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


    MKIII VTA help with negative feedback.

    Share

    scott6058

    Posts : 37
    Join date : 2009-01-21

    MKIII VTA help with negative feedback.

    Post by scott6058 on Sat Mar 28, 2009 2:00 pm

    Is there a value for the negative feedback wire going to the pc board on the VTA MKIII? My second amp performs flawlessly now but the first amp has a low level oscillating buzz and distortion only off the 4 ohm output terminal. I tried swapping tubes.
    With 120.4 volts Ac in I have-
    Half circle 539v dc
    Square 533v dc
    Triangle 490v dc
    Circle 496v dc

    B+ 496v dc
    Pins 2 & 7 6.6v ac
    Old diode 57.3v ac


    Hope maybe someone can make some sense of this Thanks Scott


    Last edited by scott6058 on Sat Mar 28, 2009 7:22 pm; edited 1 time in total (Reason for editing : mistake)

    tubes4hifi
    Admin

    Posts : 1286
    Join date : 2008-11-30

    Re: MKIII VTA help with negative feedback.

    Post by tubes4hifi on Sat Mar 28, 2009 9:16 pm

    Hi Scott,
    the NFB resistor is on the PCB and is a 10K (R7) resistor, along with a 680pf (C11) capacitor.
    These values are optimum when the NFB wire is connected to the 16 ohm tap,
    as is the normal practice.
    If you are taking the NFB connection DIRECTLY from the 4 ohm connection,
    you need to half the value of the resistor to 5.1K and double the capacitor value.
    From your note, you have two MK3 amps and one of them works fine, so you should be able to compare the two directly to find any problems. It seems your B+ voltage is a little high, and if you are still using an original quad cap you may be stressing or overloading it, as they are rated at 525v.
    That would definitely cause a hum problem. But you are saying you only hear this noise when connected to the 4 ohm tap, and only on the one amp?
    Roy www.tubes4hifi.com

    scott6058

    Posts : 37
    Join date : 2009-01-21

    Re: MKIII VTA help with negative feedback.

    Post by scott6058 on Sat Mar 28, 2009 10:11 pm

    Hi Roy
    I am taking the NFB from the 16 ohm tap
    as is normal.
    I have two MK3 amps and one of them works fine, so I should be able to compare the two directly to find any problems. I replaced both quad caps with new Authenticaps surge to 600 volts
    I only hear this noise from the speaker terminal at 4 ohm impedance. Dans advice cures this problem by replacing the R7 resister from 7.5k to 3.9k. I found my pre-wired board had a 10k in this position. Thanks Danf Roy and Bob for all your help.

    Best Regards Scott

    danf

    Posts : 58
    Join date : 2009-01-19

    Watch out for 500V on VTA board!

    Post by danf on Tue Mar 31, 2009 10:32 pm

    Hi Scott,

    You might want to increase the 6.8K resistor on the last section of the filter cap feeding the VTA board. The VTA board wants to see 400V - 440V, and 490V could overstress the 450V rated on-board filter caps. I would guess that 15K to 20K ohms is about right. Please be sure that the filter caps are discharged if you decide to make the change.

    Regards,
    Dan

    scott6058

    Posts : 37
    Join date : 2009-01-21

    Re: MKIII VTA help with negative feedback.

    Post by scott6058 on Tue Mar 31, 2009 11:36 pm

    Hi Dan
    Thanks again.
    Will do it tomorrow and let you know
    Thanks again for all the help.
    Regards Scott

    scott6058

    Posts : 37
    Join date : 2009-01-21

    Re: MKIII VTA help with negative feedback.

    Post by scott6058 on Sun Apr 05, 2009 11:13 am

    Hello Roy Bob and Dan
    My Dyna MKIII's are super.
    The VTA boards really bring them up to speed. The transformation is incredible from original. Perfect in delicate detail. I can't thank you guys enough. Changing the 6.8k resister to 20k at the quad cap brought the B+ in around 400v also. Thanks Dan for your expertise. I'm in the market for new KT88's now and then I'm done. The only thing I will have left is to sit enjoy and show them off.
    Great people and forum. Best regards Scott

    Sponsored content

    Re: MKIII VTA help with negative feedback.

    Post by Sponsored content Today at 8:05 am


      Current date/time is Tue Dec 06, 2016 8:05 am