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Thread: Biasing pots and where/how to measure

  1. #1
    Join Date: Apr 2015

    Location: london

    Posts: 112
    I'm martin.

    Default Biasing pots and where/how to measure

    Would anyone know how would I go about biasing the valves of the amp as per photographs. I believe that the area with little pots is what I'm looking for, just not sure what to connect the multimeter to.

    Cheers!
    Attached Images Attached Images

  2. #2
    Join Date: Dec 2017

    Location: Ontario, Canada

    Posts: 43
    I'm Dan.

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    First of all be very careful. LETHAL voltages inside this amp. If you don't know what you are doing, find someone who does.

    Now that the friendly warning is done

    If you look near the output tubes you will see 10 ohm resistors connected to pins 1,8, and to ground. That is measurement point. You can us any common ground. A black binding post will work, or the bare copper wire. Measure from the pin1 side. Your measurement in Volts DC divided by 10 (10 ohm resistor) will give you the current through the tube. Adjust the associated trim pot ( they appear to be labelled) to get the manufacturers recommended voltage or current.
    If you don't have the recommend bias information then it will need to be calculated. For that more information is needed.

  3. #3
    Join Date: Apr 2015

    Location: london

    Posts: 112
    I'm martin.

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    Quote Originally Posted by Blackdog View Post
    First of all be very careful. LETHAL voltages inside this amp. If you don't know what you are doing, find someone who does.

    Now that the friendly warning is done

    If you look near the output tubes you will see 10 ohm resistors connected to pins 1,8, and to ground. That is measurement point. You can us any common ground. A black binding post will work, or the bare copper wire. Measure from the pin1 side. Your measurement in Volts DC divided by 10 (10 ohm resistor) will give you the current through the tube. Adjust the associated trim pot ( they appear to be labelled) to get the manufacturers recommended voltage or current.
    If you don't have the recommend bias information then it will need to be calculated. For that more information is needed.
    Wow, what a great answer! Straight to the point! Many thanks for this. I don't have the values, but can at least compare them. It's 4 EL34s that I have in there.
    Cheers!

  4. #4
    Join Date: Dec 2017

    Location: Ontario, Canada

    Posts: 43
    I'm Dan.

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    You can't just compare to another amp, but we can calculate a reasonable value. First thing we need to know is the plate voltage. This is measured at pin 3 of one of the EL34's. This could be as high as 500VDC, so be very careful. Once we know that then we can calculate the required safe current.

    What make is the amp? Or do you have a schematic?

  5. #5
    Join Date: Apr 2015

    Location: london

    Posts: 112
    I'm martin.

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    Quote Originally Posted by Blackdog View Post
    You can't just compare to another amp, but we can calculate a reasonable value. First thing we need to know is the plate voltage. This is measured at pin 3 of one of the EL34's. This could be as high as 500VDC, so be very careful. Once we know that then we can calculate the required safe current.

    What make is the amp? Or do you have a schematic?
    Hi Dan, The make is Diva Audio by TS Lim - the model is Blue Sky - this is one reference http://www.echoloft.com/soulofmusic/divafaq.htm

    I'll measure the plate voltage and report back ;-)

    Thanks!

  6. #6
    Join Date: Dec 2017

    Location: Ontario, Canada

    Posts: 43
    I'm Dan.

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    Boy, they're not much on information on the that website are they?
    You could also just email them and see what the recommended bias setting is.

  7. #7
    Join Date: Feb 2008

    Location: Near Accrington, Lancs, UK

    Posts: 299
    I'm Ralph.

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    I used to have one of Lim's original AVC EL34 amps and the output sections looks pretty much like the OP's Diva. Bias was stated as 39mA but I used to set mine at 36mA. If I can find my original circuit diagram I may be able to confirm the anode voltage.
    Ralph.

  8. #8
    Join Date: Apr 2015

    Location: london

    Posts: 112
    I'm martin.

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    Quote Originally Posted by CornishPasty View Post
    I used to have one of Lim's original AVC EL34 amps and the output sections looks pretty much like the OP's Diva. Bias was stated as 39mA but I used to set mine at 36mA. If I can find my original circuit diagram I may be able to confirm the anode voltage.
    Thanks for this. I haven't had a chance to do the measurements this weekend, but will confirm what are the findings ;-).

  9. #9
    Join Date: Apr 2015

    Location: london

    Posts: 112
    I'm martin.

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    Quote Originally Posted by CornishPasty View Post
    I used to have one of Lim's original AVC EL34 amps and the output sections looks pretty much like the OP's Diva. Bias was stated as 39mA but I used to set mine at 36mA. If I can find my original circuit diagram I may be able to confirm the anode voltage.
    BTW - Are you sure that T S Lim of Singapore has anything to do with AVC?
    http://www.audiosalon.com.au/Diva_Audio.html
    http://www.enjoythemusic.com/magazin...aseduction.htm

  10. #10
    Join Date: Feb 2008

    Location: Near Accrington, Lancs, UK

    Posts: 299
    I'm Ralph.

    Default

    Quote Originally Posted by martinswimmer View Post
    BTW - Are you sure that T S Lim of Singapore has anything to do with AVC?
    http://www.audiosalon.com.au/Diva_Audio.html
    http://www.enjoythemusic.com/magazin...aseduction.htm
    Thats a good point and I'll admit that when I saw the name Lim I just made an assumption. However, looking at your amplifier it looks pretty much like most Chinese made EL34 amps. I'll still have a search for the diagrams tomorrow though.
    Ralph.

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