Could anyone give me a procedure to ajust the bias of the tubes (KT90 / 12AX7EH) of my JOR amplifier. I called Jadis, and they asked me to bring them the machine, but its 1000Kms from my place. And the audio people I met in Paris said to me "do it yourself". I know you helped a guy (mares-mares) 6 years ago, but I didn't find the pictures that could help me. Thank you for any help Jacques |
Weissenb, are you comfortable with working with high voltages? The JOR is 450+V, certainly a more than dangerous level. If you don't have experience with this, you should have a local tech handle it for you. You need to take the amplifier apart, removing the side and bottom plates. The most difficult part in the exercise is reassembling the JOR. Personally, I believe if you're going to bias the amplifier yourself, while you have the bottom plate off, expand on the vent holes to allow access to the bias potentiometer and plate resistors involved in the procedure, and convert this into an easy process. The bias potentiometers will be obvious to you. The plate resistors involved in the procedure are 5.62Ohm, and connect the anode/plate (pin 3)of the output tubes to the output transformers. I apologize, but off-hand, I forget if it's the yellow or orange wire; just confirm the resistor value, and you know you're dealing with the right one. While measuring the voltage across this resistor, adjust its respective potentiometer to the factory recommendation 90 - 120 mV (ideally 110 mV). Personally, I would give it a listen at 150 mV, still an incredibly conservative rating (yeah, yeah, I know - WE'RE ALL GOING TO DIE, WE ARE ALL GOING TO DIE!!!), and you'll hear much sonic improvement. You can also read this thread http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1336583789&openfrom&15&4#15 |
Thank you Trelja for the advice. I first will wait to find long clips for my voltmeter and avoid to put my fingers in too irritable compagny. Just one more question, probably obvious for you.. This bias correction has to be done with speakers and peripherals connected or not.. Its just a bit more complicated to do it with all the cables connected. Best regards Jacques |
Thank you, Jacques. Yes, for the sake of the output transformers, which are a bit of an achilles heel of the JOR, you should always connect a transformer coupled tube amplifier to a load. Loudspeakers will work fine. You don't need to have any input connections coming into the amplifier, however. If you feel comfortable with the procedure, you might want to bias to the specified value, let the tubes break-in, and they increase the bias to 150 mV, and see if you notice a nice sonic improvement. By the way, in my opinion, properly tubed and set-up, the JOR is one of the finest sounding amplifiers one will encounter. Best of luck! Joe |
