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.