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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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    Well, I found a pair of MK III's

    pjp3
    pjp3


    Posts : 39
    Join date : 2013-03-19
    Age : 62
    Location : Pelham, AL

    Well, I found a pair of MK III's Empty Well, I found a pair of MK III's

    Post by pjp3 Wed Oct 02, 2013 2:14 pm

    It's an interesting pair of MK 3's to say the least, the amps where modified by Mike Moffat of Schiit audio fame. They sound amazing!
    Not sure exactly what he did, some of you may. There is no can, but a lot under the hood.

    Pic here

    Well, I found a pair of MK III's Amp210
    peterh
    peterh


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    Location : gothenburg, sweden

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    Post by peterh Wed Oct 02, 2013 4:39 pm

    There seems to still be a selenium rectifier

    Change this to a SI ( 1n4007 will be excellent ) ASAP. If it fails it might ruin your tubes !

    (It's the rectangular thing in the middle that has a bolt and nut through it. )
    pjp3
    pjp3


    Posts : 39
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    Post by pjp3 Wed Oct 02, 2013 5:01 pm

    Got it! Thanks!
    avatar
    GP49


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    Post by GP49 Thu Oct 03, 2013 10:21 am

    Lots of 330V photoflash capacitors, connected in series for 660 working volts.
    pjp3
    pjp3


    Posts : 39
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    Post by pjp3 Thu Oct 03, 2013 1:32 pm

    GP49 wrote:Lots of 330V photoflash capacitors, connected in series for 660 working volts.
    and that's a good thing... right?

    I plan on replacing the boards with the octal upgrade from Roy and that should eliminate
    the selenium rectifier.
    peterh
    peterh


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    Post by peterh Thu Oct 03, 2013 1:37 pm

    pjp3 wrote:
    GP49 wrote:Lots of 330V photoflash capacitors, connected in series for 660 working volts.
    and that's a good thing... right?

    I plan on replacing the boards with the octal upgrade from Roy and that should eliminate
    the selenium rectifier.
    wize move. You also get individual bias adjustment possibilities.
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    GP49


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    Post by GP49 Thu Oct 03, 2013 2:07 pm

    pjp3 wrote:
    GP49 wrote:Lots of 330V photoflash capacitors, connected in series for 660 working volts.
    and that's a good thing... right?
    Photoflash capacitors were a popular way to add lots of capacitance in a small space.  Since most consumer
    photoflash tubes operate at 300 volts DC, the capacitors are made at a working voltage of 10% higher, or 330 DC.  

    For the B+ voltages in amplifiers such as the Mark III this is not enough so they get connected in SERIES
    (with a voltage equalizing resistor across each one).  The resultant capacitance is HALF what's printed on
    the capacitor.  So the two big ones in the photo are the equivalent of 375μF, 660 DC working volts.

    The bottoms of my modified Mark II amplifiers have photoflash capacitors in them, too. The original quad
    cans are disconnected. They are there just to maintain stock appearance at the top of the chassis.
    hawaii.ken
    hawaii.ken


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    Post by hawaii.ken Thu Oct 03, 2013 11:48 pm

    Photoflash capacitors were never intended for continuous ripple current such as found in power supply applications..
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    GP49


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    Post by GP49 Fri Oct 04, 2013 12:39 am

    That may be true. In their normal applications, they didn't see 120Hz as put out by a full-wave rectifier, but instead the high-frequency pulsating DC produced by the chopper power supplies in photoflash units. Mine have worked just fine and have been absolutely reliable...not one failure. I've bypassed them with film capacitors, as I would any power supply filter capacitor.

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