ultrsonic parts calling bath circuit

I presume you are asking, because you would like to repair it. Your best chance is to track down the company, which made it, or a dealership selling it. The reason for my advice in this direction is, that just any old circuit diagram will not help you . When such a device fails, in my experience it fails first the seal around the piezo element, and let any electronics beneath it get wet with (usually electrically conducting) corrosive fluid, usually ruining it. The second likely way for failure is the erosion of the metallization on the piezo disk with time: then it just stops working. You can replace that allright when you know the model of the disk. Not all disks of the same size are identical electrically.

If you want to drive a piezo disk yourself, get a simple audio oscillator (maybe on Ebay, cheap) good up to 50kHz or better. Get a simple 10 - 20Watt audio amplifier card. It may be good enough as is to 33kHz, or one or two component needs to be taken out. Then you are ready to experiment. Turn it on at low power, and tune it. At some frequency around 33kHz you will notice the characteristic foaming in the liquid. Adjust power setting. When too high, you can shatter the piezo disk. When you get used to it, you can readjust it just by looking at the characteristic foaming.

If your needs are simple, just look up "electric fogger". It is an inexpensive all in one ultrasonic fogmaker. If you put it deeper in water, it still stirs it up, without producing fog.