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Thread: Harmony H303A voltages/quiet amp

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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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    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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    Sorry for the long wait.

    I got a few NOS RCA 12AU6 and no luck. Ended up increasing the grid leak value to 10 Meg and got the plate voltage up to 30 V. Changed some caps and it sounds better. Still weird its not going any higher than that.

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    Don't forget the joker g1's Avatar
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    Did you change C1 and C2 ?

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    A leaky C3 would hardly change V1 voltages. What about a leaky C1?

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    Yes, I tried disconnecting both C1 and C2. No change in DC voltages. IIRC disconnecting the C2 did lower the gain of V1.

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    I would try replacing the plate resistor. It's already high, and may be going higher only when subjected to voltage.

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