The Dynaco Tube Audio Forum

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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


4 posters

    Polk cobra cables

    tubenutr
    tubenutr


    Posts : 78
    Join date : 2015-01-25
    Age : 66
    Location : Northern Ontario

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    Post by tubenutr Sun Dec 13, 2015 11:37 pm

    Has anyone ever tried these cables with a VTA 70?
    corndog71
    corndog71


    Posts : 840
    Join date : 2013-03-19
    Location : It can get windy here

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    Post by corndog71 Mon Dec 14, 2015 4:32 pm

    Never heard of those. I'm a fan of Kimber 4TC or 4VS if you want to save a few bucks.
    deepee99
    deepee99


    Posts : 2244
    Join date : 2012-05-23
    Location : Wallace, Idaho

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    Post by deepee99 Tue Dec 15, 2015 1:07 pm

    I have had nothing but good luck with Blue Jeans cables. They are the only folks who use Belden wire, which is the best copper on the planet and their terminations are very good, cold-welded.
    tubenutr
    tubenutr


    Posts : 78
    Join date : 2015-01-25
    Age : 66
    Location : Northern Ontario

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    Post by tubenutr Tue Dec 15, 2015 4:44 pm

    I've had a pair sitting around for about 25 yrs and was just wondering if they will cause any problems
    with a VTA 70
    deepee99
    deepee99


    Posts : 2244
    Join date : 2012-05-23
    Location : Wallace, Idaho

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    Post by deepee99 Tue Dec 15, 2015 5:55 pm

    Never heard of copper rotting, but I guess there's a first time for everything.
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    GP49


    Posts : 792
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

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    Post by GP49 Tue Dec 15, 2015 7:07 pm

    I wouldn't touch them.   Polk had an idea that they could reduce the cable inductance by making the cable out of a multitude of individual insulated strands...half of them comprising one conductor, the other half, the other.  They succeeded in reducing inductance but greatly increased capacitance, like hooking up a big capacitor across the output of the amplifier.  This drives some amplifiers nuts.  Some amplifiers oscillate under a capacitive load (stock Dynacos do not).  Some transistor amplifiers would BLOW UP!  Polk finally tried to compensate by adding a resistor/inductor network, which would defeat the whole purpose of the design to reduce inductance in the first place.

    Even if a high capacitance load doesn't cause the amplifier to oscillate, it can roll off the high frequencies at the output, affecting the feedback loop's operation and altering the amplifier's sound.

    If the enamel insulation cracked or was otherwise compromised, there was the chance of a dead short across the amplifier output terminal to ground.  Not good under any circumstance.

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