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Old 4th November 2012, 03:23 PM   #1
Mayank is offline Mayank  United States
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Question Variac question

Hi:

I have a 500VA variac with a 5A fuse.

I need to safely power on a power amp which has just had its output transistors replaced. The amp has a 15A line fuse and two 12A DC rail fuses.

How can I calculate the maximum voltage I can take the variac up to before blowing the fuse(s)?

Should it be 500VA/12A=41.6V?

Rgds
Mayank
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Old 4th November 2012, 05:20 PM   #2
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You need to provide more information about the amp. I don't think your equation is correct (apparent power divided by DC rail fuse rating??). For a class AB amp that will draw no more than a few amps from the AC line at idle and outputting a few watts to the load, things should be fine for testing. You could replace the 5A fuse with a smaller one for a little extra protection. At 120VAC your variac is rated for about 4A.
Of course a light bulb tester would alert you to catastrophic shorts.
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Last edited by sofaspud; 4th November 2012 at 05:22 PM.
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Old 4th November 2012, 05:32 PM   #3
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i agree a light bulb tester or homebrew current limiter is the way to go
especially when you've paid through the nose for transistors and aren't sure if the driver stage is o.k.
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Old 4th November 2012, 06:46 PM   #4
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Quote:
Originally Posted by Mayank View Post
Hi:

I have a 500VA variac with a 5A fuse.

I need to safely power on a power amp which has just had its output transistors replaced. The amp has a 15A line fuse and two 12A DC rail fuses.

How can I calculate the maximum voltage I can take the variac up to before blowing the fuse(s)?

Should it be 500VA/12A=41.6V?

Rgds
Mayank
Hello,

The fuses are for search currents and prevent fires.
Unless your amplifier is "class A" you'll never run near that current.
The method I use is:
turn on the amplifier
be sure the voltage of your variac is set to zero volts.
plug the amplifier into the variac.
turn up the voltage slowly from zero to about twenty volts, watching the current.
The current should move upward to a few amps while the capacitors charge up, but then return to a much lower current.
If the current stays at several amps or at any time the current jumps up,
immediately turn the voltage down to zero and find the problem.
That should work for you.
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Old 4th November 2012, 07:46 PM   #5
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myhrrhleine
your method is quite sound but after many years in the service business i've come to rely on on my homebrew current limiter because in many cases some devices are not tolerant of starting at very low voltages soft start psu being one i can think of and some of the newer class d or t amp as well
the visual indication i get from my limiting lamp is faster than trying to watch a current meter
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Old 6th November 2012, 12:37 AM   #6
Enzo is offline Enzo  United States
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If the amp is operating properly, it is at idle and will not draw much current from the variac. It only draws a lot of current when amplifying large signals into load.

But I would disagree with your approach. I would not try to calculate some voltage and expect a particular result. I would be monitoring the mains current draw as I slowly ramped up the variac voltage. I can then see if the current is trying to soar or if it is going to politely charge up, meaning it is working. The AC curent meter is an important part of a variac setup. I watch the meter, not the variac knob.

Oh, I see that is essentially what myhrr said.

I also agree that a "light bulb limiter" (google that phrase) would be a good addition to your shop.
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Old 7th November 2012, 12:43 AM   #7
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The "Light Bulb Tester" page:

Light Bulb Tester
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Old 7th November 2012, 01:43 AM   #8
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I found a good test was to route the VAS output back into the LTP return without any output transistors in circuit.
If this is good (amplifies ok on the scope) and the bias voltage is good only then risk output transistors with a light bulb in the mains feed.

I also monitor the output volts on a scope on power up, anything wrong and I immediately power off.
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Old 11th November 2012, 07:43 PM   #9
Mayank is offline Mayank  United States
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Thank you for all your comments & suggestions.

The amplifer under test is Adcom GFA-565 monoblock. I had replaced the output transistors after finding 4 of the 10 having their B-C short.

A quick stop at the local Home Depot and my Light Bulb Limiter is now a permanent fixture on the test bench!

Powering on with a 100W bulb I got an initial bright glow which quickly "simmered" to a faint red.

I am planning to now power on gradually with the variac (per myhrrhleine's process) and measure voltages at the Bias and Voltage Gain stages.

Rgds
Mayank

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Last edited by Mayank; 11th November 2012 at 07:47 PM.
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Old 11th November 2012, 08:59 PM   #10
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nice build!
i'm glad to see you've seen the light as to this device's usefullness!
turk
ps hey don't give that away! i like the veil of my personna to be a do-gooder!
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