Plate current

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by AL1, Feb 25, 2003.


  1. AL1

    AL1 Member

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    Would someone explain to me in detail how to measure the plate voltage and cathode current with my digital meter. I know you take a reading from pin 3 to ground for the plate current, but are the power tubes in the amp or do you have to remove them? Should each of the 2 6V6 tubes show the same voltage? Be as detailed as you possibly can. This is for a '65 fender deluxe reverb reissue. I assume the speaker shoud be left connected. Thanks in advance.
     
  2. PaulC

    PaulC Member

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    Actually it's the plate VOLTAGE you measure from pin 3 to ground. If you try to hook up an ammeter (curent meter) like this you'll put a dead short from the tranny winding to ground. Best case this pops the fuse in the meter. Worst case and the trannies could be damaged. To measure current you have two ways to do it. One is to put an ammeter in series with the B+. this means breaking a connection and inserting the meter, or shunting the output tranny. The other way is to install 1ohm resistors from the cathodes to ground (or cathode resistors in cathode biased amps) and measure the voltage drop across that resistor. Ohm's law will let you sub in the current for the voltage. ex: 33mv = 33ma. With this methode you're only as accurate as your resistors, and the ability of your meter to read mv's. Also this method adds the screen current in to the plate current. Call it about 5ma on average.

    I use the transformer shunt method myself. It's the most dangerous, but it's also the most accurate and fastest for me. Then tweak by ear, but recheck to make sure I've not gone over the limits of the tube.

    But if you don't have a steady hand then I'd put in the bias sense resistors, or get the bias probes which do this in their sockets.

    Later - PaulC
     
  3. AL1

    AL1 Member

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    Paul, do you leave the tubes in when when checking the plate voltage from pin 3 and do you keep the speaker connected? Thanks.
     
  4. PaulC

    PaulC Member

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    Yeah - the tubes have to be in. That's what biasing is all about. You're setting the operating condition of the tube to the amp. I use a load resistor in place of a speaker hooked to the amp. I'm going to be blasting a sine wave into the amp when checking for x-over distortion, and I don't want to hear that! After I've got thinks looking the way I want them then I'll hook up a speaker and listen and tweak some more. Paul
     
  5. sws1

    sws1 Member

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    Regarding the measurement of plate voltage, if you connect a multi-meter between the plate and ground, doesn't that cause a direct circuit through the multimeter to ground? Or is there such a large resistor inside the multimeter that not very much current would flow?

    A very dumb question: Is B+ another name for the plate voltage?

    1 more: If electrons from from the cathode to the plate (anode), why do we measure current on the cathode? The cathode simply connects to ground (via some resistors and capacitors). From what I've seen, the cathode isn't fed from anything. Correct?
     
  6. John Phillips

    John Phillips Member

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    A multi-meter (especially a modern digital one) has an extremely high resistance when used as a voltmeter, so it draws an insignificant current from what you're measuring.

    Don't ever connect one set as a current meter like that though, as Paul says you'll fry it. I did this by accident to an old analog meter (luckily not a very good one) by simply misreading the dial - it made a noise and a smell like a bad firework and the needle remained stuck at 'full deflection'... oops.

    B+ is the power supply voltage, not the plate voltage - nearly the same, but they differ by the DC voltage drop across the output transformer primary. In fact, you can use this voltage drop to calculate the idle current more safely than the 'shunt' method, if you have already accurately measured the DC resistance of the OT when the amp is off.

    Measuring the cathode current works because all the currrent that flows in at the 'positive' end of the tube (ie plate and screen-grid) has to come out again at the cathode - the cathode is 'fed' from the tube itself; currents in an electrical circuit must all 'add up'. Measuring at the cathode is far safer than at the plate because of the very much lower voltage, but as Paul says you do need to allow for the screen-grid current (another reason why setting idle current to an exact number of milliamps is fairly pointless).
     

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