Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Amplifier Discussion => Topic started by: J M Fahey on March 17, 2011, 12:23:04 AM

Title: Light Bulb Limiter
Post by: J M Fahey on March 17, 2011, 12:23:04 AM
Some images:
(http://www.antiqueradio.org/art/DimbulbSketch.gif)
(http://t1.gstatic.com/images?q=tbn:ANd9GcStK2e4J8TEDl2ZMJxwrYeYUlE-OZWDDZfUtYscZJryhhyMnF0&t=1&usg=__8-AH1UrP33N06hBfx9ChIRBdR1w=)
(http://www.vintage-radio.com/images/projects/lamp-limiter-circuit.gif)
I don't like the switch bypassing the bulb shown in the lower image, simply because it's too easy to switch it on by mistake.
Suggested bulb: 25W for small SS amps (10/30W) ; 100W for high power ones (60/200W); a tube amp might require a 150W to 200W bulb, simply because it eats too much on idle, because of all those filaments.
Most sites state that the bulb burns if a short is present; it does not; worst case it shines brightly, with full power.
If the amp is fine, the bulb usually lights barely visible red.
It allows to measure many things (do I have power rails? do I have DC on the speaker out?) which a fuse would not let you to, because it burns in a millisecond.
Remember all voltages will be low, the bulb is eating part of the wall voltage, do not aim for "schematic perfect" voltages but a reasonable proportion.
Example: power amp /main PSU states +/-40V DC
If I have a dark red or weak orange bulb, measure +/-25V, have no DC on output jack, amp is probably fine.
If I measure +/- 1 or 2V rails or +/-25V but have 24V on speaker out: bad sign.
In this last case, if I have 49V across the lower output transistor and 0.12V across the upper one, I can be pretty certain that the upper one is shorted, which will be confirmed later with an ohmmeter test with the amp off .
That's the general idea.
Much faster and better than:
replace one transistor -> power on -> blown fuse ... replace another -> power on -> blown fuse ... (repeat 25 times) power on -> nothing burns ... so THIS was the bad transistor !!!!  :grr :grr :grr
Title: Light Bulb Limiter
Post by: joecool85 on March 17, 2011, 08:42:45 AM
Thanks Juan, I think I will rig something like that together for testing purposes.  In all reality I probably should have done this long ago but I've been lazy - time to get my butt into gear :trouble
Title: Re: Light Bulb Limiter
Post by: joecool85 on March 21, 2011, 07:35:55 PM
Alright, I finally built one.  I've attached the pics.  It is an old "spot light" interior lamp that I had while in college and hadn't used in several years.  I disassembled and then reassembled as necessary.  Now the "globe" the bulb used to rotate in to aim it is now a shield to keep the bulb from getting broken by my dog and my future children.  Works really well.  Just barely glows with a soldering iron in, blips for a second when my Vox turns on and shines brightly when shorted with a piece of wire.  I used a 75w bulb since that's what I had on hand.
Title: Re: Light Bulb Limiter
Post by: J M Fahey on March 22, 2011, 12:43:01 AM
As the cat said: "it's purr-fect"
Title: Re: Light Bulb Limiter
Post by: JPHeisz on March 22, 2011, 07:12:03 AM
Nice!
I built mine into my homemade lab supply. There is a plastic tube that channels the light to the front panel:

Title: Re: Light Bulb Limiter
Post by: joecool85 on March 22, 2011, 08:05:54 AM
Nice!
I built mine into my homemade lab supply. There is a plastic tube that channels the light to the front panel:



How many watts is that bulb?
Title: Re: Light Bulb Limiter
Post by: JPHeisz on March 22, 2011, 05:09:55 PM
It's 60 watts - good for most everything I do. Convenient and out of the way, it's always ready. It feeds an outlet on the front of the supply, top one is limited by the bulb, bottom one has unlimited mains voltage.
Title: Re: Light Bulb Limiter
Post by: Casey4s on March 24, 2011, 09:37:53 PM
This is just my second post here, but I couldn't help notice this thread.   I have used a ligt bulb limiter several times, it's a quick easy way to safely control startups and testing.

Here's my take on the limiter...

(http://i238.photobucket.com/albums/ff243/Casey4s/GKB/Stock%20photos/Currentlimiter.jpg)
Title: Re: Light Bulb Limiter
Post by: rowdy_riemer on March 25, 2011, 12:14:15 AM
Sorry for the late comment, but this is cool, JM.  :tu:
Title: Re: Light Bulb Limiter
Post by: DrewV on March 27, 2011, 08:28:14 PM
I built one a few years ago. Works like a charm. I wouldn't want to make smoke without it! dv :lmao:
Title: Re: Light Bulb Limiter
Post by: polo16mi on November 11, 2011, 08:28:34 PM
It´s a great stuff this bulb. Saves me (at least) several fuses today. :tu: . I see trouble for it at future.... no more regular bulb in our country, ..just compact fluorescent lamp...
Title: Re: Light Bulb Limiter
Post by: joecool85 on November 18, 2011, 05:34:48 PM
It´s a great stuff this bulb. Saves me (at least) several fuses today. :tu: . I see trouble for it at future.... no more regular bulb in our country, ..just compact fluorescent lamp...

