| Not only high gain but high power also. Shot noise was originally attributed to electrons hitting the anodes of vacuum tubes, but transistors turned out to make the same kind of noise so it applies to them too. And it is current-dependent so sometimes you can be quieter at idle by biasing lower. For resistor noise, nothing wrong with a megohm referencing input to ground since there's not any significant current flowing there. But in a tube preamp the typical 100K anode resistor will conduct a bit and can get hot. These should be carefully auditioned. Plus higher wattage rating parts give less noise in this service than they put in most commercial amps. If you increase gain using something like 150K, 220K, or even 330K there will be less current (through the resistor and the tube) but the increased amplification factor will equally multiply any noise which occurs before that gain stage. Then with power you've got the ability to broadcast audio, naturally over extremely short distances (compared to radio frequencies which hopefully you are filtering out) but those are the distances inside your chassis where different parts of the wiring layout can interact beyond a cetain point as broadcast and receiving antennae, and either provide negative or positive feedback, stabilizing or destabilizing respectively parts of the circuit whether you intended it to happen or not. On top of the expected acoustic mechanical feedback from the speaker at high volume, which reverses polarity based on distance, you've also got magnetic feedback. Once a very high-power output transformer goes wild it can reach out a lot further and touch your pickups directly from a few feet away. At this point the noise at idle is usually as loud as as ten pounds of bacon frying and I hate that. So get out the soldering iron and fix it so you can only tell it's on if you put your ear close to the speaker, when it's actually set loud enough to play with a heavy drummer. Hearing protection required beyond this point. |