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Thread: 5150 bias questions

  1. #1
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    5150 bias questions

    Hello everyone! First post. I am new to toying around with amps but eager to learn and would greatly appreciate any advice you can help me with. I am going to add a bias pot to my 5150. There are a million posts out there with directions on how to do it. But the problem I have is, I dont know how to bias the amp once I have done the modification. I am a diesel mechanic ( heavy equipment ) so I have a multimeter in ye ol tool box but no test probe. Can anyone school me on this a little bit? I have read people explaining ohms law (which I understand) and adding a 1 ohm resister, i have herd people saying to uses a bias probe, others a tube tester. and no matter what tool I use I still dont know were the test points are. I don't know what to do with the meter or probe of whatever is this best tool for the job haha. Thanks in advance for any advice or shove in the right direction you can give me/ Book recommendations to get me started on the tube amp building/modification hobby would also be greatly appreciated!

  2. #2
    Senior Member Enzo's Avatar
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    Hi. Welcome.

    You have read up on how to modify the amp, but you need to read up on what bias means.

    ANd here is a general article about making the readings and adjustments.
    Bias FAQ

    You need a way to measure the flow of current through your tubes, either directly or indirectly. The 1 ohm resistor in series with the tube is a time honored way to do that. But the 5150 has serious physical reasons that it is difficult to add to the amp. There is no easy way to add the resistors in this amp.

    Think of a fuel line to an engine you work on. It is just a tube or hose or little pipe. You want to know how much fuel flow there is in the line. SO imagine you cut out a small section and added in a little flow meter of some sort. That is what we are doing with the 1 ohm resistor. We cut open the current path and add in the resistor. All the current flows through it, but it has such low resistance that it doesn;t interfere. And there is a VERY fundamental concept known as Ohm's Law that tells us how to easily convert a voltage reading across that resistor into the current flowing through it.

    And just to complete my analogy, the 5150 is like a fuel line running inside a frame member where you can;t get at it to add the flow meter. CLose enough.

    You can't add a resistor, at least not at your skill level, but you can use a bias probe. Bias probes come in simple adaptors for your hand held meter up to pieces of test equipment with multiple meters right on them. But the essence of all of them is an adaptor. You pull a power tube, plug the adaptor into the empty socket, then plug the tube into the adaptor. And inside the adaptor? A 1 ohm resistor. SO we accomplish the same thing but without having to modify the amp in that way.

    ANy bias probe or bias meter you buy will come with instructions, and I suspect any you are considering would have the owner manual available to read so you can get an idea what you are going to get.

    The method I use and a lot of other techs as well is the "shunt method." I recomment you maybe get a little more experience before using that. It involves taking readings across the output transformer leads - directly on your high voltage. The voltages inside these amplifiers can KILL YOU. So the bias probe is recommended. It operates at low voltage and also doesn;t require taking readings right on the tube socket pins.
    Education is what you're left with after you have forgotten what you have learned.

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    Thanks abunch for taking the time to write that out for me. It was very helpful, as was the link provided. I just read through it and it covered a lot of the questions I had. Just so I know i read that right. When they say to read at pins 3 5 and 8 it does not matter what tube is in the amp, so long as it is one of the ones listed in the link, the pin positions are the same. Correct? Jcm800 or 5150 read it at the same place? Thanks again Enzo!

  4. #4
    Senior Member Enzo's Avatar
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    Tubes come in types: 6L6 or EL34 or whatever. All 6L6s are wired the same way, they are not made for specific amps or anything. SO any amp that uses 6L6 will have plate on 3, screens on 4, control grids on 5, cathodes on 8. EL34 is the same, plus pin 1 must also be wired to pin 8
    Education is what you're left with after you have forgotten what you have learned.

  5. #5
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    Thanks again enzo! Really appreciate it! btw I live in Detroit. Just caught that you are in Lansing.

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