I don't know about where you are at, but here in the US they are getting rid of standard incandescent bulbs, but not halogen bulbs.  There is no reason a halogen wouldn't work just fine.  Also, if you can't find a "light bulb", buy a "heat lamp".  Basically the same thing except a heatlamp is red and won't be outlawed because it isn't a "lighting appliance".
Title: Re: Light Bulb Limiter
Post by: DrewV on November 19, 2011, 09:55:16 AM
You could probably stock up on a 20 year supply of incadescents for not much $$
Title: Re: Light Bulb Limiter
Post by: teemuk on November 19, 2011, 10:41:34 AM
Someone also instigated a rebellious act to sell those light bulbs as "heat balls" ...or something in that manner. Really a genious idea to get around the loopholes.

I'm all for protecting environment and saving energy but I don't support total ban of something that in the end is just a drop in the ocean when considering the big picture. The electricity consumed by my house's light bulbs is peanuts compared to street lights not to mention some industrial plants that alone may draw as much as electricity as mid-sized towns.
Title: Re: Light Bulb Limiter
Post by: Tann-74 on February 10, 2012, 06:57:13 PM
Hello to all...


we used to do the bulb limiter thing when working on tv's to check the ht feed to the loptie..

It's a handy plus cheap way of limiting the ac voltage in to a peace of equipment being worked on.
Title: Re: Light Bulb Limiter
Post by: polo16mi on February 15, 2012, 09:58:47 PM
Hi JM

You are a guru of amp repair!.

i had mounted it as you suggested time ago, and it save me several fuses, and now i happy, running  my LM3886-SansAmps Clone pre. (comming soon pictures and samples).

It is a "to do" homework for any that want throubbleshooting amp help.

Regards.
Title: Re: Light Bulb Limiter
Post by: J M Fahey on February 16, 2012, 11:25:38 AM
Well, thanks !!!
The only problem with that is that Gurus are required to wear a towel wrapped around their heads, sleep often on a nail bed but, worst of all, weigh around 40Kg.
In that case, either I am 60Kg over limits *or* I should be properly called "Oh 2 1/2 Gurus". :)
And , even worse, Gurus are strict Vegetarians, can´t eat "asado"(BBQ) or "choripán" (coal grilled spicy sausage sandwich). No way José. !!
Title: Re: Light Bulb Limiter
Post by: joecool85 on February 16, 2012, 01:59:07 PM
Well, thanks !!!
The only problem with that is that Gurus are required to wear a towel wrapped around their heads, sleep often on a nail bed but, worst of all, weigh around 40Kg.
In that case, either I am 60Kg over limits *or* I should be properly called "Oh 2 1/2 Gurus". :)
And , even worse, Gurus are strict Vegetarians, can´t eat "asado"(BBQ) or "choripán" (coal grilled spicy sausage sandwich). No way José. !!

Juan, you should add that as your Personal Text in your Profile, "2 1/2 Guru" hehe.
Title: Re: Light Bulb Limiter
Post by: noddyspuncture on April 17, 2012, 10:30:34 AM
What are your thoughts on adding an AC Ammeter to the circuit..?
Would that be useful in telling you anything extra to what you can find probing with your meter..?


Some images:
(http://www.antiqueradio.org/art/DimbulbSketch.gif)
(http://t1.gstatic.com/images?q=tbn:ANd9GcStK2e4J8TEDl2ZMJxwrYeYUlE-OZWDDZfUtYscZJryhhyMnF0&t=1&usg=__8-AH1UrP33N06hBfx9ChIRBdR1w=)
(http://www.vintage-radio.com/images/projects/lamp-limiter-circuit.gif)

Title: Re: Light Bulb Limiter
Post by: J M Fahey on April 17, 2012, 10:59:33 AM
You already *have* one ... it's the Lamp brightness.
Normal is dark orange, red or almost invisible; a shorted amp is bright shining as if it were connected to illuminate the room, it will usually blink in rhythm with the music.
Title: Re: Light Bulb Limiter
Post by: noddyspuncture on April 17, 2012, 12:15:02 PM
Yes I get that... but I am still a little confused... I jumped in here  from my own thread about my faulty amp so I'll give you a scenario...

What would happen in this case:

OK, so fix a shorted output transistor fault. You put the amp on via the bulb - the lamp lit bright, so you knew there was a short. Got your meter out and found both o/p trasistors shorted. So you replace both transistors but you don't yet know if that's the fix. And, unknown to you there is "another fault" which would blow the transistors right away - normally. So, you reconnect the amp via the bulb and switch on. Now there is no short, as you've just fixed it - so what would you expect to happen here then... a dim bulb? - making it "appear" that everything is OK? Because, if so then as soon as you remove the bulb - those new transistors would blow.

And it wouldn't be a bright bulb as you've just fixed the short!

Sorry, but do you see my confusion?

