How to lower noise floor through mods?

T_bone, if you have an older piece that you are wanting to update, the first thing to do is make sure that its actually working right first. To that end replacing the filter caps is the most important thing to do. You can get a lot of the aluminum can style caps from tubesandmore.com

Its not likely that improved resistors will be a whole lot quieter, but they will sound better. If you change coupling caps, which is where you will hear the most improvement, its a good idea to keep the following in mind: You can get better bass with larger caps, but you will pay a price in transparency and if the RC time constant associated with the cap is lower than that of the power supply low frequency pole, you will do your self a disservice. So I would stick with stock values for the most part.

Going to HEXFREDs is a nice touch although tube rectifiers are often lower noise. However, the higher output voltage that you get from semiconductors might allow you to install a high voltage regulation scheme. If you can do that, you will be instantly rewarded as long as the result is a B+ that is at least as high as stock! There used to be a device called the VB408 that was a high voltage regulator, very nicely suited for tube work. If you regulate, thats where you get the nice low noise floor.

Don't make a rat's nest of the insides. Keep your layout neat, with minimal wiring. It is a good idea to use a star-grounding technique and avoid using the chassis as ground. Its not OK even if the original manufacturer did it. That will help you get low noise too.

Have fun!!

IMO/IME its a Bad Idea to bypass coupling caps. The result is smearing. The more resolution your system has, the more you hear it. On low resolution systems it may sound like that cap got speeded up a little.

Your best shot is to put in the best coupling cap that you can.

If the power supply does not have the tube visible in plain sight, then its not there. A can over the tube would overheat it!

If you decide that you really want to tinker with this preamp yourself, **do yourself a huge favor**: go out and buy a kit from PAIA.com or bottlehead, build it up and make it work. It should look like it was built by an expert inside. Seriously, this is important because you can get in over your head very quickly modding things! There can be shock and fire hazards, both to yourself and others, plus in the real world probably only about 5% of the total modified units ever wind up being better than stock- in the other 95% the mod is a destructive act.

After you have built the kit, look into how it is built and see if you can mod that to do better. If not, don't attempt it on something that came ready-built! Above all though, this is supposed to fun. Don't loose sight of that either.