I usually play a tenor trombone or baritone. I'm completely new to the concept of bass trombone. All i know is that its obviously in Bb, with an F attachment key. Im also not used to playing notes below the staff, as i am the only soloist for our band. What should i do to have a better tone on low notes and what is the second key for? I've tried asking people in the band to help and nobody has any experience on it nor has anyone ever seen one.
This question is really broken into two parts:
1) What are the functions of the first and second valves on a bass trombone?
2) Why do I not sound like I do on a tenor trombone on a bass trombone?
Bass Trombone Valves
Virtually all bass trombones come in one of three types of configurations, based on the number of rotors/valves they have. "Plug" is a slang term that you might see used as well, and simply can be used interchangeably with the term valve. These variations are:
A single rotary bass trombone is typically pitched in Bb/F, just like a conventional tenor trombone. This means that without the valve depressed, it is in Bb, and with the valve depressed, it is pitched a fourth lower, in F.
The Dependent configuration has two valves, and there are several variations. Bb/F/Eb and Bb/F/D are the most common, with D being more common than F on modern horns. Dependent configurations are not as popular as they once were, but you still see a number of them come up used and some companies still produce them from student through professional horns. The way you'd interpret this is that with no valves depressed, it is in Bb. With one, it is pitched in F as with a single horn. With both valves depressed, it is in Eb or D, respectively. This gives access to CC in 4th position on a Bb/F/D horn.
The reason that dependent horns were once popular and are still used by some today is that the trombone neckpipe (the tube that touches your neck) is not a purely cylindrical tube. It is actually cone shaped on the inside, but very gradually. When you put two rotaries on a trombone, the each rotor can take up to a few inches of space, turning the instrument from cylindrical to conical. Or in simpler terms, it reduces the amount of taper. Some players don't mind this, others do. Those that do prefer dependent because you get the benefit of having easier access to lower notes but you dont' give up that extra inch of tapered tubing. This can have an influence on response if you are very familiar with your instrument. Many players will not be able to perceive this difference.
Independent, which is what your configuration is, is often pitched in Bb/F/Gb/D. Meaning without the valve open, it is a Bb. With the thumb valve alone depressed, it is pitched in F. With the Gb alone (the ring/middle finger) depressed, it is in Gb. With both valves engaged, it is in D.
This means you can play F in first with the first valve depressed and Gb in first with the second valve depressed. With both of them depressed, you can play D in first position.
So what you can do with this is use alternative positions with the valves to facilitate technique. For example, Bb at the second line of the bass clef staff can be played in a flat 3rd position on the F attachment, making a pattern such as Ab -> Bb -> D in the positions 3 -> Flat 3 -> 4, which is easier than 3 -> 1 -> 4.
There are many possible combinations, and if you're interested it would probably be good to ask another question such as what the possibilities are because one could easily fill up an entire answer on the uses of the various turnings.
Bass Trombone Sound
In some ways, a bass trombone is a big tenor trombone, at least in a contemporary context. However, there are some key differences that you should be aware of that give bass trombone a sound that is distinct from a tenor trombone.
First, Jin Bao do occasionally make a good horn, but they can be inconsistent. Depending on how you acquired it, it may be a "dud." There are companies such as Wessex and Mack Brass that get Jin Bao instruments and play test them before shipping. Retailers on eBay have a tendency to not do this. It is not unheard of for people to receive horns with rotors that are not aligned properly, for example, which can make a horn very difficult to play - especially in the lower register. I would have a tech check your instrument for defects to make sur eyou aren't having trouble on something that even a professional world be having trouble with!
Secondly, your mouthpiece choice for baritone and tenor trombone is very much not likely to work, or at least not produce a characteristic trombone sound. The smallest mouthpiece out of any professional I am aware of playing is a Bach 2G. Typically when you start on bass, most pedagogues will start out around there, or perhaps a 1.5G, although there are some players who double that use a smaller mouthpiece. I used a Doug Elliott 104 rim size, similar to a Bach 3G rim size, on bass when I was primarily a tenor trombonist. It was fine when paired with a deeper cup, but it doesn't produce as characteristic bass trombone quality as something even just a little bit larger. If you put in a large shank 6.5AL and were wondering why you couldn't play low notes, while it is possible to play a bass trombone with a size that small, it is not typically done and will take much more work to make it work than by using a larger mouthpiece.
Finally though, if you are used to playing on something small, don't expect to be able to switch back and forth without putting time into both instruments. Most players do not switch back and forth without putting in at least half an hour on each rim size per day. Or perhaps less once they have a very developed embrochure for both sizes. This would be work outside of your band classes as you aren't really putting in much face time in an ensemble setting as a developing musician.
It can greatly expedite the learning process to find a teacher, in person if possible or online if not, who is very knowledable at putting out other good bass trombonists. There are lots of people with opinions and there are lots of good teachers out there as well. Being able to get even a few lessons off of the latter is going to be way better than studying weekly with the former.
All modern bass trombones are basically large-bore Bb tenor trombones, plus attachments. They read the same notes, at the same pitch, but are optimised for lower notes. The basic bass trombone is the same as a tenor trombone with F attachment, just a bit larger-bore and with a larger mouthpiece. Modern ones usually have a second valve, taking it down to E or D. This allows a fully chromatic range down to the pedal notes - with just an F extention low B natural is missing. There are other, more complex arrangements possible. See http://www.rathtrombones.com/custom/bass-trombones/ for some examples.
You get good at the lower notes by practicing them!