Cheers,
Tom


You already *have* one ... it's the Lamp brightness.
Normal is dark orange, red or almost invisible; a shorted amp is bright shining as if it were connected to illuminate the room, it will usually blink in rhythm with the music.
Title: Re: Light Bulb Limiter
Post by: J M Fahey on April 17, 2012, 02:53:10 PM
Quote
So you replace both transistors but you don't yet know if that's the fix.
True. The bulb is an auxiliary device, but your measuring instrument is the multimeter. Use it.
1) Measure both power rails, they should be somewhat lower than normal, because the bulb "eats" some "wall voltage"
Example: normal rails are +/-40V.
If you find something between 25 and 35V each, fine. Reasonable.
If you find , say, +/- 5V .... *who* is eating so much?. Abnormal.
In another Forum a guy's Aguilar 750 amplifier (over +/-70V rails) had perfect symmetical ... 1.9V rails. *Big* trouble.
2) measure DC voltage at the speaker out. You should have close to 0V there. Less than 100 mV in any case.
3) the fire test.
If your amp passes tests 1 and 2, hook the speaker and have it play some music.
You should be able to get at least 5 or 10W out of it, which is quite loud in a room.
If it passes the #3 test, only then plug it straight into the wall, without limiter.

Do not skip repair stages, or you may make an expensive mistake.
You *may* get lucky of course, but don't count on that.
Title: Re: Light Bulb Limiter
Post by: noddyspuncture on April 17, 2012, 04:38:03 PM
Thank you..!
Now I understand...;c)


[/quote]
True. The bulb is an auxiliary device, but your measuring instrument is the multimeter. Use it.
1) Measure both power rails, they should be somewhat lower than normal, because the bulb "eats" some "wall voltage"
Example: normal rails are +/-40V.
If you find something between 25 and 35V each, fine. Reasonable.
If you find , say, +/- 5V .... *who* is eating so much?. Abnormal.
In another Forum a guy's Aguilar 750 amplifier (over +/-70V rails) had perfect symmetical ... 1.9V rails. *Big* trouble.
2) measure DC voltage at the speaker out. You should have close to 0V there. Less than 100 mV in any case.
3) the fire test.
If your amp passes tests 1 and 2, hook the speaker and have it play some music.
You should be able to get at least 5 or 10W out of it, which is quite loud in a room.
If it passes the #3 test, only then plug it straight into the wall, without limiter.

Do not skip repair stages, or you may make an expensive mistake.
You *may* get lucky of course, but don't count on that.
[/quote]
Title: Re: Light Bulb Limiter
Post by: DrewV on April 17, 2012, 10:57:08 PM
I'll just make a couple comments here. Regarding the level of bulb brightness, I have found that  different amps draw more or less current while "idleing" causing the bulb glow a little brighter or dimmer. Second, to prevent a situation like JM described always use a variac to bring the voltage up gently. If the amp is drawing excessive current you'll see it before you raise the V to high.
Title: Re: Light Bulb Limiter
Post by: noddyspuncture on April 18, 2012, 01:16:12 PM
Ahhh! So you are saying it's a good idea to use a combination of Variac and Light Bulb..!?

That makes sense, I take it you would start with the Variac first, feeding your Light bulb set up from it - and then feed the equipment you are working on from the Light bulb...?

Cheers,
Tom




I'll just make a couple comments here. Regarding the level of bulb brightness, I have found that  different amps draw more or less current while "idleing" causing the bulb glow a little brighter or dimmer. Second, to prevent a situation like JM described always use a variac to bring the voltage up gently. If the amp is drawing excessive current you'll see it before you raise the V to high.
Title: Re: Light Bulb Limiter
Post by: DrewV on April 19, 2012, 12:58:17 PM
Tom, Yeah that's how I've always done it. You'll know right away by the brightness of the bulb if excessive current is flowing.
Title: Re: Light Bulb Limiter
Post by: noddyspuncture on April 20, 2012, 07:09:37 AM
Which one first though - I was earlier thinking Varic first - but now, thinking about it the Bulb first would seem to make more sense... or does it on matter?


Tom, Yeah that's how I've always done it. You'll know right away by the brightness of the bulb if excessive current is flowing.
Title: Re: Light Bulb Limiter
Post by: joecool85 on April 20, 2012, 01:16:30 PM
Which one first though - I was earlier thinking Varic first - but now, thinking about it the Bulb first would seem to make more sense... or does it on matter?

It may not matter, but I would put the Variac first, then plug the Light Bulb Limiter into that, and the project amplifier into the limiter.
Title: Re: Light Bulb Limiter
Post by: noddyspuncture on April 29, 2012, 07:37:53 PM
Hi again...

I just had another thought on the bulb/variac debate.
It is concerning "switch mode power supplies", and of course any gear using them... it is well documented that using a variac is not a good idea with them!

But what about the bulb-limiter? My initial thoughts were that it would be OK and work in the same way... but I thought I'd just ask and see if there were any other opinions on it?

Cheers all,
Tom


Which one first though - I was earlier thinking Varic first - but now, thinking about it the Bulb first would seem to make more sense... or does it on matter?

It may not matter, but I would put the Variac first, then plug the Light Bulb Limiter into that, and the project amplifier into the limiter.
Title: Re: Light Bulb Limiter
Post by: J M Fahey on April 30, 2012, 11:30:31 AM
It depends on the PS design.
Most of them do not like anything blocking current.
Title: Re: Light Bulb Limiter
Post by: teemuk on April 30, 2012, 02:10:29 PM
Quote
switch mode power supplies

Not a good idea. Those things are often self regulating so they kinda hate everything that reduces the mains input voltage... The results might be catastrophic or the internal protection circuit might just disable switching due to low voltage condition or something.

