59 bassman 4x10"/ clark piedmont bias point

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by cygnusx1, Apr 13, 2016.

  1. cygnusx1

    cygnusx1 Member

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    Ok so I just bought new tubes for my Clark Piedmont. Its a 59 bassman clone. It has a bias pot. My question is, where the heck do I measure the bias at????? I have searched the internet but all I find is bassman ltd reissues with printed circuit boards. This is a hand wired amazing amp. I just want to know where to put my dvom leads! Please, i have biased amps before. I don't need technical calculations on how to figure it out. Just where do I put the darn lead! Lol i almost found myself believing I found the end of the internet searching for this. Thanks ahead of time
     
  2. Jeff Gehring

    Jeff Gehring Silver Supporting Member

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    You need to know if the Piedmont has 1 ohm resistors installed on the power tube cathodes or not. If it doesn't, you either need a device like a Bias-rite which inserts a 1 ohm resistor inline with the cathode circuit, or else you'd have to use the transformer shunt method of measuring plate current. I suggest you talk to Michael Clark about the amp and get his read on how you should proceed with biasing.
     
  3. pdf64

    pdf64 Member

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    No cathode sense resistors does not mean that one must use the pointlessly hazardous OT shunt method; hanging a current meter off the HT, in preference to a perfectly feasible, less hazardous alternative, may indicate a poor regard to safe working practice.

    Just measure the resistance of each OT primary leg, then the voltage drop across them, and apply Ohm's law I = V/R

    Ensure that meter probes are sharp; this facilitates them breaking through any surface layer of gunk / oxide on the terminal, and should also act to reduce the likelihood of them slipping off the terminal whilst taking a measurement (and possibly shorting on to adjacent terminals).

    From http://www.aikenamps.com/index.php/the-last-word-on-biasing

    The plate current can also be measured by first measuring the resistance across each side of the output transformer primary (it will usually be different on each side) with the power off. Make a note of the resistance on each side, and then, with the amplifier on, measure the DC voltage drop across each side of the output transformer. Divide this number by the previously measured resistance, and you end up with the plate current for the tubes on that side. Again, if there is more than one tube on each side, you must divide the total current by the number of tubes. This method is extremely accurate, and much safer than the shunt current measurement method, because a slip of the probe won't short anything out due to the high resistance of the voltage measurement setting on the meter compared to the very low resistance of the current measurement setting. You can also make a safer measurement by clipping the negative side of the voltmeter on ground, and measuring the center-tap voltage of the output transformer and the voltage at the plate of each output tube. Subtract the plate voltage from the center-tap voltage and you have the voltage drop across each side, and can then use this to calculate the current in each tube, again dividing by the number of tubes on each side.
     
    Last edited: Apr 14, 2016
    Malcolm Irving likes this.
  4. cygnusx1

    cygnusx1 Member

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    Thanks guys. Great suggestions.
     

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