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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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    Switching 5U4 rectifier for 5AR4 in Mk IIIs

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    Dingojazz


    Posts : 73
    Join date : 2010-09-03

    Switching 5U4 rectifier for 5AR4 in Mk IIIs Empty Switching 5U4 rectifier for 5AR4 in Mk IIIs

    Post by Dingojazz Tue Dec 09, 2014 11:36 am

    In an earlier post dealing with use of EL 34 and 6L6 power tubes in VTA octal Mk. IIIs, Bob L. indicated that changing the 5AR4 rectifier out to a 5U4 would drop the B+ voltage into a safer range and allow use of EL 34 and 6L6 power tubes.  

    Question: so would the rectifier swap be the only change required (i.e. no resistor or other changes needed to the circuit)?

    Thanks,

    Kevin
    peterh
    peterh


    Posts : 1823
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Switching 5U4 rectifier for 5AR4 in Mk IIIs Empty Re: Switching 5U4 rectifier for 5AR4 in Mk IIIs

    Post by peterh Tue Dec 09, 2014 12:36 pm

    the 5U4 has direct heaters, and a smaller cathode area, this gives a larger voltage drop.
    Look at it as an "built in series resistor", larger then the one in a 5AR4
    avatar
    Dingojazz


    Posts : 73
    Join date : 2010-09-03

    Switching 5U4 rectifier for 5AR4 in Mk IIIs Empty So does that mean

    Post by Dingojazz Tue Dec 09, 2014 1:30 pm

    that the "built in series resistor" of the 5U4 rectifier tube would be the only change needed to switch to running EL34 and 6L6 power tubes in my VTA octal Mk IIIs??

    (Sorry to be so literal, but with my limited knowledge, I find it the only tenable option)

    Thanks,

    Kevin
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    GP49


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

    Switching 5U4 rectifier for 5AR4 in Mk IIIs Empty Re: Switching 5U4 rectifier for 5AR4 in Mk IIIs

    Post by GP49 Tue Dec 09, 2014 1:41 pm

    Directly heated, the 5U4 also lacks soft-start but that didn't bother Dynaco, which used it in the Mark II power amplfier.  

    As a very common rectifier in tube-type color TVs, the 5U4 is relatively common and thus often cheap!
    avatar
    Dingojazz


    Posts : 73
    Join date : 2010-09-03

    Switching 5U4 rectifier for 5AR4 in Mk IIIs Empty Thanks-

    Post by Dingojazz Tue Dec 09, 2014 2:22 pm


    All good info. I have a couple of us-made labeled "Hammond" which I think are GE...

    Kevin

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