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In Reply to: RE: DC voltage drop across choke vs resistor!!! posted by TomWh on March 17, 2017 at 18:25:10
If the voltage drop is less across the choke than the resistor (less DC resistance) then the operating point of the amp will change and draw more current.
My confusion is that with everything in place with only only one power supply to one tube no other branches the current should be constant. So a resistor in series can be measured for voltage drop then ohms law we have the current in circuit or through the tube.
The question is in regards to why when you measure the voltage drop across a choke with known DC resistance it is different than a resistor with the same resistance.
So in the above example using the voltage drop the choke shows we have a 18 ma circuit and the voltage drop across the resistor shows a 16 ma circuit. This is b+ to a tube circuit.
So I am guessing the DC circuit is seeing the choke as more than just its resistance. My question is why.
"My confusion is that with everything in place with only only one power supply to one tube no other branches the current should be constant."
The amount of current the tube will draw depends on the bias condition of the tube.
If the tube has more voltage across it (plate to cathode) with the choke in place vs. the resistor in place then it will draw more current.
I'm not sure why you think the current should be constant.
Power supplies, unless they are current regulated supplies, supply whatever current the tube circuit demands.....not the other way around.
"I am guessing the DC circuit is seeing the choke as more than just its resistance. My question is why."
The DC will only see the resistance of the choke. The AC (music signal) will see the resistance of the choke and the reactance of the choke in series.
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"Still Working the Problem"
I am not explaining this correctly. The circuit is biased with a diode. This is why I do not check the voltage drop there. So I would normally would check the voltage drop across a choke with known resistance and do the math from there. A buddy told me the choke measurement is not as accurate as a series resistor. So I put a resistor in series and then measured across the resistor and the choke. After the math it was 16 ma for the resistor and 18 ma for the choke.
So everything is constant the diode is setting the bias for the tube the b+ is settled in to what ever the current is. So all I was doing was checking what the measured current is.
So my question is why would the drop across the choke DC resistance measure different than a standard resistor?
"why would the drop across the choke DC resistance measure different than a standard resistor"
It won't. Something else is happening that you're not aware of. As someone else suggested, maybe there's an AC component present.
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That is what I was thinking, in regards to dcr. There are caps to ground in this power supply. Would that cause this measurement issue.
This is only a guess, but there could be some AC ripple riding on the DC ahead of the choke that the choke is blocking and your meter is responding to. Check the voltages with a 'scope and see what you find.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
I did measure it on the 1st choke in the power supply. There are 2 more will measure there for the fun of it. Should get Closer down the line if ripple is being reduced.
Can the caps to ground cause this issue also?
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