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The Dynaco Tube Audio Forum

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


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    Dynaco Mark II to Mark III conversion

    Bob Latino
    Bob Latino
    Bob Latino
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    Location : Massachusetts
    • Post n°1

    Dynaco Mark II to Mark III conversion Empty Dynaco Mark II to Mark III conversion

    Post by Bob Latino Tue Jun 25, 2019 9:52 am

    The Dynaco Mark II was Dynaco's first product which came out in 1955. In 1957 Dynaco upgraded the existing Mark II (50 watts using EL34 tubes) into a Mark III (60 watts using KT88 tubes). Many Mark II users wanted to upgrade their amps to Mark III's so Dynaco sent out this bulletin to all their Dynaco dealers. The bulletin listed the steps that would need to be taken to upgrade their amps. In general you had to upgrade some capacitors and alter the bias system to be able to crank in more negative DC to control the newer KT88 output tubes. You also had to add a choke and use a 5AR4 in place of the Mark II's 5U4. Check out the complete bulletin below which gives more detail on the conversion.

    Bob

    Dynaco Mark II to Mark III conversion Mark-II-to-Mark-III-change


    Last edited by Bob Latino on Wed Jul 17, 2019 7:18 am; edited 1 time in total
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    harlantk
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    harlantk


    Posts : 11
    Join date : 2019-06-25
    • Post n°2

    Dynaco Mark II to Mark III conversion Empty Re: Dynaco Mark II to Mark III conversion

    Post by harlantk Tue Jul 16, 2019 1:30 am

    First I want to say thank you Bob for posting this! I was looking for a copy of the issued letter for MK II to upgrade for the alternate tubes.
    I would like to pose a question to you, concerning a MK II I am restoring. In the schematic for the MK II is a 25 pF for anti-oscillation, and for the MK III is a 12 pF, both from pin 1 back to pin 6 or plate to plate. Do you or does anyone know why they changed the value of this is my first question.
    My second question is, on the board I am looking at here, there is not a 25, or 12 pF capacitor on the board, nor are there any mounting holes for this fella. Was this an addition to the MK series after first production?
    I am curious about your thoughts here if you wouldn't mind.
    Thank you
    Harlan
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    WLT
    WLT
    WLT


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    Join date : 2013-07-13
    Location : Rochester NY
    • Post n°3

    Dynaco Mark II to Mark III conversion Empty Re: Dynaco Mark II to Mark III conversion

    Post by WLT Tue Jul 16, 2019 8:45 am

    The early, wide boards in the MK II do not have a location or holes drilled for either of the capacitor values. Only the later, narrower MK III style boards have that. I assume you have an amp with the wider board. Correct?

    I restored a pair of amps with early style boards with no feedback cap in that position. All stock value component replacements. They sound fine and I have had zero trouble with either amp. I would like to see the actual reason/value for those caps with test results to back it up. I have not tried any testing myself.
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    harlantk
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    harlantk


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    • Post n°4

    Dynaco Mark II to Mark III conversion Empty Re: Dynaco Mark II to Mark III conversion

    Post by harlantk Tue Jul 16, 2019 3:21 pm

    @WLT
    Thank you for the info, yes I do have the wider boards. this makes sense then, the wider board would be the older production then is my thinking. This indicates then they added the cap on later production, then lowered the value when producing the MK III.
    I will add the cap to the restoration and use the 12 pF for this, keeping with the findings of the engineering of the time, assuming this to be the case.
    I am going to run these through a battery of tests once I get my bench put together (Just moved and living out of boxes), and like you I would like to see  test results from the addition and change of value of this cap. I will post a photo of this board later, start a new thread for the restorations would be best for this.
    Again thank you for the information.
    Harlan  (Tim)
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    • Post n°5

    Dynaco Mark II to Mark III conversion Empty Re: Dynaco Mark II to Mark III conversion

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