Cathode Bias current draw

MikeT · 11-10-2009, 05:23 PM

I would like to know the current draw on the 6v6s in my 5e3. It has a 250 ohm common cathode resister. Could I calculate the current draw per tube as millivolts by dividing the voltage read across the cathode resister by 250 and then dividing by 2? Thanks....

Mike

cbarrow7625 · 11-10-2009, 05:47 PM

The quick answer is yes.

Using Ohm's Law will give you a rough estimate of the current through each tube. Of course, it is unlikely that both tubes will be drawing exactly the same amount of current; one will be a little higher, the other a little lower. If you are running them close to their dissipation limits, knowing the exact current through each could be critical. If they are in a nice safe area of their operating range, then it probably won't matter much for just getting a general idea of the operating current.

tubeswell · 11-10-2009, 06:14 PM

but since we are talking about a wire wound resistor here, and only a couple of hundred ohms, and about getting a measurement of a few dozen milliamps, then you should accurately measure the DC resistance of the cathode resistor first (you will need to unsolder the bypass cap to do this). Otherwise by just taking a guess that the 250R is actually dead on tolerance, your calculations could be out by something like 20% either way.

MWJB · 11-10-2009, 07:33 PM

Short, crude, rule of thumb, method...is your plate voltage <380vdc (plate to ground)...if so, don't fret it.

Might even take a few more volts before exceeding 14W per tube, if tubes are on the cool side. Either way, if you had 383 or 384v I wouldn't panic.

5E3 often slightly exceed 14W per tube.

If you want to measure plate current, then do so with a bias probe (it will come in handy in the future)...if you're happy with an appproximation, measure cathode voltage, or just the plate voltage. If you want to be "safe as houses" fit 330ohm cathode resistor & feel free to try a 5V4 recto and never worry about bias again.

11-10-2009, 05:23 PM	#1
MikeT Member Join Date: Jul 2006 Location: Upstate NY Posts: 26	Cathode Bias current draw I would like to know the current draw on the 6v6s in my 5e3. It has a 250 ohm common cathode resister. Could I calculate the current draw per tube as millivolts by dividing the voltage read across the cathode resister by 250 and then dividing by 2? Thanks.... Mike Last edited by MikeT; 11-10-2009 at 05:39 PM.

11-10-2009, 06:14 PM	#3
tubeswell Supporting Member Join Date: Feb 2007 Location: Wellington NZ Posts: 2,661	but since we are talking about a wire wound resistor here, and only a couple of hundred ohms, and about getting a measurement of a few dozen milliamps, then you should accurately measure the DC resistance of the cathode resistor first (you will need to unsolder the bypass cap to do this). Otherwise by just taking a guess that the 250R is actually dead on tolerance, your calculations could be out by something like 20% either way. __________________ Building a better world (one tube amp at a time)

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11-10-2009, 05:47 PM	#2
cbarrow7625 Senior Member Join Date: Mar 2007 Location: Philadelphia suburbs Posts: 366	The quick answer is yes. Using Ohm's Law will give you a rough estimate of the current through each tube. Of course, it is unlikely that both tubes will be drawing exactly the same amount of current; one will be a little higher, the other a little lower. If you are running them close to their dissipation limits, knowing the exact current through each could be critical. If they are in a nice safe area of their operating range, then it probably won't matter much for just getting a general idea of the operating current.

11-10-2009, 07:33 PM	#4
MWJB Old Timer Join Date: Jun 2006 Posts: 1,926	Short, crude, rule of thumb, method...is your plate voltage <380vdc (plate to ground)...if so, don't fret it. Might even take a few more volts before exceeding 14W per tube, if tubes are on the cool side. Either way, if you had 383 or 384v I wouldn't panic. 5E3 often slightly exceed 14W per tube. If you want to measure plate current, then do so with a bias probe (it will come in handy in the future)...if you're happy with an appproximation, measure cathode voltage, or just the plate voltage. If you want to be "safe as houses" fit 330ohm cathode resistor & feel free to try a 5V4 recto and never worry about bias again.

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