Posted by forum member:
I would recommend the Shuguang Treasure 6CA7-Zs, as tested and screened by Grant Fidelity, as a substitute for the EL34s. The additional cost is well worth the better Grade A. I noticed that the heat from the Treasure 6CA7-Zs is substantially and significantly less than the heat that emanated from either the Siemens or the JJ KT77s or the EH EL34s. That thermal difference could well mean a longer life-span for the Treasures.
original post here: http://www.audiokarma.org/forums/showthread.php?p=4503443