Light bublb limiter might work with some SMPS but probably the odds are against it most of the times. Therefore I don't really recommend trying.
Title: Re: Light Bulb Limiter
Post by: noddyspuncture on May 01, 2012, 07:56:37 AM
Thanks for the replies guys... OK just thought of something else. Would like to run it by you. Obviously there are amps out there with SMPS... I already came across a faulty Sub-Bass unit, but one where the SMPS was the problem, and managed to fix it. It was dead. I found some (rather thin) traces open circuit - maybe 'fused'..? I jumpered them and the SMPS sprang into life... sparks everywhere, coming from under the large 16 legged transformer on there. I ended up removing the T/X and found the arcing path... blackened and conducting. I cleaned everything up, replaced the T/X (added a few more jumpers) and everything was working fine. (I have attached a photo....) anyways, this leads me to my question/scenario...

If that SMPS had been OK and the actual amplifier needed attention, obviously I could not use the bulb-limiter on the mains input side.... BUT, I noticed that the output(s) from the SMPS were two really thick wires carrying DC (cannot remember the value) and also a small muti connector. Now then, would it be feasible to say that you could connect your 'bulb'... in line with the main HT out from the SMPS going into the amp section? It was all very obvious and modular, using spade connectors and this could have been easily done.

Just a thought for future reference....
Cheers,
Tom



It depends on the PS design.
Most of them do not like anything blocking current.
Title: Re: Light Bulb Limiter
Post by: phatt on May 01, 2012, 10:43:01 AM

Re safety on supply rails;
http://sound.westhost.com/project3a.htm

2/3rds down read *Powering up* 22 ohm in  place of fuse but this might need to be a different value for other voltage/current requirements.

I'm sure other here will know more. :tu:
Phil.
Title: Re: Light Bulb Limiter
Post by: noddyspuncture on May 01, 2012, 02:27:08 PM
Thanks for the link Phil - but I am now confused. The 2nd paragraph says to "use a bench supply and increase voltage slowly"... but didn't we say that using a Variac on SMPS was a no-no...?

Cheers,
Tom



Re safety on supply rails;
http://sound.westhost.com/project3a.htm

2/3rds down read *Powering up* 22 ohm in  place of fuse but this might need to be a different value for other voltage/current requirements.

I'm sure other here will know more. :tu:
Phil.
Title: Re: Light Bulb Limiter
Post by: J M Fahey on May 01, 2012, 03:00:49 PM
Dear noddys, as we said before, all SMPS designs are different.
Some will stand being fed from a Variac, some will not.
So trying to write an "universal truth" answer here is difficult.
Rather than asking different people what they think, which is a guide but of course depends on the particular SMPS they are working with, you'll have to test it by yourself, with the actual smps you have on your bench.
Title: Re: Light Bulb Limiter
Post by: Roly on May 02, 2012, 10:13:29 AM
Hi @noddyspuncture; Rod's amp doesn't use a SMPS, and the particular par starts;

"If you do have a suitable bench supply - This is much easier! Slowly advance the voltage ..."

He's talking about having a variable DC bench power supply, not a Variac, and bringing the DC on the newly built amp up slowly.  As you will see just below there he shows a very conventional split rail supply.


To go with my light bulb limiter I have a collection of globes of various wattages ranging from a 10 watt "pygmy" pilot light up to 100 watter, but even testing valve/tube amps I rarely use anything higher than the 40 watt - when you need limiting at all it is generally at a pretty low level.

I also have a couple of blown 3AG fuses with a 150 ohm 10 watt resistor soldered across each, and I normally use these when servicing solid-state amps, particularly rack amps, as they often have exposed fuse clips for each supply rail and I can just plug them in.

The general line of attack with an amp that used a SMPS would be to first isolate the SMPS from the amp and see if it is functioning correctly on its own ('tho it may need a small load to fire it up).

If so, then a power resistor or two can be used between the SMPS and the amp, but if you can't get something reasonable out of the SMPS then you have to start by fixing that first (and SMPS repair is a whole topic on its own).

Variacs are certainly handy, but over a lifetime of servicing I've never really felt the need to get one.  Since they are actually a transformer they are still a pretty "stiff" supply and a limiting lamp is a much more effective way of restricting the power input, which is what you really want to do to avoid frying anything.
Title: Re: Light Bulb Limiter
Post by: noddyspuncture on May 02, 2012, 11:49:38 AM
Many thanks Roly - that was very educational sir!
Cheers,
Tom

Hi @noddyspuncture; Rod's amp doesn't use a SMPS, and the particular par starts;

Title: Re: Light Bulb Limiter
Post by: Kaz Kylheku on October 09, 2012, 04:04:30 PM
The FAQ maintainer for the sci.electronics.repair newsgroup also maintains a SMPS repair FAQ.

This repair FAQ has a whole extensive section on lightbulb limiters.

http://www.repairfaq.org/sam/smpsfaq.htm

He claims that "[M]ost of the time you will get away with putting [the light bulb] in series with the AC line".

But it is cautioned that "[SMPSs] can still supply bursts of full (or excessive) current even if there is a light bulb in series with the AC line" and "at some range of line voltage, the output regulation may not work properly and the output(s) may go much higher than expected".
Title: Re: Light Bulb Limiter
Post by: electricity440 on May 25, 2013, 04:42:17 PM
This is just my second post here, but I couldn't help notice this thread.   I have used a ligt bulb limiter several times, it's a quick easy way to safely control startups and testing.

