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Thread: Biasing pots and where/how to measure

  1. #11
    Join Date: Apr 2016

    Location: Bishops Stortford

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    I'm Chris.

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    Anyone owning amps containing hum pots should give them occasional attention. Once pots are set then they are often not touched for many years. Indeed mine were a bit scratchy in the speakers when being adjusted.

    Treatment with Deoxit did the trick. A huge benefit is that the residual hum in the speakers can now be adjusted to almost zero.
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  2. #12
    Join Date: Apr 2015

    Location: london

    Posts: 162
    I'm martin.

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    Finally got round to measuring plate voltage and the bias - it seems to have been waaaaaay off!
    I just hope that the resistor is indeed 10ohms! The amp is AB.

    Plate voltage 470V
    I re-biased to 31.6mv

    The values were:

    1 was 41mv
    2 was 45mv
    3 was 44mv
    4 was 35mv
    Last edited by martinswimmer; 13-03-2018 at 15:00.

  3. #13
    Join Date: Apr 2015

    Location: london

    Posts: 162
    I'm martin.

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    Quote Originally Posted by RothwellAudio View Post
    31.6mV across a 10 ohm resistor means there is 3.16mA flowing through it. I have no idea what the circuit is but 3mA quiescent current sounds a bit lower that what is typical.

    This bit makes no sense to me:

    41mV plate voltage??
    No, I took off the plate voltage because it was 470V and forgot to delete "plate voltage" bit.
    CORRECTED!

    I used a calculator for biasing - so picked 3.16mA - would you say that's too low?

  4. #14
    Join Date: Apr 2015

    Location: london

    Posts: 162
    I'm martin.

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    Quote Originally Posted by RothwellAudio View Post
    Yes, round about 30mA sounds like a typical value. 3mA? Sounds very low.
    I suspect the OP is measuring something else entirely, though I could be wrong. Without a circuit diagram, or some brave soul with a lot of spare time to trace out the circuit, there's too much guesswork here.
    I divided the value by 10 - so 316mv

  5. #15
    Join Date: Apr 2015

    Location: london

    Posts: 162
    I'm martin.

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    Common ground.

  6. #16
    Join Date: Apr 2015

    Location: london

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    I'm martin.

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    Quote Originally Posted by RothwellAudio View Post
    31.6 / 10 = 316
    316 / 10 = 31.6

    In any case, sounds much better now.

  7. #17
    Join Date: Apr 2015

    Location: london

    Posts: 162
    I'm martin.

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    Quote Originally Posted by RothwellAudio View Post
    I was trying to be helpful but you seem unwilling to cooperate with any useful information, so I'm finished now.
    ?? Not sure how I offended you? I measured 316Mv, divided that by 10 and got 31.6Mv.
    The same when I measured the initial voltage. This is the first time I've done it, so probably not using right terminology.

  8. #18
    Join Date: Apr 2012

    Location: South West of England

    Posts: 263
    I'm Will.

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    It looks as if that amp has no protective earth connection. If this is the case this is a far more urgent issue to rectify. Any wiring fault or insulation breakdown in the transformer will make the chassis live.

    It also looks like the fuse is located between the two 120V windings which are in series, and not on the live. This is less of an urgent issue but still a problem.

    It also looks as if the power switch is implemented with a TRIAC which is a bit dodgy especially given the fact that it's hanging off the live pin of the IEC inlet with no other support than solder. If am correct on both these counts the amplifier shouldn't really be used in its current state.
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