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Mains Transformer Question
Mains Transformer Question
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Old 3rd August 2004, 04:11 PM   #11
dmfraser is offline dmfraser  United States
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Default Fuse

No, a 6A mains fuse means the primary will draw at most 3A. WHich is all the current that can be drawn without the fuse popping from a fatigue failure during the warranty period. The transformer would at most have a total VA rating of 360W. The 6A fuse just means that in case of a catastrophic failure, the transformer will take 6A for a short period until the fuse blows. Remember the fuse is sized for fire prevention and to minimize nuisance blowing. It has little to do with the power rating.

In the misc section of www.schematicsforfree.mattsoft.net is an article of determining the ratings of unmarked power transformers. I think you wil find what you need there.

However, in the case of multi winding transformers, it is next to impossible to determine the amperage rating of the lower voltage windings. However, it is almost impossible to make them capable of less than 1/2A. The VA rating you determine by measuring core size applies mainly to the highest voltage winding. Assme the lower ones are only good for 1/2A each.

Regardless, look up the article I refer to on my web site.
Dan Fraser
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Old 3rd August 2004, 04:33 PM   #12
xplod1236 is offline xplod1236  United States
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I have a kenwood ka-127 amp. The rating on the back says 120v 5 amps. The mains fuse is 6 amps. The amp is rated to drive 8-16 ohm speakers. I am driving two 3 ohm subs (one on each channel). I'm getting 700-750 wrms to each sub (measured with multimeter), so that's ~1400 watts out of the amp plus a lot of heat (have a fan on it). 1500 watts at 120v comes out to 12.5 amps. Remember that the fuse is 6 amps. Never had it blow. The wire inside the fuse is streched out and bent, but it never blew. I changed most of the wiring inside the amp, and replaced the speaker output relay (the contacts melted).

The 6A fuse just means that in case of a catastrophic failure, the transformer will take 6A for a short period until the fuse blows.
My 6 amp fuse handles 12 amps continuously without blowing....
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Old 7th August 2004, 03:13 PM   #13
reticent67 is offline reticent67  United States
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Thanks dmfraser! Most helpful article.
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Old 9th August 2004, 04:32 PM   #14
dmfraser is offline dmfraser  United States
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Default 6A Fuse


A 6A fuse is designed to blow within 10 seconds if subjected to a 100% overload and within 1 hour at a 135% overload.

However, running with audio is not the same thing as sine wave power, thank goodness. In reality, your average current is within the rating of the fuse, or it would have blown. Readings with a multimeter are not that accurrate. To get good measurements, I have to use an oscilloscope. To be really good, I also use an Audio Precision but lets face it, how many of the people in this forum can afford one.

The fact that the fuse wire has sagged means that you have pushed the limits though, and that you will have a nusiance disconnect some time in the near future.

If it were indeed true that you were able to get 1400W of power through a 6A fuse without it blowing, you would either have invalidated over 100 years worth of industryexperience in designing fuses or discovered a way to get more power out of an amplifier than you put in.

The reality is that if you measured the average power from the AC line, you are within 6A though you may peak above that for short times. Hence the sagging of the fuse wire when it had a near death experience.
Dan Fraser
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Old 6th September 2004, 11:54 AM   #15
ykohavix is offline ykohavix  Israel
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Default avr 65 service manual help here

This post is for Memeber reticent67 :
could you contact me for help getting the service manual for AVR 65 you got, i own avr65 and need to fix it.
mail me to :ykohavix@bezeqint.net
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