Marshall DSL401

Discussion in 'The Workbench' started by Carlos Rajoo, Jul 28, 2020.

  1. Carlos Rajoo

    Carlos Rajoo New Member

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    Hi

    I am working on this amp. It had 3 of the pre amp tubes(ECC83) dead. Upon checking, the bridge rectifier that provides the filament voltage has open. I replaced it as well as R133(470K) and R33(10 ohms) which are blown. Now when I turn on the power switch, after a few seconds there is sound. Strange because the standby switch is still in OFF position. I have checked the switch and it is OK. The voltage at W9 is at 460V, but the voltage at W10 (which connects to W8, the center tap of output transformer), with the standby OFF is at 380V. This 380V going to the OT is what causing sound at the speaker with the standy OFF. When I turn ON the standby switch, then the voltage at W10 goes up to 460V. However the sound is much softer and distorted. I have yet to check the bias and tube dissipation, which I will do tomorrow.

    Thanks
     
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  2. tmingle

    tmingle Well-Known Member

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    Did you resolder all the connections on the board? These amps are notorious for bad solder joints. I had 1 and had to resolder 4-5 times over the years. And yes, I know how to solder properly. The EL-84 power tubes are mounted on the bottom of the board, so all the heat rises onto the board.
     

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