Q: My amp is a little too buzzy / harsh but otherwise i think it rocks, what do i do?!?
A: Well, usually this is caused by higher harmonics which may sound harsh or buzzy. There are many ways to get rid of them, but this is the easiest and my fav. by far. What you need is one or more 1nF capacitors. Get polypropylene film type if you can, they're the best type for tube amps.
Wire one 1nF capacitor in parallel over the plate resistor of a preamp tube. Generally speaking, the second-last gain stage is the most effective point, followed by the last one. To find the second-last stage, look at the schematic for your amp and look at the plate load resistors. They will usually be 100k, but one or two of them will be larger, like 150k, 220k or even 330k. The gain stage with the large plate resistors creates the most gain and high harmonics, so this is the best place to put the capacitor (for starters). If there are two or more gain stages with large plate resistors try modding the one that comes last in the signal chain first.
Solder one end of the 1nF cap to one end of the plate load resistor, and the other end of the cap to the other end of the plate load resistor. Thats it. If you still feel you need more, try soldering another 1nF cap on top of the 1nF cap you already put in, and if that smoothed it out too much take the cap out again and try adding a 1nF cap over the plate resistor of the gain stage that comes next in the signal chain.
I uploaded a engl blackmore schematic showing where the extra capacitor goes, it's drawn in red so it's easy to make out, so you can see which resistor is the plate load resistor and which gain stage you usually want to be modding (the one with the biggest plate load resistor).
This mod will smooth the amp out quite a lot but not make it that much darker, if you want a darker sound i have another easy mod..