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    Initial Power-On Procedure

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    Yvon Massicotte

    Posts : 13
    Join date : 2009-07-28
    Age : 74
    Location : Montréal, QC, Canada

    Initial Power-On Procedure

    Post by Yvon Massicotte on Thu Jan 17, 2013 4:26 pm

    Hi,

    I've just finished building my ST-70 with the VTA board (low gain version).

    I build the amp from scratch using an unbuilt kit from the early sixties and the VTA board. I tested the output transformers beforehand but not the power transformer. So, I have no idea on how the components will react; they are new but 50 years old!

    I intend to proceed step by step in turning this amp on to avoid any catastrophic failure. These are the steps:

    .- pulling out all tubes, connect resistance loads, check the AC voltages at the different secondaries of the power transformer: the B+ one, the filament ones;

    .- if the power transformer is working fine, then I will insert the rectifier tube and check the DC voltages and inspect their forms on a scope;

    .- only then, if the values seem on range, I would insert one set of output tubes and check and adjust roughly the bias; I will do the same for the other channel;

    .- I will next insert the 3 driver tubes and, after appropriate warmup time fine-tune the bias;

    .- I will proceed to the AC balance test;

    .- I will again check voltages and finally I will inject a 1000 Hz signal and check for clipping on a scope.

    Have I forgotten something here or am I doing a useless or much worse a harmful procedure ?

    I would appreciate your feedback


    Yvon

    peterh

    Posts : 642
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Initial Power-On Procedure

    Post by peterh on Thu Jan 17, 2013 4:58 pm

    Connect power resistors as speaker load ( unless that is what you mentioned with
    "connect resistance loads," ).

    If the electrolyte capacitor is 50 years old i would suggest you start with slowly
    charging it with an external source ( increase 50V a time, allow current to diminish, then
    increase 50V again. ). Or you may start with a fresh one .. Otherwize it sound to be a good plan.


    Yvon Massicotte

    Posts : 13
    Join date : 2009-07-28
    Age : 74
    Location : Montréal, QC, Canada

    Re: Initial Power-On Procedure

    Post by Yvon Massicotte on Thu Jan 17, 2013 11:09 pm

    Dear Peterh:

    The resistance loads that I mentioned are indeed power resistors (Dale 8ohms 250 W). The QUAD capacitor is also new one bought with the VTA board.

    I appreciate you input.

    Best regards,

    Yvon

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