From Bill Perkins of Perkins Electro-Acoustic Research Laboratory! pcX now has back in stock, the "full range" of "new" updated PEARL TUBE COOLERS - without a doubt, one of the most useful accessories in tube audio.
Use of these devices result in reduced tube microphony for noticeably improved sonics; and longer tube life (conservatively triple) due to the cooler "heatsinking" the heat away from the glass enveloped, via the thermally conductive copper-finned cooler material.
Which Tube Cooler for Which Tube? - PDF (recently updated)