I have no experience with the KT88's but, I've used JJ EL34's and had the same problem as you. I switched to SED winged "C" EL 34 tubes and have had no problems at all. From what I've read, JJ tends to have a lot of quality problems. They sound good but, I'd stay away from them and change them all immediately before they damage your amp.
Expected hours for JJ Tesla KT88?
Folks -
In March 2007, upgraded stock Sovtek 6550 power tubes to JJ Tesla KT88s. The Sovteks had lasted 5 yrs or so (and still have some life left). A KT88 (JoLida 502B v8 position) failed in Sept after about 400 hours. A second (v7 position) began glowing intensely red today after about 700 hours. I powered down before anything bad happened. I'd heard KT88s were less robust than the 6550s. I do take care with proper biasing. Curious what the experience of others has been with KT88s and JJ Teslas in particular. Thanks in advance for any feedback.
In March 2007, upgraded stock Sovtek 6550 power tubes to JJ Tesla KT88s. The Sovteks had lasted 5 yrs or so (and still have some life left). A KT88 (JoLida 502B v8 position) failed in Sept after about 400 hours. A second (v7 position) began glowing intensely red today after about 700 hours. I powered down before anything bad happened. I'd heard KT88s were less robust than the 6550s. I do take care with proper biasing. Curious what the experience of others has been with KT88s and JJ Teslas in particular. Thanks in advance for any feedback.