I’ve been rebuilding one of my 6550 amps and here is an interesting info on matching power tubes.
First image shows slightly unmatched tubes (prefect match in IP, but 10% mismatched GM). Notice that the amp reached 1% 2nd distortion at 44 watts.
Second image shows “perfectly”, within 2%, matched tubes producing 65 watts at 1%.
So this is 20 watts difference with low distortion!!! Also notice that overall distortion even at very low power levels is 0.015% vs 0.15% that’s 10x difference.Matching of tubes must be done at voltages close to HT of an amp, I matched them @ 500v.
Many vendors sell tubes matched at a low voltage (~250-290v). Be aware of that and ask for how the tubes have been matched. Biasing can help with IP (plate current or cathode current to be exact) mismatch, but will not help much with GM mismatch.