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    Voltage seems a wee bit low, Did I screw the pooch or is this ok?

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    mcgyver74

    Posts : 74
    Join date : 2017-03-24
    Location : Jersey City, NJ

    Voltage seems a wee bit low, Did I screw the pooch or is this ok?

    Post by mcgyver74 on Sat Apr 29, 2017 7:26 pm

    So I noticed the other day that my front tubes seem to be glowing less "Bright" then the rear tubes,  I felt that was odd so I decided to measure some voltages and see what's going on if anythning

    All measurements were taken with speakers connected and shorting plugs across the inputs, also the amp was allowed to warm up for 5 min before taking measurements

    Here is what i see

    Input Voltage from wall 118V AC

    Rectifier (Russian Mullard GZ34)
    Pin 4 -398V AC*
    Pin 6 - 398V AC *
    Pin 2 - 455V DC
    Pin 8 - 457V DC
    (Note, 4 and 6 are hit via yellow sheet mod diode)

    *The power transformer going into the rectifier is putting out 398v ac on pins 4 and 6 BEFORE the yellow sheet mod diodes, on ACTUAL pins 4 and 6 I see 48v AC    I assume this has to do with the diode only letting half the wave through


    Output Tube (Sovtec 6550


    If I am looking at the amp from the front, clockwise starting from lower left

    Lower Left
    Pin 3 - 453V DC
    Pin 4 - 455V DC
    Pin 5 - -57V DC
    Pin 6 - -57V DC
    Pin 8 -  .448 V DC

    Upper Left
    Pin 3 - 455V DC
    Pin 4 - 455V DC
    Pin 5 - -58V DC
    Pin 6 - -58V DC
    Pin 8 - .441V DC

    Upper Right
    Pin 3 - 455V DC
    Pin 4 - 456V DC
    Pin 5 - -59V DC
    Pin 6 - -59V DC
    Pin 8 -  .441V DC

    Lower Right
    Pin 3 - 455V DC
    Pin 4 - 456V DC
    Pin 5 - -55V DC
    Pin 6 - -55V DC
    Pin 8 -  .430V DC

    The filament voltages are all good at the 6550's (within 0.05 of each other) and around 6.5 volts



    These are slightly lower then the manual states I should have (Plate and screen for example on the outputs should be between 470 - 520)
    For the rectifier, the plate2 and cathode (Pins 2 and 8 ) should be between 490 and 530v)

    Is this something I screwed up or is this normal for 118v Input?
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    Bob Latino
    Admin

    Posts : 2506
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Voltage seems a wee bit low, Did I screw the pooch or is this ok?

    Post by Bob Latino on Sat Apr 29, 2017 10:26 pm

    Hi,

    This is pretty normal for 118 VAC input to the amp ... If your VTA ST-120 amp play fine, I would not worry about it. The voltages shown in the manual are for 120 VAC input to the amp. Upping the AC input just a couple of volts will cause a rise of 30 to 40 volts or more on the DC voltages on pins 3 and 4 of all the output tubes. With 120 AC, in the AC voltage on pins 4 and 6 of the rectifier socket should be about 400 - 420 VAC.

    Bob
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    tubes4hifi
    Admin

    Posts : 1343
    Join date : 2008-11-30

    Re: Voltage seems a wee bit low, Did I screw the pooch or is this ok?

    Post by tubes4hifi on Sat Apr 29, 2017 10:58 pm

    most any tube amplifiers run perfectly fine with voltages +/- 10% of nominal spec

    mcgyver74

    Posts : 74
    Join date : 2017-03-24
    Location : Jersey City, NJ

    Re: Voltage seems a wee bit low, Did I screw the pooch or is this ok?

    Post by mcgyver74 on Sun Apr 30, 2017 9:57 am

    Awesome thanks!! Smile

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    Re: Voltage seems a wee bit low, Did I screw the pooch or is this ok?

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