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Sooo...
The 19-21mm Perantouched B&S F tubas *were* an improvement over the 17-21mm models!!!

Got it. Thanks!

I can pick up a brand new one and still suck on low c

I'm sorry.. I couldn't help but poke a little fun.

We actually mostly agree on this stuff and I'm not looking for an argument over relatively subtle differences that I might change my mind about later anyhow. Your comment on sweetness vs power resonates as true to me.

For the moment I still prefer my specific PT15 over the couple 17-21mm horns I've played BUT there's a good chance they either weren't good examples or needed work to play their best. Also, as with many things in life, we tend to prefer those things with which we've become the most familiar especially if we spent a lot of cash to own.

What is your procedure for cleaning the horn?

I work in a repair shop that will forever remain nameless.. don't ask. Not a sponsor and no plans to be one. I'm posting here as a tuba guy only and the shop I work at has all the tuba business it can handle.

The modern ultrasonic machine is 'fine' but VERY expensive and to really get all the lime deposits out of a horn you would probably need to run it longer than you should with many older horns.

Our method as best as I can tell somewhat unique:
1) Fully disassemble and run through a tuba sized ultrasonic machine for about five to ten minutes.
This machine is an older model, it doesn't have the frequency sweep or filtration system of newer ones. It was purchased used off eBay. We use a simple green solution. The purpose of this step is mostly to knock things loose from places a brush won't reach and to start the degreasing process. Running it longer doesn't clean it any more.
2) Rinse and brush out the entire instrument.
Get rid of all the sludge and crap that's just been knocked free.
3) Chemical bath.
Honestly, this does the bulk of the work. Horns comes out clean, degreased, and decrustified.
4) Put the slides and smaller parts in a vibratory tumbler.
I love the tumbler for polishing slides. I've left old mouthpieces in it for days without it removing plating. As best as I can tell it doesn't remove metal but just 'burnishes it smooth'. Polishes inside and out of slides but leaves the original protective patina so it's less prone to oxidize. Polish level comes out equivalent to what I would consider a 'yellow' polish.
5) Rinse, dry, reassemble.

Using the ultrasonic machine first allows our chem bath to last a lot longer and do a better job. I think it's a fairly elegant solution but if someone has a better idea I'm open to learning.

I'm certain Bloke knows all this but for those that don't.. modern ultrasonic machines use an acid chemical degreasing solution and basically combines a few of the steps I listed above. They prolong the life of the chemicals(which are VERY expensive) by filtering it. To be completely honest, the sound waves in an ultrasonic machine are more likely to cause damage than the acid in the chem bath. Horns with significant red rot in the leadpipe can develop pinhole leaks from just a few minutes in an ultrasonic machine but frankly.. that just meant they had already failed and the only thing plugging the hole was corrosion. Most horns could be left in for many hours with zero damage of any kind. We often warn customers when we see a questionable leadpipe and depending on their response might just skip the ultrasonic step entirely. The newer machines use a 'frequency sweep' to avoid hot spots but the dangers are still the same. Most of the real cleaning that you are paying a tech to accomplish is done by the chemical bath anyhow.