# Measuring Plate Current



## keeperofthegood (Apr 30, 2008)

Hey oh

OK OK I know the answer is already been posted about a billion times here. Search shows me nothing even close to this.

I want to use the 1 ohm resistor off the plate to measure current. What specifications of that 1 ohm resistor apply, wattage and tolerance? Would 1Ω 5W Wirewound Flameproof be overkill, or underkill? >.< probably a really simple answer but I have had a fever for a week and am not thinking fully 100%


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## Wild Bill (May 3, 2006)

keeperofthegood said:


> Hey oh
> 
> OK OK I know the answer is already been posted about a billion times here. Search shows me nothing even close to this.
> 
> I want to use the 1 ohm resistor off the plate to measure current. What specifications of that 1 ohm resistor apply, wattage and tolerance? Would 1Ω 5W Wirewound Flameproof be overkill, or underkill? >.< probably a really simple answer but I have had a fever for a week and am not thinking fully 100%


Don't put the resistor on the plate, Keeps! That would mean the HV would always be at that test point, just waiting to zap you silly!

Put it from the cathode pin #8 to ground. This means you will always measure only millivolts, which across a 1 ohm resistor you can read as milliamps of current.

This method actually reads cathode current, which is plate current plus screen current. For biasing amps, the screen current at idle is only a couple of milliamps so we usually ignore it and accept cathode current as plate current.

As for the resistor rating, do the math! Power in Watts = Current ^2 times Resistance. Let's assume we have a pair of 6L6's. They are never likely to see more than 200 ma of current on peaks. We know this because we looked up the data sheets and saw the typical currents for typical plate voltages for the usual 40-50 watts of power they deliver in the typical guitar amp.

200 ma. is .2 amps. .2 times .2 (or .2 squared) is .04 A times the resistance of one ohm = .04 watts! That's mice nuts! Remember, we're talking the current drop across a resistor of only 1 ohm! That's next to nothing!

So 5w resistors are WAY overkill! Use 1/2 watt! This has the added bonus of the resistors also acting like fuses. As long as things are ok they will never blow but if you did have a bad tube that decided to short it would blow that 1 ohm 1/2 watt cathode resistor to smithereens, stopping the current through the tube dead and saving the amp from any major damage. You just replace the bad tube, replace the 10 cent cathode resistor and the amp will be fine! That 5 watt resistor would cheerfully let the amp burn itself up!

Hope this helps!

WB


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## keeperofthegood (Apr 30, 2008)

Thank you much Wild Bill

Cathode not Plate, maybe why I had a hard time searching for it.

100 units now ordered


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