Tests performed by Herbie's Audio Lab with Amperex 6922 PQ input tubes yielded these glass bulb temperatures:

Bare tubes: 57.2º C (135º F)
With Pearl Tube Coolers: 68.3º C (155º F)
With brass Tube Anchors: 76.1º C (169º F)
With Duende Criatura Tube Rings (placed near top of tubes): 57.9º C (136º F)
With UltraSonic Damping Instruments: 56.6º C (134º F)

Glass temperatures of Svetlana EL34 "Winged-C" output power tubes:

Bare tubes: 114.7º C (238º F)
With Pearl Tube Coolers: 110.3º C (231º F)
With UltraSonic Damping Instruments: 114.1º C (237º F)

All tests were conducted with Sonic Frontiers SFS-80 amplifier in ambient room air (74º F) with 60 minutes warmup. Results are averages of four tubes of each type tested. Amplifier tube bias set at mid-line 50 Milliamperes.

Glass bulb temperatures vary considerably at different locations of the bulb; all measurements were taken at the same relative locations on each tube.

(Re-tests conducted to verify these results showed a margin of error of approximately +/- 1ºC with small-signal tubes and +/- 3ºC with output tubes.)

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Cool Tube Dampers by E.A.T.

Tests performed by Herbie's Audio Lab with Amperex 6922 PQ input and driver tubes, SFS-80 amp, semi-enclosed space, ambient room air (74º F):

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