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Old 26th July 2006, 10:15 AM   #1
senderj is offline senderj  Hong Kong
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Default tub pre psu

I would like to use this psu for a tube pre with 2X 12AX7 and 2X 12AU7. the B+ is 275VDC and no B-. I need help on:

1. The 10H choke is too expensive for me. Can I replace it with R? What is the Ohm value?

2. For the 6.3V 3A from the transformer, is 3A enough for all (4 tubes) filaments? It should be 8X 6.3V filaments.

3. What is the purpose of the 50Ohm 5W pair in the 6.3V 3A?
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Old 26th July 2006, 10:41 AM   #2
EC8010 is offline EC8010  United Kingdom
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The 10H choke and 50 Ohm resistors are the least of your problems. The 317 will explode on switch-on. Search for "Maida regulator".

Each valve draws 0.3A heater current. 4 x 0.3A = 1.2A, so 3A is fine.

1/2W will be fine for those 50 Ohm resistors.

Frankly, the "design" appears to be a disaster conjured up by someone who doesn't know what they are doing. Given your level of experience, I'd advise you to leave it well alone.
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Old 26th July 2006, 12:18 PM   #3
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It's a bit scary because the drawing style of the schematic makes it look like it was copied from a manual. You have to wonder if anyone built it.
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Old 26th July 2006, 05:35 PM   #4
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what's wrong with it?
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Old 26th July 2006, 07:09 PM   #5
Tweeker is offline Tweeker  United States
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The LM 317 doesnt want to see more than 35 Volts, your missing some protection circuitry.
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Old 26th July 2006, 08:43 PM   #6
Sheldon is offline Sheldon  United States
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More specifically, the 317 can't take more than 35 volts from the input to the output or to the ground connection. I suppose it's possible for a circuit like this to function, if the timing of the input voltage rise is such that the caps on the output and ground pin could charge up before the 35V difference is exceeded. This issue is the same in reverse too. So if the input cap discharges while the output cap is still charged - poof, out goes the magic smoke. Don't count on it to be reliable, and don't hook up anything to it that can't stand the full voltage output with the regulator shorted. Or, use a better design. As EC8010 suggested, look up Maida regulator. http://www.national.com/an/LB/LB-47.pdf
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Old 28th July 2006, 01:48 AM   #7
senderj is offline senderj  Hong Kong
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Thanks for all the replies. I'm glad that I ask before action.

But the Maida regulator, from the link, only output up to 160V. I need 275V. How can I mod it to this voltage?
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Old 28th July 2006, 02:25 AM   #8
SY is offline SY  United States
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I'd read the app note thoroughly before embarking on a design. All the design equations are in there.
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Old 28th July 2006, 02:06 PM   #9
jarthel is offline jarthel  Australia
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Quote:
Originally posted by SY
I'd read the app note thoroughly before embarking on a design. All the design equations are in there.

I was reading the PDF Sheldon linked and I can't see any app notes. can you tell me where it is? thank you.
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Old 28th July 2006, 02:13 PM   #10
SY is offline SY  United States
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http://www.national.com/pf/LM/LM317.html

This page gives you links to all of National Semi's datasheets and app notes for the 317.
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