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    Well, I found a pair of MK III's

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    pjp3

    Posts : 39
    Join date : 2013-03-19
    Age : 54
    Location : Pelham, AL

    Well, I found a pair of MK III's

    Post by pjp3 on 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


    peterh

    Posts : 679
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Well, I found a pair of MK III's

    Post by peterh on 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

    Posts : 39
    Join date : 2013-03-19
    Age : 54
    Location : Pelham, AL

    Re: Well, I found a pair of MK III's

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

    Got it! Thanks!

    GP49

    Posts : 733
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Well, I found a pair of MK III's

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

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

    pjp3

    Posts : 39
    Join date : 2013-03-19
    Age : 54
    Location : Pelham, AL

    Re: Well, I found a pair of MK III's

    Post by pjp3 on 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

    Posts : 679
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Well, I found a pair of MK III's

    Post by peterh on 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.

    GP49

    Posts : 733
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Well, I found a pair of MK III's

    Post by GP49 on 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

    Posts : 157
    Join date : 2012-01-31

    Re: Well, I found a pair of MK III's

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

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

    GP49

    Posts : 733
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Well, I found a pair of MK III's

    Post by GP49 on 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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