I have found this tube to be very poor as far as reliability goes. With a cool bias these tubes are dying at a 40% rate in vintage equipment I service. Customers like this option due to the size being close to original 7591 tubes. The failures and frequency of failures has forced me to replace any JJ 7591 that comes in the shop with the much taller EH tube. I wish the reliability was better as this size option sure does work well with gear in tight cabinets, JJ might want to look into the high failure rates or EH can make a more compact version.