ohms law fun!

[geoff 1965]

right guys,
try as i may i just can't get my head around calculating resistance etc using ohms law,even though maths was my best subject at school!
so here is a little test for you guys who can,i'm fine tuning my B+ and i need to adjust my el84 grid voltage.
at the moment i have 340V on the plates and 331V on the grids, this is with a 1K grid resistor and i want to reduce the grid voltage to 325V.
so using ohms law tell me the resistance i will need to reduce 340VDC to 325VDC.
i've patched my trusty 0-10K variable resistor in place of the 1K and adjusted it to get 325V on the grids so lets see who gets the closest!

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[Crabman]

Did you mean the screens? Then ... assuming you have a single screen dropping resistor and also a constant current flowing through both EL84 screens (I calculated it to be about 9 mA)..... to drop 15 volts should require a resistance of about 1.67K.

[JMPGuitars]

You can also cheat and use: http://www.ohmslawcalculator.com/ohms-law-calculator

[geoff 1965]

i love it!
not bad Dave for a calculation!

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actual voltage at the dual can cap was 341 so thats a 16 volt drop,just ordered a 1K8 with 5% tolerance which should get me in the ballpark.

[Crabman]

Yep ... a 16 volt drop would bring the calculation to 1.78K so you definitely ordered the correct resistor!