Carbon comp applications

[msclford]

I posted this over at Harmony Central, but I think you all might be able to help out more on this one. According to R.G. Keen's article, there are very few places in a tube amp where carbon comp resistors may be beneficial. He states:

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.

So what are some applications that would be useful for a tube amp? It seems like he is saying that they might be useful, but there isn't really any place to use them. Any ideas?

[tubeswell]

There have been several discussions on this board on this topic. I suggest you do a search. As for the 'best' place, well R.G.'s article you just quoted says where that is.

[Rhodesplyr]

I think you've found good information already. I tend to use carbon composition resistors occasionally for what the author you cite recommends: plate resistors with a high voltage drop and large signal. For example, in tube amps in Leslie speakers, I use them for the 6SN7 or 12AU7 56k plate resistors. This is where you get the effect of carbon comp's "voltage coefficient of resistance," which produces a little euphonic 2nd harmonic distortion. The catch is, the larger the wattage rating of the resistor, the lower this effect, so you want to go with the smallest safe value.

They are also recommended by many for grid-stop resistors, if the circuit uses them. Grid-stop resistors are the ones between a grid-leak resistor and a tube grid to prevent parasitic oscillations. Ideally, they are installed as close as possible to the control grid's pin on the tube socket. This is reportedly due to their non-inductive characteristics at radio frequencies, although I've seen some impedance vs. frequency graphs on metal films that's somewhat at odds with this claim.

And in musical instrument amps, I might use them in the signal path if it's a place where precise values aren't critical or there's little voltage drop to produce noise.

Carbon comps will tolerate momentary overloads that can cause some metal films to open up.

I often see them used in vintage repairs in places where they do absolutely nothing for "tone," i.e., voltage dropping resistors between power supply sections.

That said, there's a lot of difference between bargain-basement metal film resistors and decent quality. I like the PRP or the Dale/Vishay CMF types.

Carbon film can be a good compromise. Riken Ohm resistors were sweet, but they are out of production and pricey now. KOA Speer makes some good ones, too.