Need help from amp builders

Discussion in 'Amateur Radio Amplifiers' started by KN4CTD, Nov 16, 2018.

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  1. KN4CTD

    KN4CTD Ham Member QRZ Page

    Thank you for also taking time to explain this to me. I’m convinced now, that I really need to sit down at the bench and figure all of this out before attempting to proceed any further with the amplifier project. Is it safe to use a dvom to read the internal resistance of the meter movement? Or do I need to use an analog vom for this? When you say “without pinning the meter movement “ do you mean the movement of the meter I’m trying to measure or the vom? The meter Id like to use to measure plate current is scaled 0-1 amp but says nothing on it anywhere else.
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    It's "possible" the meter has an internal shunt and really is 1A full scale...but I think it's unlikely. More likely it's really 0-1 mA and calibrated for use with an external shunt; this makes meters a lot more "versatile," since you can use the same movement for almost anything and just produce a bunch of different "scales" for it.

    Since you can damage sensitive meter movements (the coils) with excessive applied voltage, which will produce excessive current through the coil, it's best to start out on a "high resistance" setting of the 50K, 10K, or whatever it has. If the meter needle doesn't move, switch to the next lowest resistance setting, which will provide higher probe voltage. I wouldn't start out on an R = 1x setting, as that can provide so much current you might blow the meter (although that surely would NOT happen if it really was a 1A meter with an internal shunt).

    Some meters don't have terminal markings for polarity. If you see the meter needle move "backwards," reverse the test leads.:p
    WA7PRC likes this.
  3. AF7XT

    AF7XT Ham Member QRZ Page

  4. KM3F

    KM3F Ham Member QRZ Page

    Good now that you realize the need to have the knowledge before working such a project.
    That was the point in my previous reply.
    Any ohmmeter uses an internal battery to force current through the unit under test.
    If the meter under test is a low enough value, it will peg the needle full scale and possibly bend the needle, then the meter is ruined.
    A low reading meter needs a different test procedure to measure it's internal resistance.
    [How it can be done:]
    With 1.5 volt battery, connect a 1 meg ohm potentiometer (one end and 'center connection' across the meter to be tested.
    (Make sure the pot is set to it's lowest resistance on those same terminals.) Use your DVM to check and set it.
    Apply the battery voltage across the meter and Pot together all at the same time, then slowly turn the pot until the (meter under test reads full scale.) 'not pegged' against it's internal stop.
    What you now have done is divided the battery current equally through both the Pot and the meter.
    This being the case, the pot, if you measure it's resistance of the Pot separately across the same terminals used across the meter under test, the Pot now EQUALS the meter resistance.
    This does no damage to the meter under test but you must take care to shunt the meter under test with the pot setting so excess current cannot flow through the meter under test and damage it.
    What you do by this is divide the current equally through two resistances that are the same value. This makes the Pot resistance the same as the meter resistance. Measure the Pot resistance with your DVM or Analogue Ohm meter function. meter and you now have the meter resistance value to work with in calculating meter multiplier resistances and current shunts values.
    A 1 amp meter reads 1000 milliamperes.
    Good for tube plate current but poor choice to read grid currents and other low values of current below 1/4 of it's scale.
    Good luck.
    KN4CTD likes this.
  5. K9STH

    K9STH Platinum Subscriber Platinum Subscriber QRZ Page


    For the 1-ohm resistors, that I use to measure the voltage drop which is then "calculated" to read the current, I just use 10-watt, 5% tolerance, "cement" type wire-wound power resistors. The 5% tolerance is "close enough for government work" as far as I am concerned since the regulations now restrict the power output and not, like in the "goode olde dayes", power input from the final amplifier stage.

    Of course, there are a number of ways to read the current. It is just the fact that, by using 1-ohm resistors, there is a direct correlation between current and voltage.

    In fact, by using 1-ohm resistors, it would be very easy to use those cheap Chinese digital voltmeters to give a visual, "calibrated", reading. If the voltmeter has 3 decimal places to the right of the decimal point, then the 3rd digit represents 1 mA, the 2nd digit represents 10 mA, and the 1st digit represents 100 mA. If there are only 2 decimal places to the right of the decimal point, then the 2nd digit represents 10 mA and the 1st digit represents 100 mA. Using the digital voltmeters would make for a very "modern" looking panel!

    Glen, K9STH
  6. WA7PRC

    WA7PRC Ham Member QRZ Page

    When I setup to run Tony W4ZT's bias board in my Heath SB-220, I treated it as an "adjustable" Zener diode. I connected it across a LV lab power supply, in series with a resistance to limit the current. Then, while monitoring the voltage across it, I adjusted the board 'til I saw 5.1V. When connected and operating the amplifier, Zero Signal Anode Current (ZSAC) was close enough. :)
    KN4CTD likes this.
  7. WA7PRC

    WA7PRC Ham Member QRZ Page

    Alan W2AEW is a Field RF Application Engineer for Tektronix and has created MANY very good tutorials on YouTube ( Two that apply here:

    KN4CTD likes this.
  8. KN4CTD

    KN4CTD Ham Member QRZ Page

    Is there a way to do this accurately without a 1 meg ohm pot? I tried the only 1meg ohm pot I found in some junk......whenever I get the meter moment close to reading full scale using an AA battery, it just wants to slam the needle past fsd. I’ve measured the internal resistance of the meter movement with my old run of the mill dvm. With it set on a 200 ohm scale, it reads .7 ohms? I’ve also set up kind of the same procedure with a really good 10k pot using the 1.5v AA battery but the voltage is still too high and buries the needle. I then added a couple of diodes in series to drop the voltage to the point that I can now use the 10k pot to dial the movement in to and stop dead on the full scale reading. If I measure the voltage across the meter at full scale, I read 37mv. Maybe I’ll figure this out one day!
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Your 1 Megohm pot might be "audio" taper (very non-linear) which is why a very small turn makes a big change in resistance. You can check that with your DMM on a resistance scale.

    The .7 Ohm reading doesn't sound right at all unless it really is a 1A FS meter...but then, you wouldn't be able to "pin" it with a 1.5V AA battery, at least not for very long.:p
  10. KN4CTD

    KN4CTD Ham Member QRZ Page


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