Power Tubes cathode biased w/ shared vs individual R/C?

greg · 08-13-2007, 02:17 PM

Hello.

I have been reading here and there about taking a set of cathode biased power tubes and putting them on their own individual set of cathode bypass resistor/capacitor.

The main reason I want to do this is that when separately biased, the wattage of the amp can be reduced greatly by switching out the bypass caps, which Carr is doing on one of their models.

I have tried this on a single ended 5 watt amp and it worked quite well.

My question is what values to use? Do I simply half the shared resistor value and keep the cap values the same?

At present, the amp I want to perform this mod to has a pair of cathode biased 6V6s, using a 300 ohm shared cathode resistor with a 25uf cathode bypass cap.

Thanks,

Greg

DirtyGeorge · 08-13-2007, 02:37 PM

By moving to split cathode resistors you cut the current through each resistor by half. So to get the same bias voltage you must double the resistance. To keep the same time constant you would divide the bypass cap value by 2.

greg · 08-13-2007, 03:19 PM

Thanks for the quick reponse!

Greg

08-13-2007, 02:17 PM	#1
greg Member Join Date: Aug 2006 Posts: 97	Power Tubes cathode biased w/ shared vs individual R/C? Hello. I have been reading here and there about taking a set of cathode biased power tubes and putting them on their own individual set of cathode bypass resistor/capacitor. The main reason I want to do this is that when separately biased, the wattage of the amp can be reduced greatly by switching out the bypass caps, which Carr is doing on one of their models. I have tried this on a single ended 5 watt amp and it worked quite well. My question is what values to use? Do I simply half the shared resistor value and keep the cap values the same? At present, the amp I want to perform this mod to has a pair of cathode biased 6V6s, using a 300 ohm shared cathode resistor with a 25uf cathode bypass cap. Thanks, Greg

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08-13-2007, 02:37 PM	#2
DirtyGeorge Member Join Date: Jun 2006 Location: Germantown, MD Posts: 59	By moving to split cathode resistors you cut the current through each resistor by half. So to get the same bias voltage you must double the resistance. To keep the same time constant you would divide the bypass cap value by 2.

08-13-2007, 03:19 PM	#3
greg Member Join Date: Aug 2006 Posts: 97	Thanks for the quick reponse! Greg

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