I use Ultra Sonic tube dampers on all the small signal tubes with great results; most noticeable is tighter bass.
The reason not to use dampers on 6550's or other power tubes is to allow the tubes to present harmonics and overtones. New production power tubes are low in microphonics and don't need damping. However, I can see how a damper might be needed for some NOS tubes.
The reason not to use dampers on 6550's or other power tubes is to allow the tubes to present harmonics and overtones. New production power tubes are low in microphonics and don't need damping. However, I can see how a damper might be needed for some NOS tubes.