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Thread: Adjustable bias and measuring points Valve King 112

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    Member torcamaniac's Avatar
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    Adjustable bias and measuring points Valve King 112

    Hi guys,
    I'll be adding a bias pot and measuring jacks at the back panel of the amp.
    Most people added one 1Ohm cathode resistor on pin 8 of power tubes, each. I am not planing on adding balance pots for each tube (and my tubes are well matched)so my first question is if it's okay to add cathode resistor to only one tube and its measuring point respectively, for bias monitoring?
    My second question is if it is okay to add a measuring point from pin 3 to the back so I can monitor plate voltage from outside while biasing?
    This way I would have 3 banana jacks and a hole for cermet bias pot at the back. 1 jack for bias, 1 jack for ground and 1 jack for plate voltage...

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    Lifetime Member Enzo's Avatar
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    Or just a few screws and the chassis comes out for easy access.

    You may not plan to make individual adjust controls - and I myself think that would be overkill - but really, add the 1 ohm resistors to all four sockets, even if you only bring one to the panel. It will give you current reading access for troubleshooting later in life. And resistors are cheap.

    My personal choice would be pin jacks rather than banana. A pin jack takes your probe point. My concern is that you want a jack that has no chance comeone can touch it with a finger, and is not readily poked my anything conductive. That is going to be 400-500 volts, which is lethal. Or thinking about it, maybe a terminal strip behind a rectangular chassis hole, and a screw-on cover plate. Then you remove the cover plate, and can probe all four points, well five including your tube plate.

    One could have four test points brought out to one jack with a four way selector switch next to it.

    I personaly am not comfortable with plate voltage on a test point, but I am sure many guys do it. All it takes is one moron with a paper clip sticking it in ther thinking "Oh these test points only have a few millivolts..." Help, I've fallen, and I can't get up.

    As far as the amp is concerned, test points don;t exist, so add them as you like. I explained my thoughts on it, but none of the arrangements affect performance, so one post or four, go ahead.

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    Member torcamaniac's Avatar
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    Quote Originally Posted by Enzo View Post
    Or just a few screws and the chassis comes out for easy access.

    You may not plan to make individual adjust controls - and I myself think that would be overkill - but really, add the 1 ohm resistors to all four sockets, even if you only bring one to the panel. It will give you current reading access for troubleshooting later in life. And resistors are cheap.

    My personal choice would be pin jacks rather than banana. A pin jack takes your probe point. My concern is that you want a jack that has no chance comeone can touch it with a finger, and is not readily poked my anything conductive. That is going to be 400-500 volts, which is lethal. Or thinking about it, maybe a terminal strip behind a rectangular chassis hole, and a screw-on cover plate. Then you remove the cover plate, and can probe all four points, well five including your tube plate.

    One could have four test points brought out to one jack with a four way selector switch next to it.

    I personaly am not comfortable with plate voltage on a test point, but I am sure many guys do it. All it takes is one moron with a paper clip sticking it in ther thinking "Oh these test points only have a few millivolts..." Help, I've fallen, and I can't get up.

    As far as the amp is concerned, test points don;t exist, so add them as you like. I explained my thoughts on it, but none of the arrangements affect performance, so one post or four, go ahead.

    Thanks Enzo.
    I got vk 112 so only two power valves.
    The banana sockets I got are insulated and I have test cables for multi meter with banana jacks on other end.
    Concerning the plate voltage banana socket it would be safe from anybody poking there.
    It's just that I've never seen people doing bias mods and adding plate voktage test points and that's why I asked.
    Anyway I'll add a resistor to the second tube also when I am already inside.
    One thing I don't like is drilling but I have to do it hahaha...
    One for the MV pot, 3 for bias and 1 for boas pot...

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    Lifetime Member Enzo's Avatar
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    My take on safety:
    A friend of mine had a 3 year old son, and the kid liked to play with dady's key ring. Shiny and jingley. One day the kid wandered into the room, and said "Daddy, it hurt me." He got up to go see, and found the kid had inserted a key into a wall outlet hot side, and the sparks had burned little fingers. Who would'a thought?

    Sure, guys have put plate test points on their panels. COmmercial amp makers wouldn't do that though.

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    Member torcamaniac's Avatar
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    Quote Originally Posted by Enzo View Post
    My take on safety:
    A friend of mine had a 3 year old son, and the kid liked to play with dady's key ring. Shiny and jingley. One day the kid wandered into the room, and said "Daddy, it hurt me." He got up to go see, and found the kid had inserted a key into a wall outlet hot side, and the sparks had burned little fingers. Who would'a thought?

    Sure, guys have put plate test points on their panels. COmmercial amp makers wouldn't do that though.
    I guess... Better safe than sorry. I'll better put a plastic plug in that socket...
    Thanks Enzo

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    Quote Originally Posted by torcamaniac View Post
    Better safe than sorry. I'll better put a plastic plug in that socket...
    Thanks Enzo
    I don't like the idea of having the plate voltage on a banana socket either. It will have to withstand a peak voltage of nearly 2 x plate voltage at full output. I'd use a 1000:1 divider (1M:1k) to the socket and use the meter on the mV scale to measure volts. That would make it safe (450mV) and short circuit proof.

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    Quote Originally Posted by Dave H View Post
    I don't like the idea of having the plate voltage on a banana socket either. It will have to withstand a peak voltage of nearly 2 x plate voltage at full output. I'd use a 1000:1 divider (1M:1k) to the socket and use the meter on the mV scale to measure volts. That would make it safe (450mV) and short circuit proof.
    Thats a great idea Dave...
    So a 1Meg to pin 3 than 1k from it to ground and then a wire to the socket from the point between the two resistors.
    1k/1.001M x 450V = 0.449 Volt
    Thanks man, should've thought about it myself...

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    VK112 Bias Test Points

    Thought I'd register as a MEF member so I could chime in on this thread. I just added both a bias pot and bias test points to my VK112. I didn't find any web references to adding bias test points to the VK112 so I thought I'd document what I did.

    I drilled through the PCB trace coming off pin 8 from each of the 6L6, this was the 'iffy' part when I first considered this mod. Works great!

    I can't seem to insert a photo from my local drive...

    BTW, the bias was cold as suspected; 22ma/24ma

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    Last edited by LesPaulLarry; 04-17-2019 at 03:37 PM. Reason: trying to insert picture

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    Supporting Member TomCarlos's Avatar
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    Welcome LPLarry and thanks for the post.

    When you reply, make sure you click on the "Go Advanced" button - that will bring up the tools for uploading photos.

    I want to make sure I understand what you did. Instead of running wires to the pins of the tube sockets, you drilled through the trace and connected wires directly to the trace? I would worry that the trace could be compromised over time.

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    Member torcamaniac's Avatar
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    Sorry cant upload anything it seems... I'll try to upload the file and post a link later

    So it works now.
    Attached Files Attached Files

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    Last edited by torcamaniac; 04-17-2019 at 04:29 PM. Reason: upload files

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