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    Switching 5U4 rectifier for 5AR4 in Mk IIIs

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    Dingojazz

    Posts : 39
    Join date : 2010-09-03

    Switching 5U4 rectifier for 5AR4 in Mk IIIs

    Post by Dingojazz on 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

    Posts : 641
    Join date : 2012-12-25
    Location : gothenburg, sweden

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

    Post by peterh on 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

    Dingojazz

    Posts : 39
    Join date : 2010-09-03

    So does that mean

    Post by Dingojazz on 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

    GP49

    Posts : 717
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

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

    Post by GP49 on 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!

    Dingojazz

    Posts : 39
    Join date : 2010-09-03

    Thanks-

    Post by Dingojazz on 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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