Quick question about light bulb limiter

[BigE]

Hey guys, I gotta quick question I was hoping that some of you might help me out on.

I had a 40 watt amp brought to me that was blowing the fuse whenever you flipped the Standby On.  So I hooked it up to my light bulb limiter with a 60 watt bulb in it and fired it up.  It held the fuse even when you turned the standby on.  So I plugged the amp strait into the wall and it blew the fuse once you flipped standby on.  Turns out it was a bad tube.

Question is, why didn't it blow the fuse when it was hooked up through the bulb limiter?  Was it just not getting enough current to heat up the bad tube enough?  Should I have been using a bigger wattage bulb so it would draw enough current to heat up the tube and make it blow? Otherwise I could of possibly had to go through a ton of fuses before I found the culprit.(not very economical)

Let me know your thoughts.

thanks to everyone in advance!

[Iannone]

The light bulb current limiter just limits the amount of current the circuit can carry.  In the case of a 60 W bulb, 500 mA is all the current that will flow through the circuit.  The fact that the fuse didn't blow tells you only that the fuse is rated for more than 1/2 A.  the way the current limiter works is this:

The current limiter is taking advantage of Kirchoff's current law, that tells us that the amount of current flowing through a circuit is the same at all points in the circuit.  What we are doing is placing the light bulb in SERIES with the the amp so that we know that the same amount of current is flowing through both the bulb and the primary of the Power Transformer.  The current limiter, therefore, is limiting the amount of current going through both the amp and the bulb to the amount of current drawn by the lesser of the two loads.  If the amp wants more current than the bulb does, the current will be limited to the 500 mA drawn by the bulb.  If the amp wants less than 500 mA, than whatever it is that the amp is drawing is all that will be going through the bulb as well.

The way to use a current limited is to look at the light bulb.  Pull the output tubes (most likely cause of the fuse blowing) and connect the amp up to the limiter and switch it on.  If the light bulb glows very dimmly (and you may have trouble telling that it is on at all), then you can conclude that it was a blown output tube that was drawing the excessive current.  If the bulb glows at its regular 60 W intensity, then there is some other reason that the amp is drawing this much current, and additional troubleshooting is necessary.

[Jack1962]

Great answer on the light bulb limiter.   :)

[PRR]

In overload:

Fuse blows.

Lamp glows.

> why didn't it blow the fuse when it was hooked up through the bulb limiter?

What Iannone said. 120V 60W passes 0.5 Amperes maximum at full glow. A 1A fuse can't blow.

The lamp alerts you to "Trouble" and does not need constant replacement. The amp is still sick.

> It held the fuse even when you turned the standby on.  So I plugged the amp strait into the wall and it blew

You aren't trying to diagnose the fuse. It just done its job.

I ass-ume the lamp was glowing good?  Then you just changed fuse-blow to lamp-glow, you still had a problem.  As Iannone says: pull some big bottles, check some voltages. The lamp has allowed you to try different things and measure stuff with "partial power". If you expect 280VAC at PT HV and find 150VAC, that may be "OK". If you expect 400V and find dead-nuts Zero Volts, you probably have a dead short there.