Bias adjustment for Jadis DA5

I would be very grateful, if someone could explain me how to check and adjust the bias setting on my Jadis DA5 with 6550 tubes (also plan to test the KT-88).

Thanks in advance,
get into, look for audio gallery on the left column. get nearly at the end of the page and you will find a very nice foto showing the place to adjust. note there is only one point to adjust the four valves at the same time.
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

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
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

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!
So Trelja, I did the job..
You are right, the worst was the opening..(I modified also the bottom, making 8 one inch holes in regard of the resistance and the potentiometers)

Talking about the job...The 5,62 resistances gives 6.2 when measured. The all four..(old age??)

The 2Ko potentiometer were a bit touchy.. Not so easy to make the step 2 of the bias equilibration (OmV between the two pin 3..) Nevertheless I put 145 mV bias (and less the 10mV between the 2 pin 3)

You are right the listening is incredible.. very clear and precise. At least with Jazz (Exemple Midnight Sugar, or Yim Hok-man drums incredible record). Then I went back to Piano and discovered that the "Lang Lang Chopin" (a Sony recording) is somewhat dumped..(etouffé, struggled... I don't know the english name)., It was better with other piano recordings. But neverthess, not so clear as usual..May be.. I have to let time to it...

Can this 145mV bias stay so or is it safer to go back to the traditional 110 mV bias to preserve tube longvity (KT90)

Thank you any way for your help
Congratulations, Jacques! Sounds like you've done a fantastic job. From now on, biasing your JOR will be a walk in the park.

Glad to hear you like the sound of the amp. You can keep it biased at 145 - 150 mV. It's still running exceptionally conservative at these settings.

If you want to improve the sonics, when the time comes that the KT90 need to be replaced, I would look into a quad of JJ E34L. The midrange you'll get from this amplifier will be about as good as you will encounter. And, one upgrade you can do right now is to try a pair of Shuguang 12AX7B, which might really open the sound up for you. And, these Shuguangs are incredibly low cost at that.

Merry Christmas!