Here's my take on the limiter...

(http://i238.photobucket.com/albums/ff243/Casey4s/GKB/Stock%20photos/Currentlimiter.jpg)

what software do u use to make that- i really need a good schematic designing program
Title: Re: Light Bulb Limiter
Post by: J M Fahey on May 26, 2013, 04:12:26 PM
Chiming in uncalled  :loco

Don't know what he used , but for a very similar look I downloaded Express PCB and use their ExpressSch module.

Very easy and clean.
And you can draw from scratch any special symbol you need and add it to the library.

Just as an example:
(http://img15.imageshack.us/img15/9496/12ax7tester.png)
Title: Re: Light Bulb Limiter
Post by: tubeAMP on December 09, 2013, 08:48:07 AM
yes in tube radio world it is known as a Dim Bulb Tester.  high current is dropped across the bulb protecting your equipment instead of high current across expensive delicate components such as transformers.  you can gage the bulb size by device power consumption.  100W power consumption = 100W bulb.  not to be confused with power output which is not close to power consumption :duh
Title: Re: Light Bulb Limiter
Post by: Roly on December 09, 2013, 06:08:44 PM
transformers transistors

Start low, work up.  Particularly with a solid-state amp that has just had an output transistor transplant I always start on 40 watts or lower.  If you start too high you defeat the idea.  It only needs a trickle of power to bring an un-driven amp up; with S.S. amps the idle power is typically quite low, only a few watts.
Title: Re: Light Bulb Limiter
Post by: ilyaa on February 23, 2014, 02:14:41 AM
just a question about the function/purpose of the light bulb limiter (finally made one after a couple stupid mishaps):

does it function as BOTH an

1) indicator - brightness giving you some info about current draw in amp
2) safety measure - if amp IS drawing too much current, will the bulb actually dissipate it - as power/heat in the filament - and thus prevent dangerous currents from getting into the amp itself? in this case its working as a sort of fuse that doesnt blow?

??
Title: Re: Light Bulb Limiter
Post by: DrGonz78 on February 23, 2014, 05:30:19 AM
I can state this much... 40watt bulb with 120volt AC line will limit the current to 333ma. 60w at 120vAC will limit current to 500ma. 100w at 120vAC will limiter current to 833ma. When connecting any one of those those rated light bulbs in series through the light-bulb limiter that is the limit of current through the amp when the bulb is glowing fully. So if you have a 1 amp fuse in the amp, it won't blow. The fact that the bulb is glowing bright indicates a short in the device and now is drawing excessive current, if it was not limited it would cause further damage to the amp. The light bulb stands in the way of the draw of current and limits it just in the way I just described. Well that is just my way of explaining it and I am sure other's here will explain it better.

Title: Re: Light Bulb Limiter
Post by: Roly on February 23, 2014, 06:35:01 AM
(1) yes, but the indicator function is just a bit of a bonus really.

(2) is spot on.  You can consider it to be a sort of self resetting "fuse".

The important detail here is that the resistance of a filament lamp changes quite radically from cold to operating temperature.  Its resistance at rated voltage is easy to work out from its wattage rating, but the cold resistance you measure with an ohm meter will be somewhere around one-tenth (depending on the rated wattage).  Generally the higher the wattage the greater the difference.

Aside: in big theater globes (e.g. 500W - 2kW) the cold inrush is such a problem that lighting desks often have a "keep alive" minimum setting that holds the filament at a (very) dull red heat to reduce the inrush if they are just banged on.  Taking such a theater light and plugging it directly in to an ordinary outlet is a good way to blow the globe.  This is one reason they often have oddball plugs that are incompatible with normal outlets.  {another is that, in 240V countries at least, you encounter half-voltage lamps that are intended to be connected in series pairs.}
Title: Re: Light Bulb Limiter
Post by: ilyaa on February 23, 2014, 12:33:28 PM
cool - thats what i thought!

so if i turn on this amp thats been sitting around in an unknown condition and start to troubleshoot it and the bulb is glowing brightly (this is all hypothetical right now) it's going to be alright to leave it on as i try and figure out the problem? i was imagining id have to shut it off if there was excess current, but i guess if the bulb has an actual the limiting function things should be all good, riiiight?
Title: Re: Light Bulb Limiter
Post by: J M Fahey on February 23, 2014, 02:08:19 PM
Anyway be prudent.
Turn it on, measure, guess what's wrong, turn off and replace what you think is bad.
Turn on again, recheck, and so on.
And there's many measurements which are made with the amp off, such as continuity, resistance, transistor shorts, etc.
And some damage can be had even with bulb limiter inline.
Example: you put an electrolytic the wrong way.
It won't *explode*  but it may very well overheat and vent.
Or any part which can be damaged if it receives the full 25 W allowed by the bulb.
So use it, it's a tool, definitely not Magic.
Title: Re: Light Bulb Limiter
Post by: ilyaa on February 26, 2014, 11:42:30 PM
Last question:

is the light bulb limiter only useful in a preliminary testing stage - making sure youve got the right voltage on rails and such things, no shorts, no faulty or open spots - but should be removed before actually testing the amp in a playing situation?

