vacuum tube plate current?

[skal1]

What is the safest and accurest method to measure plate current usage at a op tx or at the  plate load resister.

cheers

skal1

 

[ruffrecords]

If the tube in question is a triode, the safest place to measure it is in the cathode circuit. As often as not there is a cathode resistor and it its value is known it is a simple matter to measure the current by measuring the voltage across this resistor.

Cheers

Ian

 

[skal1]

thanks ian , so if measuring the voltage drop across  this resistor then divide it by the the resistor i should get the total current drawn at idle.is this correct, i am a bit simple.

cheers

skal1

 

[gemini86]

That is correct.

 

[CJ]

you can also carefully hook an ammeter across the output,

one lead on the red wire, one on the blu, fire it up,

the current takes the path of least resistance which is the ammeter,

won't this short out the tube?

no, the tube is biased to conduct a certain amount of current,

measuring across the output will not mess up the bias, so the tube will put out as much current as it normally does when going thru the output,

note that the meter leads will be at high voltage so take the normal precautions,

the beauty of this method is that you do not have to unsolder the output transformer lead to get the meter in series,

full props going out to Gerald Weber for teaching me this trick in his books,

,

 

[skal1]

Hi thanks lads


skla1

 

[mjk]

you can also carefully hook an ammeter across the output,

one lead on the red wire, one on the blu, fire it up,

the current takes the path of least resistance which is the ammeter,

won't this short out the tube?

no, the tube is biased to conduct a certain amount of current,

measuring across the output will not mess up the bias, so the tube will put out as much current as it normally does when going thru the output,

note that the meter leads will be at high voltage so take the normal precautions,

the beauty of this method is that you do not have to unsolder the output transformer lead to get the meter in series,

full props going out to Gerald Weber for teaching me this trick in his books,

,

Clipping an ammeter across the OPT primary will only work in circuits where the plate current goes through the OPT. It depends on the internal resistance of the ammeter being way lower then the DC resistance of the OPT primary winding. This method won't work if you have a plate load resistor (i.e. parafeed output circuit) because with parafeed there is no DC current through the OPT. I mention this because the OP asks about the plate load resistor. If you do have a plate load resistor you can measure the voltage drop across it and calculate current using ohm's law. That is the easiest.

 

[CJ]

typical shunt in an ammeter might be 0.1 ohms,

typical dcr for a output primary will be at least 100 ohms,

so you have 1/1000 ratio or 0.1 percent accuracy.

yes, a para feed circuit will require that you put the meter across the choke instead,

if using ohms law, measure the ohms on the resistor with the power off,

color code calculations are usually fine, but you never know.