I agree with Newbee. It's possible that the window of bias adjust,i.e. negative voltage, is insufficient to properly bias the KT-88's you received. Therefore the tubes are running away. A tech needs to measure the maximum bias voltage at the tube socket to see if it's sufficient. If it isn't, a different resistor could be installed to 'open' the bias window to allow for a wider, higher, bias voltage adjustment provided the minus bias voltage was high enough to begin with from the power supply. The example would be, with the KT-88's you received and based on the plate voltage your amp runs the output tubes at, you need a bias voltage of -55Vdc. Your adjustment maxes out at -48Vdc, so the tube runs away.
Can't bias tubes on amplifier
Hello all:
I've been here before regarding an issue I am having with a Tube Audio Design TAD-60 amplifier. It is designed to take EL-34, 6CA7, or KT-88 tubes. The problem I am having is that while I can easily set the bias for EL-34 tubes, the KT-88 tubes heat up way past 600 very quickly and no matter how far down I turn the bias adjustment screws, I can't get the tubes to stop overheating.
I've ordered new KT-88s thinking that it may have been the tubes, but they still overheat. The guy who built the amp, Paul Grzybek, has passed away. Any thoughts would be welcome, as well as someone who is good at fixing problems like this. Thanks in advance.
I've been here before regarding an issue I am having with a Tube Audio Design TAD-60 amplifier. It is designed to take EL-34, 6CA7, or KT-88 tubes. The problem I am having is that while I can easily set the bias for EL-34 tubes, the KT-88 tubes heat up way past 600 very quickly and no matter how far down I turn the bias adjustment screws, I can't get the tubes to stop overheating.
I've ordered new KT-88s thinking that it may have been the tubes, but they still overheat. The guy who built the amp, Paul Grzybek, has passed away. Any thoughts would be welcome, as well as someone who is good at fixing problems like this. Thanks in advance.