the reason im asking: i was thinking about it today and it occurred to me that if the bulb limits current to below 1 mA, the amp cant actually function properly, can it? depending on the amp, of course, but most need more current than that at least through the primaries, right??? in which case if you had the limiter inline, could you actually test the amp beyond taking voltage measurements?

does this question make sense?
Title: Re: Light Bulb Limiter
Post by: DrGonz78 on February 27, 2014, 01:04:04 AM
The current will not be below 1ma but I think you meant to type 1 amp. The idea is to have no load until the DC offset is at least below 500mv on the output. You would then put a dummy load on the output to make sure the DC offset is still at least lower than 500mv. At that point you can use a scope to look at the wave form on the output and anywhere through the amps signal chains. Say you have a healthy amp and connect it to the speaker then yes you can play the amp, but it will not sound very good. It at least can confirm that it is passing signal, but if you have a scope you will know that already.

Juan Fahey has in the past given advice about connecting the speaker after the unit is powered up. I can't explain his thoughts on connecting the speaker while after turning the amp on... He has his reasons for this and I hope he can explain that part for us. So I guess we both have a question about this topic.
Title: Re: Light Bulb Limiter
Post by: ilyaa on February 27, 2014, 02:31:45 AM
Oh yeah I meant 1 amp -

In the case of a tube amp, though, I'm obviously not turning it on without a load, anyway....

But just to be clear, you are confirming my suspicion: an amp with a light bulb limiter inline will not actually SOUND like it should, even if everything in the circuit checks out, correct?
Title: Re: Light Bulb Limiter
Post by: DrGonz78 on February 27, 2014, 04:58:07 AM
Yes not only does the bulb limit current that can get through, but it is limiting voltage as well. So power rails through the amp will go down by a certain percentage too. Basically the sound of the amp will be limited too. So yes a healthy amp won't sound like it should with the bulb limiter in line.

Edit: I am referring to an example of using the light bulb limiter in regards to a solid state application in my previous post.
Title: Re: Light Bulb Limiter
Post by: J M Fahey on March 03, 2014, 10:17:59 AM
The "connect load *after* turn on" bit comes from the fact that most SS amps start "stupid" (imagine yourself being shaken away in the middle of the night and asked something within seconds)

In the case of the amps, normally there are a couple large value electrolytic caps which take a couple seconds to be charged and reach operating voltage.

During that time it's common that the still unbalanced amp places full +V or -V at the speaker terminals, which is heard as a turn on loud Pop.

Which means it's pulling *big* current from one of those rails .... often more than the bulb limiter will allow...... so rail voltage collapses.

On *some* amps, that collapsed rail does not allow some important capacitor to charge, it never stabilizes, bulb keeps shining and amp does not work.

Yet same amp, if turned on without load, does not collapse and in a couple seconds is stable.

So the drill is:
Turn amp on with limiter, no load
Measure voltages, specially no DC on output (less than 200mV)
If normal, connect speaker
Listen for hum/hiss/buzz/whatever
If all fine, play a couple chords at a couple watts volume (which is quite loud inside a small room)
If all fine, plug amp straight without limiter.
Connect load.
Enjoy .  :dbtu:
Title: Re: Light Bulb Limiter
Post by: Jack1962 on June 23, 2014, 12:51:53 PM
Be using a Light Bulb Limiter for years , sure saves on fuses lol
Title: Re: Light Bulb Limiter
Post by: joecool85 on June 25, 2014, 12:16:35 PM
Be using a Light Bulb Limiter for years , sure saves on fuses lol

And smoke detector batteries!
Title: Re: Light Bulb Limiter
Post by: Roly on January 25, 2015, 07:35:23 AM
Internet/YouTube advice
Twice this week folks have turned up with repairs using a limiting lamp rated at 250 to 300 watts.

Well, like some TV detectives I don't believe in co-incidence, so I wasn't too surprised to discover not one but several YouTube videos on how to build a limiting lamp (or dim bulb tester) that advise using globes of such high wattage.  In one of those videos Uncle Doug even says of the globe "the bigger the better", so we have people specially seeking out a globe of 300 watt or more rating.

Since this is horribly bad advice for anybody trying to repair a solid state amplifier I decided to collect some actual numbers to show why.


Temperature co-efficient of resistance
The first thing we need to be aware of is that a tungsten filament lamp is not a pure fixed resistance, its resistance changes with temperature, and since it goes from room temperature to about 2500-3000ºC when running it shouldn't be too surprising that its resistance also changes, almost as dramatically as its temperature, typically increasing by a factor of between x10 and x20 times.

It is this fact that we make use of with a filament lamp as a power limiter.  If we are careful with our choice of lamp then it will be cool and low resistance if the amplifier is operating normally, but will light and go much higher resistance if the amp has a fault.  This means that when cool most of the voltage will appear across the amplifier, but if hot most of the voltage will be dropped across the lamp.  The lamp will operate somewhat like a self-resetting fuse.  This is why other types of lamps such as fluro's or LED's won't do.

So a filament lamp is a non-linear resistor, and as such we can't simply apply Ohm's Law.

But just as a fuse must be selected to be large enough not to blow during normal operation, yet small enough to blow when there is a fault, so our limiting lamp wattage must be selected to pass enough current for amplifier idle operation, but to light and limit if the current gets too high.  You wouldn't say "the bigger the better" with a fuse, and the same applies with a limiting lamp.


