set up a $2,500 max bid to be placed 10 seconds before the auction ended. When I woke up the next morning, I had bought it for $1,860. I felt both happiness and regret. What had I gotten into?
When the amp arrived in Oslo several weeks later, I was thrilled to see an all-original 1964 Vibrolux Reverb with Jensen C10N speakers—highly desirable among Fender amp players and collectors. I pulled out the chassis and noticed a well-preserved circuit board, with the death cap wired to the ground switch and a non-grounded two-prong power cord. The brown electrolytic Mallory DC and filter caps looked surprisingly nice and were not leaking. The resistors on the power tube sockets also seemed to be in good shape. It even had factory-original RCA tubes.
When I woke up the next morning, I had bought it for $1,860. I felt both happiness and regret. What had I gotten into?
If you see a leak on a 30-year-old electrolytic capacitor, I strongly recommend replacing it. Old electrolytic caps can mean little clean headroom and farty bass, since they can’t hold the required DC voltage when you strike a chord and the massive current starts flowing through the power circuitry to the tube plates. But I decided to not replace any tubes, caps, or resistors before testing the amp. And the grille? Wow! I don’t think I have ever seen such a dark brown—and nice—piece of cloth, with just minor rifts.
I uninstalled the speakers and noticed the cones were marinated with a thick layer of dirt, dust, and smoke particles, probably from a long life in smoky clubs and bars. I screwed them back on the baffle without cleaning them. The wood was still whole and robust, but the Tolex had many scars and cigarette burns, and the faceplate and knobs indicated heavy but not rough usage. Surprisingly, the pots rotated very smoothly. All this indicates that an amp has been played on a regular basis. It looked like a true warrior.
I found a 230/110V step-down transformer and flipped power and standby on for a 15-second interval. Without proper grounding, I was careful to not touch any other electrical equipment in the room, since you don’t know what voltage guitar strings might carry when connected to a non-grounded amp. I expected the regular background noise—scratchy pots and pop and crackle from bad tubes—but the amp was dead quiet! I stroked a heavy E chord and got a loud, mellow, and very dark and midrange-y tone. I flipped the bright switch on and increased the treble to 5, which is normally an extremely bright setting on Fender amps.
The dusty speaker cones on these old and inefficient speakers filter out the sharp treble—a truly desirable feature in vintage amps. They really make your guitar and pedals sound smoother and creamier, and this was the darkest sounding Fender amp I have ever come upon. What makes the Vibrolux Reverb so good is the balance between the attack and responsiveness of the lightly driven 10" speakers, and the compression from the smaller power and output transformers. I think Fender nailed it with the size, weight, and power of this 35-watt, dual-6L6GC creation.
Later, I installed a grounded power cord and disabled the death cap and ground switch. I got a 230V high-quality power transformer from Mercury Magnetics. It’s 10 years later, and the amp has, incredibly, never failed me. I play it at carefully selected gigs with the original speakers, tubes, and caps still in place. Someday I might consider installing a 25k mid switch or pot on the back in the ground switch slot. This is a must-have and easily reversible mod for Fender amps lacking a mid-pot. It makes them break up much sooner, with a crunch outside the clean Fender tone borderline.
An important point of this story is that we can’t typically expect this kind of luck with vintage amps. Some maintenance is usually required and will make an amp more reliable and durable. Be sure the electrolytic caps are in good condition, and always bring spare tubes to gigs and practices, or bring a backup amp.
64 Fender Vibrolux Reverb vs 65 Super Reverb speakers
Removing or replacing a single component in your amp can have significant impacts on both its tonal character and the amount of gain or headroom on tap. Here we guide you through several easy projects you can do in relatively little time with a few basic tools.
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.
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.
Swap Negative-Feedback Circuit Resistors to Adjust Headroom
Another way to increase your amp’s headroom is to adjust the size of the negativefeedback resistor. Because the earliest tube guitar amps from the 1950s weren’t intended to overdrive (though it wouldn’t be long before rock ’n’ roll pioneers harnessed the glorious sound), the negativefeedback circuit was implemented as a way to reduce distortion. It does so by taking a very small signal from the amp’s output and injecting it back into the gain stage— only it’s out of phase with the output. This causes phase cancellation and affects the amp’s overall gain character.
The negative feedback resistor located off of the amplifier’s output jack. Decreasing its value will increase your amp’s overall headroom. In the photo above, the feedback resistor is located between the top two blue coupling capacitors—it’s the component with (left to right) gray, red, brown, and silver bands on it, and one of its leads is being gripped by needle-nose pliers. (For complete information on how to read resistor color codes, visit wikipedia.org and search for the “Electronic color code” entry.)
