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    VTA 70 bias question

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    tubenutr

    Posts : 62
    Join date : 2015-01-25
    Age : 59
    Location : Northern Ontario

    VTA 70 bias question

    Post by tubenutr on Wed Jun 10, 2015 3:26 pm

    Greetings,

    My '70 loses bias almost on a daily basis. My first indication
    is a distorted sound and measuring the bias, its far less than
    .40 V.

    Driving me nuts, its driving me crazier.
    Any help would be GREATLY appreciated.

    Thanks,

    Mike

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: VTA 70 bias question

    Post by Bob Latino on Wed Jun 10, 2015 3:53 pm

    Depending on the age of the amp, either a bad rectifier tube or a bad quad cap can cause a loss of bias voltage. Try replacing the rectifier tube first because doing so is cheaper and requires less effort ..

    Bob

    tubenutr

    Posts : 62
    Join date : 2015-01-25
    Age : 59
    Location : Northern Ontario

    VTA 70 bias question

    Post by tubenutr on Wed Jun 10, 2015 3:59 pm

    Bob,

    The tube has tested very good and the quad cap is a new CE.
    Can't seem to track down the problem. I have checked all solder
    joints, wiring connections, parts placement on VTA board and all
    seem to be fine.
    Any other recommendations????

    Mike

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: VTA 70 bias question

    Post by Bob Latino on Wed Jun 10, 2015 4:13 pm

    Mike,

    When the bias on your amp does drop, how low does it go from the .400 VDC position and does it do so on ALL output tubes? > Check the line voltage to see if that dropped also. If the line voltage does not drop > Next > check the bias system.

    Pull out all the tubes. Set the bias pots to their mid position. Remove the bottom cover, turn the amp ON and measure ..

    1. The AC voltage to chassis ground where the RED/BLACK wire connects to the driver board. (you should get 50 to 60 volts or so AC). If you don't get this then the power transformer has a bad bias winding.

    2. The DC voltage to chassis ground on the side of the diode that is furthest to the right when the power transformer is facing you. (you should get 70 to 80 volts DC). If you don't get this then the diode is bad.

    Bob

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