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Old 9th March 2008, 10:32 AM   #1
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Default calculating total current draw

Hi all,


Could someone check this out for me - I think I must be wrong.

I have an amp that uses 4 6L6, 2 6FQ7 and 1 12AT7. All valves are 6.3V and the total current draw for them all is 3.6+1.2+0.6= 5.4A at 6.3V.

Now I want to roughly figure out what current this amp will draw from the mains supply. A quick calculation on ohms law says that that equals 34W. Now I know the B+ and various other bits and pieces will add a bit more, but the plate on the back of this amp says 250W. Fuse fitted is 4A.

Whats the correct way of getting a reasonably close estimate on this?


Fran
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Old 9th March 2008, 10:48 AM   #2
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Say you will have 450 V of anode supply voltage and the peakcurrent of 1 tube is 100 mA then you will have 180 watts of peakpower more.
The transformers mostly have an effectivity of 80%, so don't forget then too.
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Old 9th March 2008, 11:07 AM   #3
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Ah! I knew I was missing something. I am pretty new to this and always have thought of the heaters as the main current draw. Neglecting the low current draw of the B+ - just because its usually in mA. But that was forgetting about the high voltage!

Many thanks,

Fran
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Old 9th March 2008, 11:11 AM   #4
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Quote:
Now I know the B+ and various other bits and pieces will add a bit more, but the plate on the back of this amp says 250W.
That 'bit more' you're talking about is the greater part of it! The heaters take only a small proportion of the total consumption, like about 20%.
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