Using the Carbon Comp Resistor for Magic Mojo
Copyright 2002 R.G. Keen. All rights reserved. No permission for local copies or serving from pages other than http://www.geofex.com.
|The carbon composition (CC) resistor has been extolled as
the paragon of pure tube tone by almost everyone who has a stake in
vintage sounding amplifiers and effects. It's reputed to be imbued with
almost mystical powers to make toneless, lifeless guitar signals take on
tonal advantages that only Zeus from Mount Olympus might achieve on his
Is this real? Are Carbon Comps really magic tone mojo?
Maybe. Like everything else, there's the hype, and then there's the real world. A good maxim to remember about electronics is that if you can't express it in numbers (that are also measurable by someone else, not just made-up numbers... there are those around too 8-) then you really don't know the thing at all, you're only believing the myth.
The vintage amps we all love had CC's in them, and they certainly
have their share of mojo, but the makers of those amps in the 50's and
60's used them because that's what was available. Today we have lots of
resistor options. What's different about carbon comp, and can we express
it in numbers so we don't keep being superstitious?
1. high voltage across the resistor is necessary, in the range of
100V on up
Guidelines 1 and 2 are simply the recognition that the voltage coefficient of resistance is not very big. In fact, although the coefficient is small, it was specified to be small by the makers and controlled tightly, indicating that it was a recognized problem. In the Radiotron Designer's Handbook ( 4th edition, pg. 1345) they list the JAN-R-11 specification for CC resistors as less than 0.035% per volt for 1/4 and 1/2W resistors, and 0.02% per volt for higher power ratings. Given that the max voltages for these parts was 1/4W- 200V; 1/2W - 350V; 1W and 2W - 500V, that works out to a 7% change in resistance for a 1/4 W part used at its max voltage, a 12.3 % change for a 1/2W, and a 10% change for bigger resistors. That's one of the thrusts of guideline 4 - pick the smallest dissipation resistor you can, to maximize the coefficient.
Of course, that's as big as the effect can get, and you would have to carefully set up the situation to get that much resistor distortion. In an amp, you probably won't be able to get that close to max voltages or signal levels. Realistic levels might be 200V across a 1/2W resistor, and a 75V signal swing. That would give you a 2.6% distortion - enough to be audible as sweetening. That's the point of guideline 3 - you have to have a big enough signal swing across the resistor to have the signal distorted significantly by the voltage coefficient.
But with a 10V signal, you only get 0.35% distortion, and it starts down the slippery slope to inaudibility. More importantly, these percentages represent the maximum beyond which a resistor would have been rejected in the 1950's. Today's CC resistors are much lower distortion. From IRC's web site, we find some numbers. A typical resistor voltage coefficient can be seen at http://www.irctt.com/pdf_files/IBT.pdf - which shows carbon comp at 0.005%/volt for that company's products. Another was 0.008%/V. These are smaller than the max allowed under the JAN military spec.
So where do they work best? Where can we use CC's for their soft distortion, and where can we sidestep them to lower noise?
First, they do no good and lots of noisy bad where the signal level is small and the following amplification is high - a classical description of an input stage. The input to an amp should probably have a metal film plate resistor to minimize noise. Grid resistors in all but output stages also do no good, because the signal level is typically too low. A 12AX7 can be driven from cutoff to positive grid voltage with a couple of volts of signal, so the grid resistor never has a big enough signal to be distorted appreciably.
Cathode resistors are another poor use of CC. They typically only have a few volts across them, and they're often decoupled with a capacitor, both of which would minimize the resistor distortion. In cathode followers, there can be substantial DC and signal voltage across a cathode resistor, but in this case, the resistor is driven by the low impedance of the cathode, and the voltage across the resistor is controlled by the grid voltage very tightly, so the exact resistor value doesn't matter much - there won't be significant distortion.
The place to use CC's is where there's big signal - plate resistors, and ideally the stage just before the phase inverter. The phase inverter would otherwise be ideal, with plate resistors carrying the highest signal voltage in the amp, but phase inverters are often enclosed in a feedback loop. The feedback minimizes the distortion the resistor generates.
Use CC's sparingly - only where your personal ears tell you that they
make a difference.
So now you know what's happening, and something of the numbers
involved. The effect is real, though slight.