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Old 16th May 2009, 08:38 PM   #1
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Default 2x 12AX7 preamp fuse value

Hello, this is a newbie question:

What fuse value do I need to protect a 2x 12AX7 standalone preamp unit? How do I calculate it? Wall voltage is 240V.

Thank you.
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Old 16th May 2009, 09:58 PM   #2
pointy is offline pointy  United Kingdom
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in the uk(230-240v) we would use a 3 amp fuse for the wall plug
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Old 16th May 2009, 10:11 PM   #3
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3 amps for a 2 x 12AX7 preamp?

I'd use a 63mA MDL slow blow fuse.

Cheers!
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Old 17th May 2009, 09:07 AM   #4
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Thanks. Could you please tell me how you get to those numbers? I'm a newbie and I'm not sure how to calculate it.

These are my numbers for the power supply needs:

From what I've read, the two heaters draw 6.3V at 600mA, that's 3.6W approx.

Plate load is around 2-3mA, less than 1W maximum dissipation for 12ax7s, so that's another 2W.

Why the 63mA fuse?

Thanks for your help, I'm a bit lost here.
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Old 17th May 2009, 09:47 AM   #5
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Hi,

Quote:
Originally posted by gottlieb
Thanks. Could you please tell me how you get to those numbers? I'm a newbie and I'm not sure how to calculate it.

These are my numbers for the power supply needs:

From what I've read, the two heaters draw 6.3V at 600mA, that's 3.6W approx.

Plate load is around 2-3mA, less than 1W maximum dissipation for 12ax7s, so that's another 2W.
So you have ~6W.

I = P/E

25mA = 6/240

allow for losses and stuff is approx X2 at these power levels = 50mA.


Quote:
Originally posted by gottlieb
Why the 63mA fuse?
Microcurrent MDL's are in increments like 32, 63, 125mA, etc., so the closest one would be 63 (some listings say 64 or 65mA, don't worry).

Fuses in this range are expensive though.

Cheers!
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Old 17th May 2009, 10:16 AM   #6
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Thanks a lot Geek.
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Old 17th May 2009, 10:32 AM   #7
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Are you really worried that your 12AX7s are going to self-destruct? It doesn't seem very likely.
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Old 18th May 2009, 03:29 AM   #8
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by barretter
Are you really worried that your 12AX7s are going to self-destruct? It doesn't seem very likely.

That probably is not the worry here. I always worry about unfused or improperly fused devices connected to AC mains - that would be my concern. Insulation fails, components short, and fires in electrical/electronic devices still cause significant property damage or worse. (Burning wall warts anyone?)
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