It is much better to use a tube tester than your amp or preamp to test tubes. And always check for shorts before testing a tube. This is a Hickok 605A tube tester in good working order. It comes with the original operating manual. The two tubes that this tester uses have been replaced and it was checked and calibrated with the 6L6 calibrating tube. The price is $450.00 and $30.00 shipping within the US. Tested with a special 6L6 used for calibration, a 12AX7 and 6SN7 and within acceptable limits of 2 to 3 percent, which is very good for any tube tester.