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Old 16th April 2008, 05:16 PM   #1
Nordic is offline Nordic  South Africa
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Default How can I calculate voltage over cathode resistor?

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...
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Old 16th April 2008, 05:53 PM   #2
SY is offline SY  United States
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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.
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Old 16th April 2008, 06:01 PM   #3
Jeb-D. is offline Jeb-D.  United States
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Yes, plotting loadlines is the easiest way to find operating points.
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Old 16th April 2008, 06:39 PM   #4
Nordic is offline Nordic  South Africa
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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.
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Old 16th April 2008, 06:44 PM   #5
SY is offline SY  United States
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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.
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Old 16th April 2008, 06:47 PM   #6
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Oh and as far as dissipation is concerned, 2.6mA times 146V is about 380mW. Use a 1W resistor.
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Old 16th April 2008, 09:37 PM   #7
Nordic is offline Nordic  South Africa
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Thanks that was very usefull, makeing better progress now...
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Old 17th April 2008, 07:40 PM   #8
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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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Old 17th April 2008, 09:26 PM   #9
SY is offline SY  United States
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Good question and I don't know. I'm a lousy circuit board designer so I use point to point by default.
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Old 17th April 2008, 09:44 PM   #10
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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

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