Typical amplifier load
I measured the idle consumption of my homebrew Twin-50, two identical 50W s.s. amps of conventional design in a single case.

My energy meter told me they were drawing about 100mA at 240VAC and a power of 15 watts.  Well these do not exactly compute, but we can get the general idea that one 50 watt amplifier draws about 10 watts at idle, that's around 50mA at 240VAC or 100mA at 110VAC.

This is for idle or very low output, and the amplifier mains input "looks" like a fairly high resistance, but if we try to drive the amplifier harder it will appear to be a lower resistance and draw more current.  This increased current must come through our lamp which causes it to heat up and increase resistance.  Actually this is an unstable flip-flop situation so that when the lamp starts to light the change in its resistance causes a regenerative or positive feedback effect where the bulk of the voltage drop quickly shifts from the amplifier to the lamp.

If we stop or reduce the drive to the amplifier the reverse will happen and the bulk of the voltage drop will shift back to the amplifier from the lamp as it cools.

So what we need is a lamp that will pass the idle current with minimal voltage drop, but if the amp tries to draw much more current than this, say due to a shorted output transistor or power supply rectifier, the lamp will light up and drop most of the voltage across itself, saving the amplifier from further damage.

Possibly the most important application is when testing a solid-state amplifier that has just been repaired.  If a damaged component, such as a driver transistor, has been overlooked then just plugging the amp in and switching on can result in all the newly fitted transistors being destroyed in milliseconds.  The correct limiting lamp will prevent that very annoying disaster.


Lamps
This table shows the marked wattage and implicit nominal resistance and current for a range of lamps.  It also shows the measured cold DC resistance, and the ratio of hot to cold resistance.

While all of these lamps, with one exception, are for 240VAC operation, the same will be true for 110VAC lamps although the resistances will be about half and the currents roughly doubled, however the wattages divided between the lamp and load will be the same on either supply voltage.

WattsOhms(hot)Amp(hot)Ohms(cold)R hot/coldRemark
2523040.104165.513.9Homebrand
2523040.104166.113.9Homebrand
2523040.104167.013.8Homebrand
3516460.146121.113.6Arlec QI downlight
4014400.167104.313.8Philips golfball
4014400.167104.213.8HPM golfball
5011520.20882.414.0GU10 QI downlight
609600.25066.314.5clear
609600.25065.314.7GE
6010420.24074.114.1Performer (250V)
757680.31352.314.7pearl
1005760.41745.612.6Mirabella
1005760.41745.812.6Mirabella
1005760.41747.212.2Mirabella
1005760.41743.513.2Mirabella
1503840.625Par 38
2752091.146Heat lamp
3751541.563Heat lamp


Choosing the right wattage lamp
We can't just apply Ohm's Law to a non-linear resistance, but a bit of electrical commonsense and intuition leads us to a rule of thumb that the limiting lamp needs to have about the same power rating as the amplifier being tested.  You can go lower or higher depending on what is to hand but reasonable limits would be between half and double the amplifier output rating, tending to the low side out of caution.

Note carefully that we do not expect the amplifier to work normally when a limiting lamp is in series with its mains supply.  Some voltage will be lost across the lamp so the DC voltages in the amplifier are likely to be similarly reduced, and while the output may be capable of something close to full voltage swing with the speaker load disconnected, the amplifier will not be capable of anything like full power output into a load, and any attempt to drive it hard will cause it to sag and the lamp to light.


Exceptions
While a limiting lamp will work fine with the vast majority of guitar amplifiers there are some amplifier circuits, particularly later models with Switch Mode Power Supplies (SMPS), that will not work at all unless the lamp is quite a high wattage.

Valve/tube amps are a bit of a different case because they have a higher idle power draw due to the valve heaters.  They also tend to be a lot more forgiving under fault conditions, inherently soft-starting as the valves warm up.  Where you get instant fuse blowing you can be pretty sure that the fault is somewhere in the power supply, and there is normally much less need to power the amplifier up limited so you can take measurements.


Note
While filament lamps are being phased out for normal lighting applications there are some exceptions, oven lights, fridge lights, and heat lamps.  In the first two applications the high and low temperatures currently prevent the use of SMPS, so that rules out CFL's and LED's.  Heat lamps of course are intended as radiant heaters so they are not "inefficient" for their application.  These specials look like being available into the foreseeable future.
Title: Re: Light Bulb Limiter
Post by: J M Fahey on February 06, 2015, 09:49:51 AM
Agree and add: so called "high efficiency" filament lamps are still being freely sold.

They actually have a small filament quartz lamp inside a standard glass bulb, with standard screw (E27)  base, for replacement purposes:

(http://upload.wikimedia.org/wikipedia/commons/4/49/Wolfram-Halogengl%C3%BChlampe.png)

They work exactly the same as the old ones, just adjust for real power: the "100W equivalent" is actually 75/80W and so on ("high efficiency", remember? ::))  but that's no big deal.

And of course, you can still use a "pencil" quartz lamp
(http://images.jr-international.fr/images_jr/jr/H100.jpg)
 although the smallest one easy to find is usually 100/150W  ... of course you'll need the proper socket ,  I strongly suggest you use it inside the proper light fixture; they get REAL hot and a fire is possible.

