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Paul,
I cannot recommend Aiken Amps website enough, it is an excellent resource for technical information on tube amplifier design, even though it is primarily focused on musical instrument amplifiers.
Good info on resistor selection and noise.
However, there is a case for using carbon composition (CC) resistors. As many builders know, CC resistors do play a vital role as grid stoppers (and gate stoppers). The resistive element is a carbon slug which blocks entry of RF signals into the grid of the signal or power tube or the gate of a FET. Conventional metal film and carbon film resistors rely on a conductive element and therefore allow RF to pass through the resistor into the circuit. In the case of high gain/transconductance tubes such as the WE417 (5842), 6C45P, triode strapped pentodes (6688, 7788, D3A, E280F, etc.), using CC resistors as cathode resistors in whole or in part as cathode stoppers also eliminates this as an entry point for RF into the circuit. Usually for these tubes, there are multiple cathode connections, so attaching a CC resistor to each with the paralleled value biasing the tube will reduce the DC current through the resistor, which should decrease noise.
Unfortunately, carbon comp resistors seem to be passing out of the market, major parts suppliers (Mouser, Digikey, for example) are not stocking them and are back ordered or state the manufacturer has ceased production. I have had to buy NOS AB, Ohmite, and others on eBay, though some suppliers like TubeDepot catering to amp builders seem to have some stock.
David
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