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Thread: Vox AC 100 Mk2

  1. #1
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    Vox AC 100 Mk2

    Hey folks,

    My names Rant and I've been building this project for a few months now,
    its a Vox AC 100 Mk2 and is a college project for my final year and my first valve project (quite a large project, for my first but it too late now).
    My intention is to make the bias adjustable in the long run but time restrictions as its a college project are forcing me build the original bias.


    Here is a link to the schematic I've been working from. The problem i'm having at the moment is that when the amp is turn on fully and is idle one of the 100 ohm resistors on the anode of the EL34's starts smoking and then the HT fuse blows. I am thinking of increasing the power rating on these resistors but from reviewing out pictures of the amplifiers on websites it looks like the power rating on these resistors (R26, R27, R28, R29) or around 5 - 7 watts and the power rating of the resistors I have in place at the moment are 5 watt.
    I measured the voltage across one the these resistors when I turned it on and it rose to 16 volts before the HT fuse blow. The bias produces -35V.

    Any observations on this would be very welcome.


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  2. #2
    Supporting Member Jazz P Bass's Avatar
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    Dec 2008
    Wernersville, PA
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    That tube is no good.
    The one that is frying the resistor.

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  3. #3
    Senior Member
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    Welcome, to this sizey forum... There are different sections for different subjects. This thread for example would probably get more replys in debugging your build. (Even if it turns out to be that tube.)

    Good luck with your build!

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    In this forum everyone is entitled to my opinion.

  4. #4
    Old Timer
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    Cornelius, Oregon
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    I modded a Sovtek Mig 100 into a clone of a Vox AC100 myself. It isn't done yet but works well enough. On mine I used relay switching to switch between cathode bias and fixed bias, and with the fixed bias setting I have an adjustable bias in there rather than something hard set. The EL34's and KT77's made today can't take such a high idle current as what the -35v provides, and so I would guess that is a large part of the problem. In one of the Gerald Weber books, (2nd one I think) Ken Fischer talks about the vintage AC100's and says that they would often change to -42v bias and the vintage Mullards would be happy...I'd guess with today's tubes that a cooler bias than that might be good tho. I'm running around 470v plate and around -42v bias when in fixed bias on my clone amp and it is happy with JJ KT77's. I [I][B]think[B][I] I am using 5 watt metal oxide plate resistors on mine, but they could be more than that too...I would have to check again. They are fine in the amp though, but anything lower than them would smoke even with good tubes once the volume was up a bit and the amp was played for awhile.

    I'd try a new set of tubes and change your bias circuit to get something adjustable if possible, and get something that will get a higher negative voltage than the isn't enough.


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