# Need help from amp builders

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

1. ### KN4CTDHam MemberQRZ Page

I have an al-1500 that I use in my station, yet I rarely turn it on or even transmit for that matter. There are few hams in my area (that I’m aware of) and I am genuinely more interested in learning above all else as far as this hobby is concerned. Hopefully, others can see this by looking at how many times I’ve posted concerning something that is probably “elementary “ to more experienced hams.

2. ### KN4CTDHam MemberQRZ Page

I told you I was terrible at math! . Thank GOD for the online calculators and such!

3. ### KN4CTDHam MemberQRZ Page

So.....if my meter is a 10ma movement and it takes 37mv to provide full scale deflection, do I need to double the resistance to make it read full scale by adding another 3.5 ohms in series with the movement to make it read 2 amps full scale? I am truly sorry that I’m having this hard a time understanding, but I’m the kind of person that once it all makes sense and clicks, I’ll never forget it.

4. ### WA7PRCHam MemberQRZ Page

No. In the beginning of his first tutorial, W2AEW shows how a panel meter displays current directly:

When you add a "voltage multiplier" resistance in series, the meter displays voltage. For example, 10 mA (0.010 A) thru a resistor w/ 1000V across it is 1000 / 0.010 = 100,000Ω. Practically speaking, we can ignore the meter's 3.5Ω but, if you wanted to account for it, to be precise, R would be 99996.5Ω.

When you add resistance in shunt (parallel), the meter's full scale current increases. In the case of your 10ma/3.5Ω meter, every 3.5Ω added in parallel increases the full scale current by 10 mA. W2AEW works thru "current shunts" in the second tutorial:

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5. ### KN4CTDHam MemberQRZ Page

So, according to the video, is this equation correct for calculating parallel shunt resistance? Rs= Vfs/Ifsd-Ifs. Ok, let’s say I want this meter to measure 2 amps full scale. .35Vfs/2Ifsd - .1Ifs. = shunt resistor value? Please correct me because I have a hard time keeping my decimal points in the correct places. That doesn’t seem right to me. Where is someone gonna find a resistor that low in value?.....

6. ### KN4CTDHam MemberQRZ Page

Is this a case where a multiplier would be added to increase the resistance of the meter circuit thus increasing the full scale sensitivity to a higher voltage so that a more common shunt value could be used?

7. ### AF7XTHam MemberQRZ Page

The correct formula for the shunt is meter voltage-sensitivity divided by desired current minus meter current. Where E = meter sensitivity in volts, I=current being measured, and i=meter FS current, we have: E / I - i = shunt resistance .
You almost always get some odd resistance so...

Increase the voltage

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8. ### KN4CTDHam MemberQRZ Page

Ok, that part makes much more sense now. So I would add a series resistor (multiplier) to increase the full scale meter sensitivity in volts. This will then allow me to choose a higher, more common resistor value for the shunt???? Now if this is correct, then I need to learn how to meter grid current next. Thanks for all the help and suggestions everyone.

9. ### WB2WIKPlatinum SubscriberPlatinum SubscriberQRZ Page

To change the indicated range of a 10mA meter with ~40mV sensitivity to read 1A full scale, you only need a very common 0.04 Ohm resistor wired across the meter terminals (or anywhere that places it the same position in the circuit).

This is not an odd value, it's a common meter shunt value available for a couple of bucks from a variety of sources, like this one: https://www.surplussales.com/Resistors/WireWound/WW0-0499.html

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10. ### KN4CTDHam MemberQRZ Page

Ok I think I’ve got it figured out so that I understand how to make the meter read correctly. I’ll have to make sure, I get it wired into the power supply B- correctly when I have more time to work on it. In the mean time, I think I will sit down and attempt to draw what I’ve built in a schematic so that I may post it for critique by others and to be sure I’m being safe. Then if I have problems/issues the next time I attempt to apply hv and set the bias, someone on here may have a better shot at troubleshooting any issues. I appreciate all who have replied with suggestions and to help me learn.