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    High bias on Mark IV amps

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    stevenally

    Posts : 3
    Join date : 2013-12-07

    High bias on Mark IV amps

    Post by stevenally on Thu Nov 06, 2014 4:52 pm

    Just picked up a set of Mark IV amps. Prior to plugging them in I replaced the Quad caps and selenium rectifiers (Dynakit parts) and the 100UF caps.
    I am having a problem with high bias voltage. With the pot turned down all the way I am seeing about 3.7 volts at the test point.
    Have tried several sets of power tubes (EL34 & KT66) and several rectifier tubes (new repro and vintage) with no change.
    Can anyone point me in the right direction to start troubleshooting the amps ?

    peterh

    Posts : 643
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: High bias on Mark IV amps

    Post by peterh on Thu Nov 06, 2014 5:23 pm

    You should have tested it before changing components !

    The probem you see might be an effect of the Si diod has less forward drop then the
    Se you replaced. Thus you will have to adjust the resistor 10K to ground, replace with 5.6k 1/2w.

    tubes4hifi
    Admin

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    Join date : 2008-11-30

    Re: High bias on Mark IV amps

    Post by tubes4hifi on Mon Nov 10, 2014 1:23 pm

    Peter has the correct answer!

    stevenally

    Posts : 3
    Join date : 2013-12-07

    Re: High bias on Mark IV amps

    Post by stevenally on Mon Nov 10, 2014 4:58 pm

    Thanks. Changed resistors and all is well. After a bad experience with selenium rectifiers in a pair of Mark III amps I rebuilt , I make it a point of replacing them before I even put power to them. Also of the 6 or 7 Dynaco units I have rebuilt I have always had to change the quad caps and 50uf caps if present as they are always way out of spec. This is the first pair of mark IV amps i have rebuilt and I never ran into this before on the mark III or st70s. Again , thanks for your help.Just finishing up a pas 2
    with Zmod boards and new power supply from tubes4hifi to pair with the mark IV amps. Can't wait to dust off some vinyl and see what it sound like.

    howlin' hoosier

    Posts : 29
    Join date : 2012-04-30

    Re: High bias on Mark IV amps

    Post by howlin' hoosier on Fri Feb 19, 2016 6:47 pm

    Is the 10K to 5.6K 1/2w resistor swap the 10K resistor on lug #5 to the bias pot lug #1?

    tubes4hifi
    Admin

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    Re: High bias on Mark IV amps

    Post by tubes4hifi on Fri Feb 19, 2016 9:05 pm

    Howlin'
    uhh . . . gee . . . you're the one with the MK4 amp, is that the resistor that is grounded on one end?
    If you're not sure, guess I'll have to find a manual and get back to you on that . . . .

    (Peter said . . . adjust the resistor 10K to ground, replace with 5.6k 1/2w )

    howlin' hoosier

    Posts : 29
    Join date : 2012-04-30

    Re: High bias on Mark IV amps

    Post by howlin' hoosier on Sat Feb 20, 2016 5:17 pm

    tubes4hifi wrote:Howlin'    
    uhh . . .  gee . . . you're the one with the MK4 amp, is that the resistor that is grounded on one end?
    If you're not sure, guess I'll have to find a manual and get back to you on that . . . .

    (Peter said . . . adjust the resistor 10K to ground, replace with 5.6k 1/2w )

    Sorry if I asked something that's old hat to you- I wasn't sure if both 10K resistors needed to be pulled, or just the one that goes to the bias pot.

    tubes4hifi
    Admin

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    Join date : 2008-11-30

    Re: High bias on Mark IV amps

    Post by tubes4hifi on Sun Feb 21, 2016 2:02 pm

    Howlin' . . . (and everyone else that wants to understand how the bias circuit works) . . .
    looks like Steven who posted the original question followed the suggestion and the fix worked.
    Now think about this logically, and the answer should come to you.
    The bias circuit has a diode, a 10K resistor, a 10K pot, and another 10K resistor to ground.
    The problem is there isn't enough negative bias voltage to bring the tube current down.
    So Peter suggested replacing the 10K resistor connected to ground to 5.6K.
    To make this really simple, assume there was about -60vdc after the diode, then divided down by the two resistors and pot.
    As was, the center position of the pot would have around -30vdc. By changing the resistor to ground,
    the center position of the pot would now have around -35vdc (more negative, which brings the tube current down).
    Or he could have changed the 10K connected to the diode to about 15K, leaving the other 10K to ground the same.
    Same effect. The center of the 10K would now be more negative.
    On my MK3 amplifier upgrade (which is the same as the MK4 circuit) what I do is change all 3 resistors to give a wider range of adjustment.
    I connect a diode to the negative bias lead of the transformer, add 100uF to ground (was 50uF originally, but don't go larger than 100uF or the bias will come up way too slow).
    Then a 1K resistor (instead of 10K), then another 100uF to ground (this removes more of the ripple voltage, making cleaner DC), then a 50K pot so you have a very WIDE adjustment range, then finally a 27K to ground. This makes the total resistance to ground 78K (instead of the original 30K), but the center of the pot is now around -40vdc, with a range of about -30 to -50vdc instead of the original narrow range of -25v to -35v. Seems newer production tubes want a more negative bias than the older tubes, plus line voltage out of the wall is closer to 120vac now than the 110vac that was 50-60 years ago.

    peterh

    Posts : 643
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: High bias on Mark IV amps

    Post by peterh on Sun Feb 21, 2016 2:47 pm

    The reason that the bias voltage increased was that the Si diode has much less forward drop then the Se it replaced. Thus the - voltage became even more.

    howlin' hoosier

    Posts : 29
    Join date : 2012-04-30

    Re: High bias on Mark IV amps

    Post by howlin' hoosier on Fri Mar 18, 2016 3:57 pm

    I appreciate the explanation of how the biasing works/needs to be altered when dumping the selenium rectifier and swapping in a diode.

    I wired in a 5.6K resistor into the bias circuit, and and then ordered a blank board from Dynakit to replace a suspect board (traces were lifting up slightly here and there). I noticed one of the sockets for the EL34s was a bit sloppy, so grudgingly replaced those as well (a PITA to be sure, but well worth the time). The bias now holds steady without creeping up after several minutes. It's nice having a pair of amps working properly again...

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