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Bias Tester

Discussion in 'Amps [BG]' started by Grissle, Dec 28, 2013.

  1. Grissle

    Grissle Supporting Member

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  2. beans-on-toast

    beans-on-toast

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    I have one. Works great.
  3. B-string

    B-string Supporting Member

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    Pretty common piece. It will allow "direct" current monitoring (current scale on the meter). Others use a 1 ohm precision resistor between cathode and ground and millivolts read equal milliamps, some also provide plate and screen voltage monitor points.

    The only thing I don't like is, if you blow the fuse for the shunt in your meter it could send you on a wild goose chase in the bias circuit.
  4. MAMMOTHvolume

    MAMMOTHvolume

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    put a 1 ohm in the cathode on all your valves, problem solved! most amps don't ave room to use a bias meter in so you have to take it out the case anyway
  5. beans-on-toast

    beans-on-toast

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    The advantage of reading current is that some of the less expensive volt meters do not measure low voltage that well.

    I see that mojo is out of stock. You can buy the same product directly from Hoffman. Here it is. Doug is fast to ship and offers good stuff on his site.
  6. Grissle

    Grissle Supporting Member

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    Sorry for the noob questions (haven't ordered the amp book yet!) but what's the advantage to having two of the same kits over just the one?

    Also any good web sites for noobs? I've really been getting into the tech side of my music in the last few years and have repaired and modded dozens of guitars/basses since. So this is the next step in my journey. Big thanks all😋
  7. beans-on-toast

    beans-on-toast

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    Haven't you ever heard that two heads are better than one? :p

    With one module (head) you can test one tube at a time. You have to power down the amp, move the module to another socket, then power the amp up to test the second tube.

    There are a number of "how to" books and videos available at Antique Electronics (www.tubesandmore.com). There is a FAQ here on TB with a section full of links on tube amp repair.
  8. B-string

    B-string Supporting Member

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    With that model as is there is no advantage to having two unless you have matching calibrated meters. Without the banana jack plugged into a meter set to current (ma) the tube is dead (out of circuit with an open cathode).
  9. beans-on-toast

    beans-on-toast

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    Here is the FAQ that I mentioned.

    For more money you can buy a prebuilt bias checker. Bias King is one example. Check out their manual to see how it works. These type of units can cost a lot though. Building the Hoffman one isn't too hard.
  10. beans-on-toast

    beans-on-toast

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    Yes, otherwise it could be done by shorting the unit that doesn't have the meter on it. Unless you have a special box that everything connects to with a switch that can select one unit and short the other, it would be safer to have just a single unit. Easier just to use a single unit.

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