To remove the current resistor and
install a new one:
• If you have a soldering iron that lets you set exact temperature, set it for between 700 and 800 degrees Fahrenheit.
• Heat the solder joint on one end of the feedback resistor and gently lift it out of the circuit, then do the other.
• Bend the tips of the new resistor’s leads to fit neatly in the two vacated solder joints.
• Snip off excess length on the leads of the new resistor.
• Heat one of the solder joints and put one end of the resistor in place, and then proceed to the other solder joint.
• Add a bit of solder to the new solder joints so that there’s a solid connection.
• Repeat the steps above with different value resistors until you are satisfied with the increase or decrease in headroom.
Swap the Cathode Resistor to Adjust Headroom
Shown here is our Fender Twin Reverb. Its 1.5k Ω cathode resistor is marked by the brown, green, red, and silver bands.
Adjusting the value of the resistor connected to the cathode (the main filament-like part that forms the core of a vacuum tube) of any of the gain-stage preamp tubes can greatly affect the overdrive capabilities and headroom. The bias of a preamp tube— how much voltage is running through it— occurs in the tube’s cathode.
Not all amps have a cathode resistor, but when they do, it’s wired in parallel with a cathode capacitor—which can also be swapped out for one with a different value to increase or decrease headroom (see Mod 4, below, for more on this).
Generally, the range of values for the cathode resistor is 820 ohms (Ω) to 10 kΩ, but the most common value is 1.5 kΩ. Decreasing the value causes the tube to bias hotter, which in turn causes the tube to overdrive quicker, yielding a hairier tone due to the increase in gain. It follows that increasing the value of the cathode resistor causes the tube to bias cooler, lowering the gain of the tube and thus increasing clean headroom. To change the value of the cathode resistor, refer to the steps in the Mod 2: Swap Negative- Feedback Circuit Resistors to Adjust Headroom section.
Swap the Cathode Capacitor to Adjust Headroom
To increase or decrease gain, you can swap out the cathode capacitor (here, it’s the black component with green writing) with one of a different value—a lower value for more gain, higher for more dirt.
As mentioned above, the cathode capacitor also has a significant effect on an amp’s available gain. The larger the value of the cathode capacitor, the more low end is accentuated in that gain stage. The smaller the value of the cathode cap, the more high end is accentuated. The typical range of cathode capacitor values is anywhere from .68 μf to 250 μf. A typical cathode cap value in lower-gain amps (including the Fender Twin we’ve been working on here) is 25 μf. In higher-gain amps such as a Marshall Super Lead, you would expect to see a cap value of .68 μf. The reason higher gain amps use cathode caps with such small values (especially in the early gain stages) is to tame the potential for too much bass to be amplified—which could result in the amp sounding too muddy when pushed into overdrive.
Some amplifiers—including old Supros and Magnatones—do not have cathode caps on the first gain stage(s). You can increase the gain of these amps by adding a cathode capacitor in parallel with the cathode resistor of that gain stage. To change the value of this cathode capacitor, follow the rules for changing a resistor in the two previous sections.
Swapping the Coupling Capacitors to Adjust Bass Response
To alter bass response, you can swap coupling caps for different values. In our Twin Reverb example, the coupling caps are the two blue cylinders at the end of the circuit board (closest to the power tubes).
The second most common request we get at our shop is to change the overall tonal character of an amplifier. As with changing an amp’s gain, small changes in the circuit can greatly affect the tone.
If you’re looking to get more (or less) bass out of your amp, its coupling caps—which act as frequency filters—are great candidates for modification. Coupling capacitors typically have values from .022 μf to .1 μf. The purpose of coupling caps is to block DC voltage and can be found in several places in the circuit. The specific ones that we’ll be dealing with are situated between the phase inverter plates and the power-tube grids. Smaller values such as .022 μf attenuate the bass in the preamp, preventing it from being passed into the power amp section. Larger values such as .1 μf allow more bass to pass through. In a bass amp, you may see up to .47 μf.
Naturally, the idea when modifying coupling capacitors is to get the great bass response you desire without causing the amp to sound too boomy. High-gain amps typically have a smaller value than clean amps for this reason.
Coupling caps are rarely electrolytic and will therefore function without regard to polarity. That said, certain types of coupling caps—including film and paper-in-oil varieties—may yield small sonic differences depending on the direction of travel.
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.