Different brands of 12AX7 tubes may provide different sonic results. I suggest you not try to adjust bias yourself. It is a electrically hazardous process if you are not aware of all the places inside where very high voltage is exposed.
Before swapping tubes, note what the idle temperature of the main heat sinks are. A very inexpensive digital temperature probe is available from Harbor Freight Tools to really nail this down.
If the temperature is about the same after swapping tubes the bias has not changed enough to worry about. If the amp now idles much hotter, you probably should not use those tubes.
Frank Van Alstine
