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    Setting Bias Step by Step Instructions

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    Dogstar

    Posts : 374
    Join date : 2014-06-23

    Setting Bias Step by Step  Instructions Empty Setting Bias Step by Step Instructions

    Post by Dogstar on Mon May 11, 2020 9:32 am

    Greetings....I'm search this forum for instructions for setting the bias for my amp but nothing seems to be coming up.

    Is there a STICKY? Because I am not seeing one either. Though I do see that it says what it should be biased too.

    So can you please reply with instructions or provide a link?

    At one time I could check and bias in my sleep. Now its been a few months and the new job really consumes a lot of my time. Before checkinI decided to put my VTA ST120 setup in my home office and thought it would be best to check and adjust the bias even though I carefully moved the amp and did not removes any tubes in the process.

    I remember how when I was a kid my father had this Grundig Console Stereo with a turntable, tuner, amp and speakers all in one beautiful wooden cabinet. I don't remember seeing my dad remove the tubes in the move. Same as the TV's (it was a looooong time ago - the late 60's). And I don't remember my dad setting the bias on that Grundig. But I do remember always checking and replacing tubes for the TV's. I don't remember whatever happened to the Grundig after he passed away and my 84 year old mom doesn't remember either.

    Anyway....these days I think I should check the bias more often than not though since I installed the last set of Genelex KT88's in has been less frequently than before.

    But posting those instructions will be great help. Thank you.
    Bob Latino
    Bob Latino
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    Join date : 2008-11-26
    Location : Massachusetts

    Setting Bias Step by Step  Instructions Empty Re: Setting Bias Step by Step Instructions

    Post by Bob Latino on Mon May 11, 2020 10:47 am

    Dogstar wrote:Greetings....I'm search this forum for instructions for setting the bias for my amp but nothing seems to be coming up.

    Is there a STICKY? Because I am not seeing one either. Though I do see that it says what it should be biased too.

    So can you please reply with instructions or provide a link?

    At one time I could check and bias in my sleep. Now its been a few months and the new job really consumes a lot of my time. Before checkinI decided to put my VTA ST120 setup in my home office and thought it would be best to check and adjust the bias even though I carefully moved the amp and did not removes any tubes in the process.

    I remember how when I was a kid my father had this Grundig Console Stereo with a turntable, tuner, amp and speakers all in one beautiful wooden cabinet. I don't remember seeing my dad remove the tubes in the move. Same as the TV's (it was a looooong time ago - the late 60's). And I don't remember my dad setting the bias on that Grundig. But I do remember always checking and replacing tubes for the TV's. I don't remember whatever happened to the Grundig after he passed away and my 84 year old mom doesn't remember either.

    Anyway....these days I think I should check the bias more often than not though since I installed the last set of Genelex KT88's in has been less frequently than before.

    But posting those instructions will be great help. Thank you.

    For the VTA ST-120 for the year 2012 to present day

    1. Set you multimeter to the 0 to 2 volt DC scale

    2. Place the black probe on the chassis and the red probe in the front LEFT tube socket pin that says "Biaset 1.56 V". Adjust the bias pot on the driver board next to the front left tube so that the meter reads .500 volts DC (that is POINT 5 volts DC or 1/2 volt DC). Next place the red probe in the front tube socket pin that is OPPOSITE the pin that you just used. Use the rear bias pot on the left channel to adjust the rear left output tube to .500 volts DC.

    3. Do the same as #2 above for the right channel

    Facts about bias below ..

    Since all tubes share the bias current > as you adjust one tube UP, the other tubes will go DOWN very slightly. Go back and forth between the four output tubes until all are between .490 and .510. That is plenty close enough. You will never all four output tubes to stay at .500 exactly.

    As the amp warms up from a cold start, the bias will rise very slightly over the first half hour. After 1/2 hour recheck the bias and reset the output tubes so that all are between .490 and .510 VDC.

    If all your tubes have a wandering bias UP or DOWN over time, then probably your line voltage is wandering UP and DOWN.

    If just ONE tube just cannot be biased at .500, then that tube is probably bad or going bad OR possibly the 10 ohm 2 watt resistor that goes from pins 1/8 to chassis ground is bad and does not measure 10 ohms anymore.

    If ALL tubes cannot be biased, then probably your rectifier has a problem.

    Check your bias about once a month. As the output tube wear in, the bias will change slightly.

    Bob
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    Dogstar

    Posts : 374
    Join date : 2014-06-23

    Setting Bias Step by Step  Instructions Empty Re: Setting Bias Step by Step Instructions

    Post by Dogstar on Tue May 19, 2020 9:51 am

    Thank you this is exactly what I was looking for.

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