6/19/2023 0 Comments Plate resistor signal path![]() ![]() ![]() However, longer exposure to high currents and voltages will definitely cause the resistor to burn up. For this reason, they are sometimes used for applications where surges are expected. They are more likely than film resistors to survive brief exposure to large currents - i.e., that they can dissipate large amounts of power in excess of their nominal rating - as long as the power surge doesn’t last for more than an instant. On the plus side, carbon composition resistors are very tolerant of large transient currents. They’re also susceptible to moisture damage, which can cause their resistance to fluctuate further. Their tolerance is typically 20%, and their resistance can change based on environmental conditions. In addition to being noisy, carbon composition resistors are inaccurate and relatively unstable. Other problems with carbon composition resistors. ![]() However, if a carbon composition resistor tests well and the circuit isn’t noisy, we’ll usually leave them alone. When restoring a vintage amplifier, we may replace certain carbon composition resistors with new metal film resistors, if we feel that it will benefit the circuit. Still, we appreciate the history and aesthetics of carbon composition. We primarily use low-noise metal film resistors in our new amp builds. Sometimes, when we are repairing a vintage amp, we’ll swap out a couple of early signal path carbon composition resistors to achieve this. But because low-noise resistors are mounted at the critical points, such as the tube grids where resistor noise is most likely to be amplified, the amp will sound cleaner. This will make the circuit look more period-correct, and will also introduce a minor amount of resistor hum. Or, you may see the opposite: a circuit with modern resistors in sensitive locations, such as the early signal path, and carbon composition resistors elsewhere. The rest of the circuit can be composed of modern resistors, which have the benefit of smaller size, closer tolerance, or better power-handling qualities. This inserts the carbon resistor into a place in the circuit where it will have the most impact, because its noise will be amplified by the gain stage. In other cases, you’ll see carbon composition resistors only at critical points in the signal path, such as at the grid of the input triode. To emulate that, some builders will use carbon composition resistors exclusively. Vintage amp manufacturers did not use carbon resistors for tone: they used them because they were the only low-wattage resistors available. Occasionally, you’ll see them throughout an amp - even in new builds. There are a couple of ways to use carbon composition resistors. Other people prefer a cleaner sound, and use modern styles of resistors instead, such as low-noise metal film resistors. Some people like the subtle lo-fi warmth that carbon resistors offer. Because these resistors are so noisy, and there are so many of them in the circuit, carbon resistors are an audible factor in a vintage amp’s tone. But is this really a bad thing? Yes and no.īefore 1970 or so, all amplifiers used carbon composition resistors. Carbon composition resistors produce the most noise. However, depending in their material composition and shape, resistors may produce other types of noise as well. All resistors produce Johnson (thermal) noise, a byproduct of the fact that resistors dissipate heat. Carbon composition resistors are those brown cylindrical resistors that you’ll see in most amps made before 1970. ![]()
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