Light Bulb Limiter for Amp Builders

[Cygnus X1]

A lot of folks building and extensively modding amps around here!

Here's a link to a very simple tool every builder should use, it can save a lot of money and safety is my full time real life job.

Although I'm guilty of not using it enough!

The Dim-bulb Radio Tester

[MagPel]

Great! This should be made a sticky.

I'll be throwing one together for sure before i power up my amp after recapping and modifying.

[LPV]

Sounds like a good idea for just a couple of bucks.

[mono]

Ok, someone needs to explain how this works as I find the diagram misleading..



Why is there a plug on the far left?
Is the lightbuld circuit sitting between the radio and Earth, so it lights up if there is a short in the radio?

[LPV]

If the amp draws too much right away it burns the bulb before it can fry the amp. Note the bulb is in series. You plug in the light bulb/switch and then plug your amp into the socket (right).

[IndianScout]

this method has been used by tv techs for years and years..

[LPV]

Here's a cleaner pic of the basic idea.

[m0b1liz3]

Damn I thought this was a thread on how to build a light bulb attenuator haha.

[mono]

Oh I get it, the plug on the left goes into the mains and then the amp plugs into the lighbulb circuit. Makes perfect sense, thanks for clearing that up.

[kec]

I use a variac.

[LPV]

Monkey see, monkey DO

[MagPel]

Monkey see, monkey WIRE!

[Cygnus X1]

Originally Posted by kec

I use a variac.

I also use a variac, but you need to have an ammeter on it to detect short
circuits.
The light bulb absorbs that and gives an immediate indicator of what is going on without being close to the amp.

[kingedb]

Very cool post!! I doubt I'll get into modding or building much. This will be great for that random vintage amp find. Thanks!

[TWANG]

use one all the time..
just a female plug/cord/bulb/cord/male plug.

[crompo]

Originally Posted by LPV

If the amp draws too much right away it burns the bulb before it can fry the amp. Note the bulb is in series. You plug in the light bulb/switch and then plug your amp into the socket (right).

Not quite !!

It is simply a current limiting device and will limit current flow dependant on the wattage of the globe used.

Imagine switching on a light in your house. The filament is connected directly between active (live) and neutral. The globe will glow at full brightness.

So a 25 watt globe at 100 volts will draw a certain amount of current as follows

P (watts) = VI
25/100 = I

I = 0.025 amps

So when used to fire up an amp the TOTAL current draw you will get should the amp be shorted internally will be 0.025 amps (not very much !!) and therefore no damage to expensive components should a wiring mistake have been made. The globe will "limit" the current draw to a maximum of the globes rating.

If you were to use a 100watt globe the max current draw would be 100mA.

What you should see when powering up an amp for the first time with a light bulb limiter is the globe initially lights bright for a split second (this will be the filter caps charging) then the globe will dim back to a dull glow. If it is full on bright you have a problem. This is of course without plugging in and playing - just at idle.

Make sense ??

Wayne

[LPV]

Wayne, I realized my error the when I used it for first power up. You are absolutely correct. I didn't take it out of the circuit after I installed the tubes and spent 20 minutes trying to figure out why all my B+ voltages were way low when I went to test. All the while I had a 40w bulb glowing like Christmas beside me! The full tube compliment will pull more than 0.04 amps.

[FourT6and2]

I made one of these and used it for my Metro build. The bulb didn't light up at all. But the amp works just fine. I plugged a lamp into the limiter instead of my amp, just to test it and the bulb lit up, but dimly (like it's supposed to with the amp). Weird.

I used a 30 watt bulb. This ever happen to anybody?

[Cygnus X1]

Never tried a 30 watt, always 75 or 100.

[LPV]

It only should light constantly if you have a short or if you are like me and screw up and forget to remove it before you put the tubes in. It should also glow for a second as the caps charge.

[FourT6and2]

Originally Posted by LPV

It only should light constantly if you have a short or if you are like me and screw up and forget to remove it before you put the tubes in. It should also glow for a second as the caps charge.

Yeah, that never happened with mine. Bulb never actually went on. Amp is fine, though.

[LPV]

Its only on the very first power up for a sec, maybe you missed it ? Anyway, its worse if its glowing to beat all hell.

[Dr.Distortion]

These work great... I've used one for years. I also use a Variac. Just plug it in after the limiter.

[FourT6and2]

Originally Posted by LPV

Its only on the very first power up for a sec, maybe you missed it ? Anyway, its worse if its glowing to beat all hell.

Yeah, I used it on the first power-up. Never saw the bulb actually go on. Oh well, no biggie! Amp sounds great, though.

[Mike-t]

Can anyone elaborate on this schematic when wiring with a three prong plug. I can't seem to get it to work and I think the ground is where I am having problems. It seems simple enough but I'm missing the plot somewhere. I don't have much experience in home wiring so I think I'm probably overlooking something simple.

[crompo]

All you need do is cut into the active (also called the live) wire and hook the globe in to that.

Earth & neutral untouched to the three pin plug that goes into the wall.

Make sense ?

Wayne

Check this out

http://tubenewbie.com/light_bulb_current_limiter.html

[Mike-t]

Originally Posted by crompo

All you need do is cut into the active (also called the live) wire and hook the globe in to that.

Earth & neutral untouched to the three pin plug that goes into the wall.

Make sense ?

Wayne

Check this out

Light Bulb Current Limiter

It does make sense, thats why I feel like such an idiot for not being able to make it work...

I think I may have located the problem however, guess I will have to wait until tonight to find out.

[V2]

Hi guys.

here is a quick drawing of what I plan to build after a trip to home depot. I'll use one box with switch and receptacle, and a separate box with the light socket. I wanted to double check my wiring, because I ended up with white wires on both hot and neutral terminals of the receptacle.

The photo quality is horrible, but hopefully it is clear enough to see the wiring labels etc.

Cheers,
John

[Six Gun]

I have the box and I use one of those Cheap Clamp-On Lamps painters use, that you buy at Home Depot. This way you can clamp the light anywhere.

And I also have the 4-gang box, with the duplex plug on one side and a duplex-light-switch with the Red LED Light indicator to show the power is applied to the box.

[schoolie]

If you plug a Kill a Watt into the Limiter, then plug your amp into the Kill a Watt, then you can monitor the voltage and current draw. The Kill A Watt unit is only around $20 and is a great setup. Also, you need to pick the correct wattage bulb. If you use a 25 Watt bulb in a 50W Marshall, it's going to glow like crazy, but there's no short.