WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
If your tube tester has KT88 on the chart, test a known good 6550 on the KT88 settings, and use that value as your reference value for good/bad tubes. The 6550 is very similar to a KT88.