import { Observable } from 'rxjs/Rx';

// ...

ngOnInit() {
    Observable.forkJoin(
        this.instrumentService.current,
        this.tuningService.tunings
    ).subscribe(
        data => {
            console.log(data);
        }
    );
}

this.instrumentService.current.subscribe(instrument => {
     console.log(instrument);
     this.instrument = instrument;
 });

 this.tuningService.tunings.subscribe(tunings => {
     console.log(tunings);
     this.tunings = tunings;
 });

private _tunings: BehaviorSubject<Tuning[]> = new BehaviorSubject([]);

/**
 * Class constructor.
 *
 * @param {ApiService} apiService
 */
constructor (private apiService: ApiService) {
    this.getAll().subscribe(tunings => this._tunings.next(tunings));
}

public get tunings() {
    return this._tunings.asObservable();
}

public getAll(): Observable<Tuning[]> {
    return this.apiService.call(this.url);
}