While browsing through Mojo’s store of resistors, you may have found yourself scratching your head over what type of resistor to select for your purpose. Isn’t a resistor just a resistor— a pawn on the circuit board? Why so many different types? Well, they actually play a big role in the overall product of an amplifier circuit.
In the early days of electronic circuitry, carbon-composition resistors were really the only thing available besides wire-wound; this was current technology. Then, over time, with the progression of electronics came different ways of producing resistors. Eventually, we were provided with more options for cheap, like carbon-film, metal-oxide, and metal-film. So, when we crack open our beloved vintage amps, we usually see an army of cool-looking carbon-comp. resistors.
Due to the carbon-comps.’ technological antiquity, there are some inherent problems in its design, though many will slap my wrist for using the term “problems.” With any resistor, there exists a voltage coefficient of resistance. Basically, it describes the varying resistance when there is an increase or decrease in voltage across the resistor. Usually, the coefficient is negative, which means that resistance falls with an increase in voltage. If it is large enough, the inconsistency will result in resistor distortion. Carbon-comps. normally have a higher V.C.R. than newer types. Barth Electronics provides an article on V.C.R. which states:
The voltage coefficient varies with different resistive
materials, and seems to be greatest for materials that are composed
of a granular conglomeration of resistive material held together
with an insulating binder. Carbon composition and cermet film
resistors use these types of resistive materials.
These days, the difference may not be much, but in the carbon-comp. days, it was enough to create audible distortion. When you see “new production” carbon-comp. resistors, that means that generally, they are going to perform a bit better in regards to the manufacturer’s specifications because of advancements in production. Included in those specifications is the V.C.R. rating of the resistor. Today, you may find the rating to be 0.005%/V compared to the carbon-comps. of the vintage era to be around 0.035%/V.
This particular “problem” with carbon-comp. resistors is one of the reasons that amp designers do continue to use them. Resistor distortion causes harmonics which can be pleasant to the ear. When used in the right areas of the amp, this “problem” can be used to your advantage. By utilizing carbon-comps. as plate resistors on the output where the voltage is high, the second harmonic coloring is maximized. Here, a well designed front end will deliver the signal to the plates of the output where the carbon-comp. can add some style. On the other hand, by placing a carbon-comp. early on in the signal path can create unwanted noise which is amplified by the output stage.
Noise is another issue to consider when choosing resistors because all resistors produce noise. Do they still seem trivial? Wire-wound resistors are the least noisy followed by metal-film, metal-oxide, and carbon-film. The noisiest— carbon comp. So, if you are concerned with noise, use metal-film resistors, especially in the input stage. Aiken Amps has a great article that goes into detail about the different types of noise that a resistor can produce. Check out the link below.
Another area of the amp circuit that should not contain carbon-comp. resistors is the power supply. Since most carbon-comp. resistors new and old are not UL94-VO protected, which is a flameproof coating, they should stay away from the B line.
One more thing to consider is the power rating of the resistor. You would think that ideally the larger the better, right? Well, it is true that larger wattage resistors with larger geometries produce less noise than smaller resistors, but at the same time, their V.C.R. tends to be larger as well. So, it is best to use a power rating that is large enough for the application without being much larger than is necessary.
Okay, so hopefully you’re a little more comfortable with what resistors to order. Basically, if you want to build a vintage replica, use carbon-comp. throughout. Remember, there’s a reason we like vintage amps; they were noisy and full of character. If you’re building an amp from scratch, use newer metal-film or metal-oxide in the input stages, and experiment with carbon comp. flavoring in the high voltage areas of the output. Finally, find a good median between the resistor’s power rating and what is required for the application. There is no real benefit of using high rated resistors everywhere in the amp.
Here’s some good reading on the subject:
Electronics Tutorials: Resistor Types
Aiken Amps: Resistor Types— Does it Matter?
Barth Electronics: Voltage Coefficient of Resistance Application Note
R.G. Keen: Using the Carbon Comp Resistor for Magic Mojo