"Hum, Buzz, and Hiss" That's actually a pretty big menu. Different sonic defects, usually from far different sources.
Hum - Hum is almost always an injection of the primary line frequency or 2x the line frequency into the signal path. Hum usually originates in one of three places. First is insufficient power supply filtering. Easy to fix; add filtering. Just remember that one needs to add filter stages and not just bigger and bigger caps. Second source is ground loops and bad grounds. When interconnecting equipment, float all signal grounds except one. Keep signal and case/safety grounds separate except for one point (even when interconnecting equipment). And use either heavy ground busses or star ground schemes for signal grounds. Third source, and hardly worth a mention in my mind, is heater hum. This comes usually from poor lead dressing within the chassis. Heater circuits should always be twisted pair, should be kept well away from the signal lines, and should enter the tube socket area at near perpendicular paths to the signal wires. In rare cases heater hum can bleed through to the signal chain due to cathode coupling in unipotential cathode tubes. But this is rare and usually can only be seen when gains get over 50dBv to 60dBv. Look at the noise specs on the 12AX7 for typical numbers.
HIss - Hiss is either thermal or shot noise coming from a formed component (usually a resistor). First rule is don't use carbon based resistors; film or otherwise. For signal paths use quality meal film resistors, for power applications use wire wound. This will usually lick most hiss problems. However, there is another source of hiss that can be a bugger to find. This is high frequency oscillation. Even at very low voltage levels this can creep in. Always check for oscillation sources (pentode/tetrode screen supply circuits can be a real problem). On high gain tubes, series screen, control grid, and plate resistors can help a lot. Especially when using transmitter tetrodes like the 807 or 1625. And watch those totem pole configurations like SRPP, White Cathode Follower, and µ-Follower. These can oscillate at high frequency (>1MHz) like nobody's business.
Buzz - This is one of three things; a bad tube, a bad ground, or sand in the signal path. The first is simple, swap tubes and see if it follows channels or goes away. The second, look for finicky grounds with a VTVM (or other high input impedance meter) and visually examine all ground solder joints. For sand this is a little different (Yes I know I just stomped on a bunch of people's toes. And yes I know if done properly sand can be used and result in a dead quiet circuit. But the key word here is "properly".). So first, always snub SS diodes with small signal caps to shunt the high frequency noise, as close as possible to the device. Second, only use linear analog SS devices. Most voltage regulators are fine but never use a switching supply and never use an active control circuit that self oscillates to maintain control. Both are bad news for noise reduction.
Follow these rules and hum, hiss, and buzz should be rare problems in your circuit. Ok; now everyone I've offended, please feel free to let fly.
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