Don't ask how I found out  xP
Title: Re: Light Bulb Limiter
Post by: galaxiex on July 26, 2015, 04:44:43 PM
PIC 01
My old (unsafe - no safety ground) Light Bulb Limiter.

Note that it has a 3 prong plug but only because that's what I had when I made it.  ::)

If ROLY were here he would give me a good verbal lashing for such an abomination.
Well... it actually works as long as you are careful... and it has saved me from some mis-wired lashups on the bench.

PIC 02
My newly constructed (safety grounded) Light Bulb Limiter.  <3)

PIC 03
Hey!!! why doesn't the bulb light up?  :lmao:
Title: Re: Light Bulb Limiter
Post by: DrGonz78 on July 27, 2015, 01:37:24 AM
Yeah I made one on an old wooden checker board that I made in shop class years ago. However, it was wired with a three prong, but still I always thought to build a more professional looking device. Still using the good ole checker board one today, lol.

If ROLY were here he would give me a good verbal lashing for such an abomination.
Well... it actually works as long as you are careful... and it has saved me from some mis-wired lashups on the bench.

Speaking of Roly... I have not seen him post here lately and hope all is well with him.

Title: Re: Light Bulb Limiter
Post by: galaxiex on July 27, 2015, 11:08:50 PM


Speaking of Roly... I have not seen him post here lately and hope all is well with him.

Yeah, me too.

Last post I saw from him ended with "I'm getting too old for this" or something like that...

I do hope he's ok.
Title: Re: Light Bulb Limiter
Post by: DrGonz78 on July 28, 2015, 01:43:44 AM
I just had a feeling something was not right... Just after reading this I remembered that Roly also took care of the Australian Valve Amps site.

http://www.ozvalveamps.org/

Just went there and right at the top of the page it reads "Maintenance of AVA has been transferred to Stephen Bruce, as Roly's health prevents him from continuing. Over the next few months I will be applying the pending updates. Please be patient. Only this index page and the submission details have been updated so far."

Well I hope he is doing okay and just had a feeling something was just not right since he has been absent as of late.  :(
Title: Re: Light Bulb Limiter
Post by: Bill Moore on July 30, 2015, 03:43:12 PM
I can't seem to add a pic, I'll try a link to my "spare parts" limiter.
http://images.lilypix.com/displayimage.php?pid=62743&fullsize=1
Title: Re: Light Bulb Limiter
Post by: nzst205 on April 10, 2016, 09:21:11 PM
The biggest lightbulb I can find is 140W. I am trying to test a Solid State Amp (Jamo) when I connect to Amp via the light build it glows slightly then when I press the ON button the the amp it glows REALY bright (doesn't blow) and the amp turn off.
Is this indicating that I need a bigger light build or indicating a direct short in the power supply.
Interestingly if a plug the amp straight into the mains the amp now works some of the time.
There are 4 10000uf caps in the power supply, perhaps this phenomina is just the caps trying to charge ? and thus a bigger build is needed perhaps.
With the amp running it is drawing 0.14 amps without speakers attached so not a big load.
Any assistance for this newbe would appreciated.
Title: Re: Light Bulb Limiter
Post by: nosaj on April 10, 2016, 09:46:15 PM
The light bulb is not supposed to blow. It is just a visual indicator of an issue if one is there and yours says yes there is a problem.
The direct short could be in the power supply or elsewhere.  Do you have a schematic or exact model number?

Thanks,
nosaj
Title: Re: Light Bulb Limiter
Post by: nzst205 on April 10, 2016, 09:53:22 PM
Yes I have a schematic, attached
Title: Re: Light Bulb Limiter
Post by: phatt on April 11, 2016, 04:24:17 AM
If the unit is working then there is very little need for the light bulb.
This is a very complex circuit with many potential points to check.
The most likely issue would be all those circuit breakers.
Check "AVR power board" and the relay board for clues.

With a wooden stick gently probe around all the relay circuits to see if you can find which one is causing the problem.
Phil.
Title: Re: Light Bulb Limiter
Post by: nzst205 on April 12, 2016, 01:08:35 PM
Thanks of your input. Taking a root-cause-analysis approach I started with the light build box and found that inside of it the power plug receptacle had blown black gunk out of the vent hole in the bottom this was why it would start up without the light bulb device but with the lightbulb in series it was limiting the power available to start up properly. Using your suggestion I found that the main power relay continued 'chattering' after the amp off power switch was replaced so that is the next replacement but probably also indicating that there is some ghost voltage flowing to the relay, perhaps the diode across the relay may also be faulty, will replace for good measure.
Title: Re: Light Bulb Limiter
Post by: substatica on September 16, 2016, 08:20:54 PM
Here's my 5 minute limiter...
Title: Re: Light Bulb Limiter
Post by: phatt on September 17, 2016, 07:10:43 AM
Yep,, that is all you need,, saves lotsa headaches. :tu:
Title: Re: Light Bulb Limiter
Post by: dlbraly on July 15, 2018, 09:45:49 AM
Here is the one I made.
I made a shorting plug as well, figured it would be easier to check to make sure the bulbs were good. Not much use if you have bad bulbs in it.

Only thing I purchased were the bulbs.