Angular 2 - Services consuming others services before call a method

I've this scenario.

backend.json

{
"devServer": "http://server1/'",
"proServer" : "http://server2/'",
"use" :  "devServer"
}

global.service.ts

import {Injectable} from '@angular/core';
import {Http, HTTP_PROVIDERS, Response, RequestOptions, URLSearchParams} from '@angular/http';
@Injectable()
export class GlobalService {
constructor(private _http: Http){}
getBackendServer(){
    return this.server = this._http.get('./backend.json')
        .map((res:Response) => res.json())
}
}

And I've this other service: search.service.ts

import {Injectable} from '@angular/core';
import {Http, HTTP_PROVIDERS, Response, RequestOptions, URLSearchParams} from '@angular/http';
import {GlobalService} from '../services/global.service';
import 'rxjs/add/operator/map';
@Injectable()
export class SearchService {
server;
constructor( private _http: Http, private gs: GlobalService ) {
    this.server = gs.getBackendServer().subscribe(
        (data) => {
            this.server = data[data.use];
        },
        (err) => {
            console.log(err);  
        }
    );
}
DoGeneralSearch(params:Search){
    console.log(this.server);
    let options = this.buildOptions(params);
    return this._http.get(this.server + 'room/search', options)
     .map((res:Response) => res.json())
} 
}

What I want to do: Looks obvious: I want to keep the information regarding the URL of the backend server in a JSON file - And the global service should return the server so this can be used in any method of the search service.

The problem is: the DoGeneralSearch() method executes before the global service is capable to resolve the task to read the JSON file and return the result. So, I've a this.server = Subscriber {isUnsubscribed: false, syncErrorValue: null, syncErrorThrown: false, syncErrorThrowable: false, isStopped: false…} instead the result itself.

I need to find some how to prevent the method DoGeneralSearch be executed just after resolved the this.server variable.

Any suggestions ?