How can I overwrite the url of OpenLayers.Protocol.HTTP? I tried

Code:
searchformPanel.protocol.url
and it works (checked with console.log), but at the end the original url is sent (in the code below: url: '/fs/') (see attached picture).

mY1pq.png

This is the code:

Code:
var searchformPanel = new GeoExt.form.FormPanel({
      border: false,
      width: 250,
      protocol: new OpenLayers.Protocol.HTTP({
        url: '/fs/',
        format: new OpenLayers.Format.GeoJSON()
      }),
      items:[{
        xtype: 'combo',
        id: 'idcombo',
        store: new Ext.data.SimpleStore({
          fields:['fsclass','ollayer'],
          data:[["Boreholes","Boreholes"],["All_layers","All layers"]]
        }),
        displayField: 'ollayer',
        valueField: 'fsclass',
        fieldLabel: 'Layer',
        emptyText: 'select a layer',
        submitValue: false,
        selectOnFocus: true,
        mode: 'local',
        typeAhead: true,
        editable: false,
        forceSelection: true,
        triggerAction: 'all'
      },{
        xtype: 'textfield',
        id: 'idtextfield',
        fieldLabel: 'Find features',
        emptyText: 'enter word',
        name: 'comments__like',
        allowBlank: false
      }],
      listeners:{
        actioncomplete: function(form, action){
          searchShowTip(action.response.features);
        }
      },
      buttons:[{
        text: 'search',
        listeners:{
          click: function(){

            var comboLayer = Ext.getCmp('idcombo').getRawValue();
            var keyword = Ext.getCmp('idtextfield').getRawValue();

            var newUrl = '/fs/' + comboLayer + '?format=GeoJSON&comments__ilike=' + keyword + '&queryable=comments';
            console.log('1:' + newUrl);

            //this gets '/fs/' from the searchformPanel
            console.log('2:' + searchformPanel.protocol.url);

            searchformPanel.protocol.url = newUrl;
            console.log('3:' + searchformPanel.protocol.url);

            searchformPanel.search();
          }
        }
      }]
  });
Please any support about this is very welcome, thank you!