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Re: Jadis Orchestra bias setup

Posted by Derek (A) on February 20, 2007 at 04:46:44

In Reply to: Jadis Orchestra bias setup posted by imfos on February 19, 2007 at 10:36:46:

Hi .

You need two digital multimeters, with insulated aligator clips on both.  Set to  200mV DC range

Allow the amp to heat up and stabilise for  say - 10 min.

First meter:

Connect one lead to  the EL34 output tube pin#3
Connect the other lead likewise to the EL34 output tube (on the same channel) also pin#3

In otherwords - you connect one meter between pin#3 on both of the tubes on the channel you are testing.

Second meter:
Connect one lead to the yellow wire end of the resistor with that is connected to pin#3
Connect the other lead to the other side of this same resistor.

In otherwords - you have now connected this second meter across either end of the resistor that is on pin#3.

Adjusting:

adjust one of the  small 22k pots until you have *0mV* on meter #1

In otherwords - you you have 0mV between pin#3 and pin#3

Adjust the second 22k pot until you have 110mV on meter #2

In otherwords -  you have 110mV across both ends of the resistor that is connected to pin#3 and the yellow wire.

Keep adjusting till you have set the  above and it has stabilised.  This will require a lot of back and forth between the  two pots and they are a pretty crude piece of work those pots!  It may also be frustrating!!!!  the smallest amount of a turn will give big changes - so be patient.

You can set to between 95 -  120mv, with 110mv being the ideal.  The 0mV is a real bitch to do but get it as close as is possible without going mad!!  It requires very fine adjustment in order to prevent and damage to the amplifier.

Needless to say.......remember that you are working with live HIGH VOLTAGES and all the necessary saftey precautions must be  followed.  Be very quiet, be  very slow and think of what you are doing.  Look and study exactly what you are about to  do and practise with no power on  to get the multimeter clips into  place.  You will of course connect the meters before you power up  I know but there is litttle room, so  mind that nothing shorts.

with all that in mind - it is really easy - honest.  Once you have done it you will  want to  do it again!

If there is anything that is not clear please just ask.

Regards

Derek

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