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Thread: Harmony H303A voltages/quiet amp

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    Junior Member Ampere's Avatar
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    Harmony H303A voltages/quiet amp

    Hi
    schematic:harmony_h303a_sch.pdf

    Having low output on this supposedly 8W amp. I'm suspecting the following.

    The plate voltage for the 12AU6 measures 18 VDC, which seems low. Datasheet suggests 250 or so. The "B" source is 118 VDC. R5 plate resistor is a little out of spec at 450K. C3 is not leaky. (disconnecting doesn't affect). I don't have another 12AU6 at hand, but the old one tested good on my Accurate Instruments 257 tester.

    The "A" voltage measured something over 120 VDC, which I assume is close to spec since the filter caps are rated 150V.

    The schematic has an error. R4 is connected to B source rather than ground.


    -Niko

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    Supporting Member Jazz P Bass's Avatar
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    The plate voltage at the 12AU6 is definitely low.
    What voltage do you read at pin 6?

    Here is a similar (H400) schematic with voltage designations.
    (Although it is a "Widow Maker")
    harmony_h400.pdf

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    Junior Member Ampere's Avatar
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    Sorry for the delay and thanks for the schematic.

    The voltage on pin 6 reads 22.4 VDC

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    Supporting Member Jazz P Bass's Avatar
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    The only way forward is to try another tube.

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