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

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).

This is the 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!