The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Dynaco VTA tube amp kits, all Tubes4hifi.com products and all Dynakitparts.com products


    Voltage question on Mk4

    Share

    harpy

    Posts : 94
    Join date : 2012-02-04

    Voltage question on Mk4

    Post by harpy on Sun Mar 18, 2012 1:01 pm

    I think I am to a point where I am starting to check voltage. GZ34 voltage:

    pin 2 440 DC measured 488 DC
    pin 4 370 AC measured 393 AC
    pin 6 370 AC measured 394 AC
    pin 8 440 DC measured 488 DC

    Is that in the ball park or do the voltages need to go down? Mods so far are Dyna kit cans.

    Thanks,
    Jim

    DynakitParts
    Admin

    Posts : 183
    Join date : 2008-11-30

    Voltages...MK IV

    Post by DynakitParts on Sun Mar 18, 2012 1:50 pm


    Jim...These are high. Are you measuring these voltages with all tubes installed & correctly biased? Is your line voltage a bit high?

    Kevin

    harpy

    Posts : 94
    Join date : 2012-02-04

    Re: Voltage question on Mk4

    Post by harpy on Sun Mar 18, 2012 2:05 pm

    Hi Kevin,

    Line voltage is 120-121 Ac. All the tubes are in and Bias is set at 1.42. Both amps measure very similar. The other amp is around 479 VDC 384 VAC.

    DynakitParts
    Admin

    Posts : 183
    Join date : 2008-11-30

    Re: Voltage question on Mk4

    Post by DynakitParts on Sun Mar 18, 2012 2:39 pm


    Jim,
    Remove the rectifier tube and measure the AC voltages at Pins #4 and #6 at rectifier
    socket V1. Let's see if these are about 10 to 15 volts higher.

    Kevin

    harpy

    Posts : 94
    Join date : 2012-02-04

    Re: Voltage question on Mk4

    Post by harpy on Sun Mar 18, 2012 3:20 pm

    Yes, pin 4 and 6 are 393 vac with tube, 404 vac without.

    DynakitParts
    Admin

    Posts : 183
    Join date : 2008-11-30

    Re: Voltage question on Mk4

    Post by DynakitParts on Sun Mar 18, 2012 4:47 pm


    Jim,
    I suspect the problem is in part the fact that the PA-135 was wound for a 117 vac primary. Wih a line voltage of 120 vac plus the secondary high voltages will be approximately 11 volts higher.

    Looking back at my original test notes on the MK IV amplifier...The secondary voltages I measured were about 13 to 15 volts higher on the high voltage secondary's under no load. This was measured on an original Dynaco MK IV amplifier.I tested several samples which confirmed these measurements to be accurate. Appears the
    published specs conflict with the actual voltage measurements.I still use these MKIV amps and have not had any major issues other than the power transformer gets quite hot.

    We redesigned our Dynakit Replacement PA-135 power transformer to operate at 120 vac while retaining the the original published secondary voltages within 1-2%.

    I would suggest adding a CL-90 Thermistor on the AC (120) vac primary side which should reduce your voltage by about 3 volts. This will bring the voltage down closer
    to the original 117 vac spec. Another option would be to use a variac which will allow you to dial in the voltage exactly.

    A third option would be to contact Sal Brisindi...He is a member of this forum. Sal has a neat plug-in soft start with a voltage reducing transformer. Send me an email
    and I will provide you will Sal's contact information.

    Regards,

    Kevin





    harpy

    Posts : 94
    Join date : 2012-02-04

    Re: Voltage question on Mk4

    Post by harpy on Sun Mar 18, 2012 5:26 pm

    Kevin,

    Thanks a lot that really helps. Just had one more question. Amp 1 measures 392 vac with rectifier removed at pins 4 and 6. Amp 2 measures 401 vac this time around. Is the 9 vac difference between the amps a big deal? They may have been from slightly different eras as the transformer covers were different. Again many thanks.


    DynakitParts
    Admin

    Posts : 183
    Join date : 2008-11-30

    MK IV Voltages

    Post by DynakitParts on Sun Mar 18, 2012 6:00 pm


    Jim,
    There could be some variables from lot to lot on transformers.I don't think your going to hear a difference from one amp to the other.

    I would install the tubes..put on some music & enjoy.

    Kevin

    harpy

    Posts : 94
    Join date : 2012-02-04

    Re: Voltage question on Mk4

    Post by harpy on Sun Mar 18, 2012 6:22 pm

    Thanks again Kevin. I am going to put on music and do just that, well when the tornado sirens stop going off.

    Sponsored content

    Re: Voltage question on Mk4

    Post by Sponsored content Today at 10:03 pm


      Current date/time is Wed Dec 07, 2016 10:03 pm