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"Finished" my AX84 Single Ended Lead build -- No sound... Next?

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  • "Finished" my AX84 Single Ended Lead build -- No sound... Next?

    Finished assembling my AX84 SEL kit from Doberman amps -- I've triple-checked the wiring and I got no output whatsoever, not even hum or hiss thru the speakers. It's a pretty basic single-ended design with a double preamp gain and a single KT88 output tube. Check SCHEMATIC if ya wana.

    The Good:

    --No blown fuses, smoke, fire, or gamma radiation, death rays, or shrapnel
    --Tubes heat normally, indicator lights up, low-volt side all tests with good results.
    --All my B+ volts are (as expected) about 10% above spec. My mains are ~126vac, using a 115v PT
    --Speakers and cables all test good with another amp.
    --Both transformers tested for shorts and opens, OK

    The Bad:

    --Obviously, no speaker output whatsoever
    --When there is any input signal at all, my OT makes a squealing/high pitched sound that varies in pitch with the knobs on the tone stack and preamps. This is mechanical noise from the trans, not speaker output. Volume increases and decreases with the master voiume control.

    I'm just looking for tips on where to go from here. One tab on my OT was bent when I got it but I don't know any other tests for transformers besides impedance and open/shorts. Suggestions appreciated!

  • #2
    Measure the resistance across the speaker terminals at the speaker with it unpluged from the amp. If it's an 8 ohm speaker, you should measure about 6 ohms. Now plug the speaker into the amp and measure again. You should measure pretty close to a short, less than 1 ohm. Now unsolder the white or yellow wire that goes from the jack to the transformer. You should now measure the 6 ohms again at the speaker. If you are still measuring a short, there is a short at the speaker jack.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

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