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Light bulb current limiter


babadono
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Hi Guys,

Esoecially you who build/modify amps....

Do you just wire a light bulb in series with the AC hot line if you're fearing your build or mod might cause an input power problem?

 What wattage? I posted over in the solid state section about a mod I'm doing but thought I might try here and get more exposure.

Thanks in advance

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It's my understanding that limiting turn-on current is primarily a tube amp repair/restoration tool. The issue there is old capacitors that turn into resistors and fry tubes if switched directly to 120V after decades of of sitting in a closet. In that case techs generally use a variac and slowly ramp up voltage - but even that can result in damage to a tube amp - thus the light bulb limiter.

 

I ran a concert production company, that means we supplied "backline", a term for rented musical equipment. This allowed musicians to travel lighter and specify certain tubed guitar amps, etc. be supplied by the promoter. Thus I became versed in tube equipment. Thus I built what you're asking about (though I still don't know if it reasonably applies to non-tube stuff).

 

The deal with the light bulb in series with the tube amp is that if it got bright, capacitors had failed and needed attention, but given that the bulb when bright increases wildly in resistance, not enough current passed to damage the tubes and other stuff that wasn't broken. If the bulb was dim, the caps were probably fine. You add a switch to short the bulb once the amp started OK so that the full 120V is then supplied.

 

A simple version. I found that a 200W bulb was required to pass enough voltage to the DUT:

 

LampBox.thumb.JPG.2fb89ad55c542f5f83b27ecdb14ce11c.JPG

 

Rocket Science Schematic:

574041424_LampBoxSchematic.thumb.png.d71000fc8eb870bcd41714103ee7be16.png

Note: be sure the ground passes from male to female Edisons (that's the correct term for the 120V 3-wire plugs we use in America). : )

 

God bless you and your precious family - Langston

Edited by Langston
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Thanks @Langston. Pretty much what I thought. Just wire the bulb in series with the hot. Wattage is probably determined by the max. the amp is supposed to draw I would think. There used to be member here and other audio forums @djk who was an absolute master and godsend with these kind of questions. Sadly he passed  few years back. R.I.P.

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I just looked through some of @djk 's posts. He truly was a master. : )

 

This forum is gaining a lot of respect from me. I'm impressed with the respect I see between different posters, particularly the knowledgeable toward the newbies.

 

Blessings - Langston

Edited by Langston
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3 hours ago, Langston said:

I just looked through some of @djk 's posts. He truly was a master. : )

 

This forum is gaining a lot of respect from me. I'm impressed with the respect I see between different posters, particularly the knowledgeable toward the newbies.

 

Blessings - Langston

He helped alot of people. Not very long ago helped a member with problems in Belgium with a concert using LaScalas for out of doors venue.

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I don't need a current limiter.  Just pointing out that it's now hard to find incandescent bulbs any brighter than 60 watts.  Do you get these brighter bulbs from specialty dealers?  I could use a 150-watt bulb for one of my studio flashes.

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They are available in a specialized field but right now I cannot remember what or where. Do some google searching and I am sure you will be able to find them. The only use for a current limiter are technicians and builders of amplifiers. If there is a dead short somewhere the build will light up drawing current away from device.  If you do neither no need to have or build one. 

 

With a variac you slowly increase the voltage while monitoring amps with an amp meter. If amperage rises very high you immediately know there is a problem and can quickly shut it down. 

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1 hour ago, henry4841 said:

With a variac you slowly increase the voltage while monitoring amps with an amp meter. If amperage rises very high you immediately know there is a problem and can quickly shut it down. 

And you can install a 1A fast or slow blow fuse in the Variac to limit current draw regardless of what the Amp meter shows.  Regarding the amp meter, I use a Fluke that measures in mA AC.  Most tube amps idle at about 1000-2000mA.  I have a dedicated VAC meter on the setup as well.

 

 

 

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On 6/9/2021 at 6:36 PM, babadono said:

 @djkwho was an absolute master and godsend with these kind of questions. Sadly he passed  few years back. R.I.P.

I did not know that @djk  passed away , his last post was Jan 26 2018 so going on 3,5 years

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8 hours ago, John Warren said:

 

And you can install a 1A fast or slow blow fuse in the Variac to limit current draw regardless of what the Amp meter shows.  Regarding the amp meter, I use a Fluke that measures in mA AC.  Most tube amps idle at about 1000-2000mA.  I have a dedicated VAC meter on the setup as well.

 

 

 

image.png.9435484f530e2abab44961b4cb297a07.png

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  • 2 months later...
On 6/12/2021 at 4:29 AM, henry4841 said:

 

 

I made one of these a couple decades ago. However, I was told to use only a 10 to 15 watt incandescent bulb. It is supposed to act like a fuse if the caps were bad and something went catastrophic. The bulb is supposed to "glow" when full input line AC voltage from a variac is achieved, but not glow "brightly".

 

Therefore, I'm confused as to the use of a 250 watt bulb. What am I missing here?

 

(this was for a tube preamp and some 50 watt tube power amps)

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The reference 250watt bulb is what you use testing tube amplifiers. For SS 100watt ones are what you want to use. At least this is what I was taught. There are many on youtube who just use 100watt bulbs for all their testing though. The point is having something that will drain some of the wattage away from amplifier being tested before it destroys itself if you have a dead short. Saved me on a few occasions. 

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On 6/12/2021 at 5:57 AM, Islander said:

I don't need a current limiter.  Just pointing out that it's now hard to find incandescent bulbs any brighter than 60 watts.  Do you get these brighter bulbs from specialty dealers?  I could use a 150-watt bulb for one of my studio flashes.

Would 2 150 watt bulbs in series provide the protection of 1 300w?

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