Just thought I'd share my recent reliability problems with a couple of tubes. Amp is a Dynaco Stereo 70 with a Dynamutt Octal driver board modified for lower gain with 6SN7 in place of 6SL7 driver tube. The amp was being used with its original Mullard rectifier and EL34s. A few years ago it went "CRACK" with a nice arc in the rectifier. I don't recall exactly how, but I traced the problem to one of the output tube sockets, and took the amp out of service to repair. I bought a quad of JJ KT77s to install, but before I could attend to the repair, my psychotic ex girlfriend shoved the amp off the kitchen table, breaking a couple of the 6SN7s and the brand new (NOS) GE GZ34 rectifier. Grrr. In my disgust, I abandoned the project for a couple of years, getting back to it about six months ago. I went through the amp, replaced the rectifier and output sockets, replaced the can cap, did not add any additional capacitance, put in old 6SN7s I had on hand, a brand new (Russian) Tung-Sol GZ34 and the quad of JJ KT77s. I biased it to the Dynaco spec (1.56 V with the original Dynaco cathode resistors), and kept an eye on the bias frequently. The amp sounded good, if perhaps a bit light in the bass. Now to the problems...
First problem was with a JJ KT77. I don't recall exactly what happened, but when checking the bias I determined that one tube (coincidentally or not, in the socket I originally diagnosed as the problem) was drawing excessive current from the bias circuit so I couldn't properly bias any of the tubes. The tube wasn't red-plating, but with that tube installed the bias voltage was being pulled way down. The tube had only a few weeks of use. I bought a replacement, and everything was fine until a few days later when "poof..." and the replacement lost vacuum due to a crack around the base. Defect? Damage in transport? Who knows. Got a warranty replacement. All good for a couple of months when... the replacement started to red-plate. (Still in that same position - but a new socket - where the original problem became evident.) I moved the tube to other positions and the red-plating stayed with the tube. OK, now I'm done with JJ KT77s. I purchased a quad of EH 6CA7s and installed them. Biased them, and they sounded fine. For a month. Until... "ZAP." One of them (in a different position!) started glowing like a barbecue. Tried it in different positions, and the red-plating followed. I've returned it for a warranty replacement, which I haven't yet received. In the meantime I've installed a pair of the original Mullard EL34s in that channel. They test pretty weak on my Hickok 6000, but they are sounding fine.
I've owned several tube hi-fi amps and about half a dozen or more tube guitar amps since 1973 when I built my first Dynaco SCA-35. I've only ever used tubes made "back in the day" until now and not once had a problem with a runaway power tube. Out of the 10 current production power tubes I've bought, 4 have gone bad in less than 2 months of use. Is this typical???
First problem was with a JJ KT77. I don't recall exactly what happened, but when checking the bias I determined that one tube (coincidentally or not, in the socket I originally diagnosed as the problem) was drawing excessive current from the bias circuit so I couldn't properly bias any of the tubes. The tube wasn't red-plating, but with that tube installed the bias voltage was being pulled way down. The tube had only a few weeks of use. I bought a replacement, and everything was fine until a few days later when "poof..." and the replacement lost vacuum due to a crack around the base. Defect? Damage in transport? Who knows. Got a warranty replacement. All good for a couple of months when... the replacement started to red-plate. (Still in that same position - but a new socket - where the original problem became evident.) I moved the tube to other positions and the red-plating stayed with the tube. OK, now I'm done with JJ KT77s. I purchased a quad of EH 6CA7s and installed them. Biased them, and they sounded fine. For a month. Until... "ZAP." One of them (in a different position!) started glowing like a barbecue. Tried it in different positions, and the red-plating followed. I've returned it for a warranty replacement, which I haven't yet received. In the meantime I've installed a pair of the original Mullard EL34s in that channel. They test pretty weak on my Hickok 6000, but they are sounding fine.
I've owned several tube hi-fi amps and about half a dozen or more tube guitar amps since 1973 when I built my first Dynaco SCA-35. I've only ever used tubes made "back in the day" until now and not once had a problem with a runaway power tube. Out of the 10 current production power tubes I've bought, 4 have gone bad in less than 2 months of use. Is this typical???