Kahlenberg D-2

[FDDMS-er]

I've mounted a Graham White 353-100 valve to it and set it on a chunk of hardwood as a base.

Nothing wrong with a G-W 353 valve for a small- to mid-size Kahlenberg, that's for sure.

For something a bit less railroad and a bit more "brown water"/harbor boat, the all-brass F.C. Kingston pull-style "whistle" type valves are a reasonably priced alternative. They come in a bunch of varieties (pull lever or push lever, various air inlet sizes and PSI ratings, etc), so a person probably needs do a bit of due diligence to find the "perfect" one for their application. If a setup has the valve placed close enough to the horn, smaller Kahlenbergs often work pretty good with a 3/8" inlet valve (as do Westinghouse A-2 horns).

They can be found on Ebay, Amazon, etc, but I personally used to get them through McMaster-Carr in Chicago. When buying, it is a good idea to get a strainer of at least equal inlet size and flow capacity to the valve and mount it somewhere upwind of the valve.

(Strainers are like the proverbial "pistol in Texas" (hat tip to Horace Kephart): you may not always need it, but when you do, you will want it bad and there is not going to be time to fetch it. Having Kahlenberg service a horn once makes most operators become believers.)

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ETA: People who want to remove moderate dents from spun brass projectors can sometimes find a musical instrument repair shop willing to take the job on. It is not much different than doing bodywork on a trombone, and is every bit as expensive.

[FDDMS-er]

Do the older ones have a different sound to them or are they just minor differences in the physical looks?

After re-reading my earlier reply, I apologize for not making as clear as I should have the fact that many people who make their living on the river will tell you - occasionally quite emphatically - that there is a great deal of difference between the sound of a cast projector and a spun brass projector even if they are the same nominal frequency. If comparing apples to apples (cast projectors) and oranges to oranges (spun brass projectors), an older horn doesn't sound appreciably different than a new one of the same designation unless Kahlenberg made changes to projector length(s) somewhere over the years... and as already mentioned, they often append the letter "A" to the model designation at the end to indicate a change to the length of one or more projectors.