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    Measuring Voltage Surge at Turn On

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    BNR_1

    Posts : 78
    Join date : 2013-06-11

    Measuring Voltage Surge at Turn On

    Post by BNR_1 on Wed Dec 18, 2013 9:46 am

    It has been noted often that the intial voltage surge on the quad capacitors on these Dynaco amps may spike above the rated capacity of the capacitors upon energizing the amp. I would like to see how high these surge rises then drops on a Mark III amp. Can anyone tell me specifically where I should be measuring to see these voltage surge? I will be using a standard digital volt meter. This is for a stock Mark III amp without any new driver board,

    Also does the tubes need to be placed in the amp to see a difference in the surge? It is a lot easier to take measurements when the amp is upside down or on its side without any tubes.

    Thank you

    Bob Latino
    Admin

    Posts : 2414
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Measuring Voltage Surge at Turn On

    Post by Bob Latino on Wed Dec 18, 2013 11:08 am

    Have all tubes in the amp and measure the DC voltage to chassis ground off pin #8 of the rectifier tube socket when you turn the amp on. See what it surges up to?

    If you don't have all the tubes in there then the DC voltage WILL go up higher than normal.

    Bob

    BNR_1

    Posts : 78
    Join date : 2013-06-11

    Re: Measuring Voltage Surge at Turn On

    Post by BNR_1 on Wed Dec 18, 2013 1:42 pm

    Thanks Bob for confirming.

    BNR_1

    Posts : 78
    Join date : 2013-06-11

    Re: Measuring Voltage Surge at Turn On

    Post by BNR_1 on Sat Dec 21, 2013 5:56 pm

    For those who maybe curious, so with the amp plugged into a bucking transformer (114V) and without any tubes I could get 0 volts at pin 8. With only the rectifier in place (JJ 5AR4) the measurement went up to 553 volts.

    Bob I guess your comment about if not all the tubes are in.....you meant at least the rectifier. I did not check the peak voltage off of pin 8 with all the tubes in place. I should have. Will try that for another day.

    Bob Latino
    Admin

    Posts : 2414
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Measuring Voltage Surge at Turn On

    Post by Bob Latino on Sat Dec 21, 2013 7:38 pm

    You should never run any tube amp with JUST the rectifier in there. If you do, the voltage will go up higher than normal and put additional stress on the quad cap and all the other electrolytic caps in the amp. Any time the rectifier is in the amp ALL THE OTHER tubes should also be in there to provide a voltage drop to keep the B+ voltages down into a normal range.

    Bob

    BNR_1

    Posts : 78
    Join date : 2013-06-11

    Re: Measuring Voltage Surge at Turn On

    Post by BNR_1 on Sat Dec 21, 2013 10:51 pm

    Thanks Bob for letting me know.

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