How can I calculate voltage over cathode resistor?

Nordic · 16th April 2008, 04:16 PM

My head is getting stuck....

Please give me a quick rough guide to calculate the DC voltage where the cathode resistor and the valve meets...

SY · 16th April 2008, 04:53 PM

This one is easy, since you know the plate voltage.

Current is (242-123)V/220k = 0.541mA. So the cathode voltage is (0.541)(2k2) = 1.19V.

If you don't know the plate voltage, you can determine the operating points by a graphical construction on the plate curves.

Jeb-D. · 16th April 2008, 05:01 PM

Yes, plotting loadlines is the easiest way to find operating points.

Nordic · 16th April 2008, 05:39 PM

Sweet SY, that was very handy...

Now if I know the B+ is 250 and there is no plate resistor and a 56k on the cathode to ground, and 144V on the grid would I need load lines?....

I had a tutorial once which got me to understand it, unfortunatley I am 10 second Tom...memory of a nougat bar.

I am sorry to be such a bother, I am not trying to design something, just to calculate appropriate component dissipations to not start fires.... when building an existing design.

SY · 16th April 2008, 05:44 PM

That's an easy one, too. You can do a graphical construction to get a precise answer. Or you can get really, really close by making some approximations.

If you've got 144V on the grid, the cathode will be a couple of volts north of that. Call it 146V, just for fun. That means the tube's current is 146V/56k = 2.61mA. If it was really 147V or 145V on the cathode, you can see that the current will still be pretty close to 2.6mA, so you don't need to sweat it.

Ohm's Law is a pretty powerful tool.

SY · 16th April 2008, 05:47 PM

Oh and as far as dissipation is concerned, 2.6mA times 146V is about 380mW. Use a 1W resistor.

Nordic · 16th April 2008, 08:37 PM

Thanks that was very usefull, makeing better progress now...

Nordic · 17th April 2008, 06:40 PM

One more question.... what is the recommended trace seperation for a 250V trace...?

I reccon point to point is not yet needed at this relatively low voltage..?

SY · 17th April 2008, 08:26 PM

Good question and I don't know. I'm a lousy circuit board designer so I use point to point by default.

porkchop61 · 17th April 2008, 08:44 PM

Look on the "PCB wizards.com web site. Towards the bottom, there is this question (and answer):
Q: Where would I find info on trace spacing based on voltage levels? How far should components be spaced for 1000V vs 4000V that type of thing?
http://www.pcbwizards.com/faq.htm

Glenn

16th April 2008, 04:53 PM	#2
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	This one is easy, since you know the plate voltage. Current is (242-123)V/220k = 0.541mA. So the cathode voltage is (0.541)(2k2) = 1.19V. If you don't know the plate voltage, you can determine the operating points by a graphical construction on the plate curves. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

16th April 2008, 05:44 PM	#5
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	That's an easy one, too. You can do a graphical construction to get a precise answer. Or you can get really, really close by making some approximations. If you've got 144V on the grid, the cathode will be a couple of volts north of that. Call it 146V, just for fun. That means the tube's current is 146V/56k = 2.61mA. If it was really 147V or 145V on the cathode, you can see that the current will still be pretty close to 2.6mA, so you don't need to sweat it. Ohm's Law is a pretty powerful tool. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

16th April 2008, 05:47 PM	#6
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	Oh and as far as dissipation is concerned, 2.6mA times 146V is about 380mW. Use a 1W resistor. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

17th April 2008, 08:26 PM	#9
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	Good question and I don't know. I'm a lousy circuit board designer so I use point to point by default. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

17th April 2008, 08:44 PM	#10
porkchop61 diyAudio Member Join Date: Aug 2004 Location: Mansfield, Connecticut	Look on the "PCB wizards.com web site. Towards the bottom, there is this question (and answer): Q: Where would I find info on trace spacing based on voltage levels? How far should components be spaced for 1000V vs 4000V that type of thing? http://www.pcbwizards.com/faq.htm Glenn __________________ "Imagination is more important than knowledge"

16th April 2008, 05:01 PM	#3
Jeb-D. diyAudio Member Join Date: Nov 2005 Location: SoCal	Yes, plotting loadlines is the easiest way to find operating points.

16th April 2008, 05:39 PM	#4
Nordic diyAudio Member Join Date: Sep 2005	Sweet SY, that was very handy... Now if I know the B+ is 250 and there is no plate resistor and a 56k on the cathode to ground, and 144V on the grid would I need load lines?.... I had a tutorial once which got me to understand it, unfortunatley I am 10 second Tom...memory of a nougat bar. I am sorry to be such a bother, I am not trying to design something, just to calculate appropriate component dissipations to not start fires.... when building an existing design.

16th April 2008, 08:37 PM	#7
Nordic diyAudio Member Join Date: Sep 2005	Thanks that was very usefull, makeing better progress now...

17th April 2008, 06:40 PM	#8
Nordic diyAudio Member Join Date: Sep 2005	One more question.... what is the recommended trace seperation for a 250V trace...? I reccon point to point is not yet needed at this relatively low voltage..?

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