@howsthefidelity, you’re right about the Mullards. Please give the Shuguang (or Psvane) triple mica 12AX7B aka Silver Dragon a go. At $10, you will absolutely not believe their performance. They easily best the Mullards, and compete with the best ever variants. Impossible sounding, I realize...
A journey of building and modding amplifiers completely rewrote my thoughts and beliefs, and I now feel fixed versus cathode bias sits among the top of the factors of how a tube amplifier performs and sounds. Fixed bias sounds a lot tighter, faster, reactive, and neutral. Cathode bias sounds more relaxed, warm, liquid, lush, romantic, and slower. Neither presents a better or worse presentation, it’s why they make vanilla AND chocolate.
If you can handle working around high voltage, you can easily bias the JOR yourself. The effort lies in taking it apart, and (especially) putting it back together. Removing the side panels and bottom plate, simply rest the amp on its side on something like a piece of carpet, with the power transformer sitting on the lower side for obvious reasons. Measure the voltage across the ~5.6R resistor that sits between the output tube’s plate (via the yellow wire) and the output transformer. Adjust the potentiometer to 90 - 120 (Jadis considers 110 best) mV, and you’ve biased the tube. Repeat for the other 3 output tubes, and you’ve biased the amp. There’s an additional step some employ, but as it confuses the heck out of people, and rarely yields anything, let’s disregard it. Beyond this, the figures I just listed leaves the amp biased extraordinarily (and surprisingly at 40% plate dissipation for an EL34’s rated 25 watts, and far colder for your KT90) cold, which Jadis acknowledges. Personally, I bias the amp a lot hotter, though still conservatively, but I won’t say anything further than that for now. Also, I’ve suggested folks consider cutting holes in the bottom plate that line up with the bias resistors and potentiometers, as the ability to bias the JOR without taking it apart would reduce it to something that takes just a couple of minutes.
Officially, the JOR uses 300W at idle.
A journey of building and modding amplifiers completely rewrote my thoughts and beliefs, and I now feel fixed versus cathode bias sits among the top of the factors of how a tube amplifier performs and sounds. Fixed bias sounds a lot tighter, faster, reactive, and neutral. Cathode bias sounds more relaxed, warm, liquid, lush, romantic, and slower. Neither presents a better or worse presentation, it’s why they make vanilla AND chocolate.
If you can handle working around high voltage, you can easily bias the JOR yourself. The effort lies in taking it apart, and (especially) putting it back together. Removing the side panels and bottom plate, simply rest the amp on its side on something like a piece of carpet, with the power transformer sitting on the lower side for obvious reasons. Measure the voltage across the ~5.6R resistor that sits between the output tube’s plate (via the yellow wire) and the output transformer. Adjust the potentiometer to 90 - 120 (Jadis considers 110 best) mV, and you’ve biased the tube. Repeat for the other 3 output tubes, and you’ve biased the amp. There’s an additional step some employ, but as it confuses the heck out of people, and rarely yields anything, let’s disregard it. Beyond this, the figures I just listed leaves the amp biased extraordinarily (and surprisingly at 40% plate dissipation for an EL34’s rated 25 watts, and far colder for your KT90) cold, which Jadis acknowledges. Personally, I bias the amp a lot hotter, though still conservatively, but I won’t say anything further than that for now. Also, I’ve suggested folks consider cutting holes in the bottom plate that line up with the bias resistors and potentiometers, as the ability to bias the JOR without taking it apart would reduce it to something that takes just a couple of minutes.
Officially, the JOR uses 300W at idle.