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    Quick troubleshooting guide for Dynaco tube amps

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    Bob Latino
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    Posts: 1961
    Join date: 2008-11-26
    Location: Massachusetts

    Quick troubleshooting guide for Dynaco tube amps

    Post by Bob Latino on Thu Feb 11, 2010 2:03 pm

    Note - This is not meant to be an all encompassing troubleshooting guide for Dynaco tube amps. It is just to show areas to check if some of the more common tube amp problems show up in your Dynaco tube amp. It also assumes that you have eliminated your preamp, interconnects, source components, speakers and speaker wires as a cause of the problem.

    1. Problem - No sound and tubes don't light up
    Areas to check - amp unplugged, fuse blown, bad power switch, bad primary winding on power transformer

    2. Problem - All the tubes light up but no sound
    Area to check - Nearly always a bad rectifier tube - just because the filament lights doesn't mean the rectifier is good. Check the bias voltage - A no bias situation = bad rectifier tube

    3. Problem - Fuse blows immediately upon turning the amp on
    Area to check - Nearly always a bad power transformer winding or one of the windings is somehow connected to chassis ground

    4. Problem - Fuse blows about 10 to 30 seconds after turn on and/or sparking inside rectifier tube
    Areas to check - bad rectifier tube, bad DC high voltage power storage (usually a bad quad cap), bad (shorted inside) output tube

    5. Problem - Amp has a hum that doesn't change in level as you advance the volume control of your preamp
    Areas to check - bad quad cap, bad ground inside the amp, bad tube filament that is leaking AC into the audio

    6. Problem - Hiss, noise, crackling sounds on idle
    Areas to check - Noisy driver tubes, noisy resistor on driver board (especially older carbon composition resistors), bad solder joint or bad solder trace on the driver board.

    7. Problem - Popping or snapping sound
    Areas to check - Usually means that high voltage is jumping to chassis ground somewhere. Could be anywhere even might be inside a tube

    8. Problem - "Motorboating" (audible osscillating sound)
    Areas to check - bad quad cap or other electrolytic or coupling capacitors

    9. Problem - All tubes have a low bias that can't be turned up high enough
    Areas to check - Weak rectifier tube, weak quad cap, bad bias circuit capacitors or the high voltage AC winding on power transformer is bad

    10. Problem - One output tube or output tube pair will not bias
    Areas to check - One or both of those output tubes are bad or that (those) tube socket(s) has a problem (usually pins 5 or 6 on an octal socket)

    11. Problem - Amp makes a "crackling" noise at high volume levels or on musical peaks (especially bass peaks)
    Area to check - Not enough DC power storage - replace the quad cap

    12. Problem - Unequal volume on the two channels of a Dynaco stereo tube amp
    Areas to check - weak tube(s), bad cathode resistor, bad resistor in the negative feedback line, bad output transformer

    13. Problem - One channel cuts out completely or drops in volume after time (as the amp heats up)
    Areas to check - Bad solder connection, bad solder trace on driver board, bad coupling capacitor

    Bob

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