I need to install a drumtrap . Any particular advices ?

Drumtraps are not code in most locales so can you consider a standard P trap? Is this s new install or a repair/replace?

I already have a P trap but the smell is coming back in the bathroom.

My house is an old house . There is only a small vent pipe for a) 1 bathroom with a bath tub,1 toilet,1 sink, and b) another bathroom with 1 sink ,1 toilet and 1 shower.. A plumber told me that a drumtrap will definitively reduce the smell.

Ask the plumber if he will guarantee ,(in writing) that installing a drumtrap will remove the smell? I think the plumber's blowing smoke up your skirt. Try this and forget the drum trap. Most bad smells in tubs, lavatorys and showers are caused by rotting hair matted with grease from the soap and from rotting food in kitchen disposals Try this for the smell. Tonight before bedtine Take a 1/2 gallon of bleach and pour it down the drains and let set over night. Next morning ,(and this is important) flush out the drain with two large pans of boiling water to loosen the grease and flush the mess out. The bleach will make the hair slippery and began to dissolve it and the hot water will melt the grease and flush the mess away.
Also, pull the lavatory stopper and look down the drain. About 4 inches down you'll see a small rod sticking out. Fish out any hair that's caught there and do the same thing to the lavatory if you detect any odor. This should make your drains smell better. With all chemicals used in drain cleaning proper ventalation should be observed. Good luck, Tomp.

Yep, drum is not the way to go. Try Tom's fix but also consider my brew. Mix 1/4 cup of table salt with a 1/4 cup of baking soda and dump into the drain followed by a cup of vinegar. Next follow it with the same pans of water used by Tom. Both work well under the right condition, mine's just more fun to watch, step back when you dump the vinegar.

I Would Slap The Stupid Off That Plumber And Make Him Smart Again