ok, I got my light bulb tester, it works and is very useful.
But I really do not understand how it is working/what's happening...
Has somebody a tutorial/explanation at hand about what there's happening? (no build instruction...)
Thank you!
But I really do not understand how it is working/what's happening...
Has somebody a tutorial/explanation at hand about what there's happening? (no build instruction...)
Thank you!
The light bulb is in series with the item under test. If the item has a fault, e.g. a short-circuit, it, or it via its supply transformer, will draw a high current and the lamp will light fairly brightly. If it is open-circuit no current will flow, and the light will remain off. Somewhere between is good!
There is a whole lot more to it than just that 😉
With your bulb disconnected go and measure its resistance.
Now calculate the current it draws based on what you measured and your mains voltage using I=V/R
Knowing that Watts equals the current multiplied by the voltage go and multiply your calculated bulb current by your mains voltage. Does that equal the bulb wattage or not?
Therein lies the first clue...
With your bulb disconnected go and measure its resistance.
Now calculate the current it draws based on what you measured and your mains voltage using I=V/R
Knowing that Watts equals the current multiplied by the voltage go and multiply your calculated bulb current by your mains voltage. Does that equal the bulb wattage or not?
Therein lies the first clue...
Oh, Mooly's getting tricky! 🙂
Hint: A bulb filament does not have the same resistance in its different states.
Hint: A bulb filament does not have the same resistance in its different states.
Yes, an incandescent light bulb's PTC characteristics makes it far more useful in this application than an imaginable ohmic resistor of a resistance that is calculated from the bulb's wattage and voltage.
Best regards!
Best regards!
You must choose a light bulb of the appropriate wattage when using the tester.
If the bulb's wattage is too low, it will light brightly even if your amplifier has no problems, and your amplifier won't play at all.
Choosing an incandescent bulb that has roughly 1.5 to 2 times the amplifier's rated wattage will allow your amplifier to play normally without fully lighting the bulb.
If the bulb's wattage is too low, it will light brightly even if your amplifier has no problems, and your amplifier won't play at all.
Choosing an incandescent bulb that has roughly 1.5 to 2 times the amplifier's rated wattage will allow your amplifier to play normally without fully lighting the bulb.
Pulled from google: Also applies to tube heaters/filaments.
In a light bulb the filament is at about 2700 oC when it is working and its resistance when hot is about ten times that when cold. (For a typical domestic light bulb the resistance measured at room temperature was 32Ω and this rose to 324Ω at its working temperature).
In a light bulb the filament is at about 2700 oC when it is working and its resistance when hot is about ten times that when cold. (For a typical domestic light bulb the resistance measured at room temperature was 32Ω and this rose to 324Ω at its working temperature).
True, but that´s not the point or main usefulness.
Bulb limiters are mainly protective devices, and such high rating will protect very little.
Nothing inside the amp that is, you might as well use a fuse.
Probably the PT but no semiconductor.
The best light bulb wattage should be such that *at idle* , no signal of course, it barely lights, or to mention some numbers, when it´s dropping, say, 10 to 20% of mains voltage.
Close enough to measure voltages , trace audio, roughly bias, etc. , even (after it´s proven reasonably functional) to pass a little audio, say 1W or so (which is surprisingly loud) to check for oscillation, popcorn, hiss, buzz, etc.
Only after that, when you "think" it´s repaired, you may, if you wish, test it with a larger bulb.
I get your 'for measurement purposes' point, but how would a layman know what that 'best' light bulb wattage might be to use in his tester?
Surely you need to have some practical guide as to what is an appropriate bulb wattage to use for a particular item of equipment?
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My advice is to start with bulbs having low power ratings. Connect the amplifier and check whether the filaments lights up. If not, take a voltmeter and measure the DC rail voltages. If these, show little drop from what they are supposed to be, then, the lamp's rating is adequate. If not, increase the lamp's power rating, but do so very cautiously, until the rail voltages rise sufficiently close to what they should be, or the bulb's filament lights up. If the rail voltages remain low and the light bulb lights up, it means there is too much current being drawn indicating a fault condition.
One of the biggest lessons I learned while using my dim bulb tester is that it can limit the power going into the amp. As an electronics novice, this wasn't immediately obvious to me and it led me to believe something was wrong with my tube amp when in fact the amp worked perfectly without the dim bulb tester in series. So often when the dim bulb tester is described/explained, the emphasis is mainly (or solely) focused on its function as an indicator - so that's how I thought about it.
See barry4's response for an example. As a novice, I read responses like that and thought "simple enough" and moved on.
The result of not understanding it completely (as both an indicator and a limiter) was quite funny and embarrassing. Witness it for yourself here Yet Another SSE Build Thread
A few posts later ... "light bulb" came on (figuratively).
See barry4's response for an example. As a novice, I read responses like that and thought "simple enough" and moved on.
The result of not understanding it completely (as both an indicator and a limiter) was quite funny and embarrassing. Witness it for yourself here Yet Another SSE Build Thread
A few posts later ... "light bulb" came on (figuratively).
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Thanks edbarx!
I have a selection (40W; 60W; 100W and 150W) of incandescent bulbs for use with my series bulb tester, which just about covers all eventualities! 😎
> both an indicator and a limiter
Also "protection".
Say the careless builder creates a ZERO-Ohm short. Plug in turn on, "infinite" current flows from the wall. Heat, flames, molten metal.
Now add in series any household size lamp. Even a 1,000 Watt beast. The dead-short build will at worst light the light, at a fraction of the safe limit of household wiring. I would have 8 Amps on a 20A line and breaker. In Luzern it would be 4.2A on a 13A breaker and line. Yes, the 1000W lamp would pass enough power to blow-up a 10W toy, but at least the house does not burn down.
Also "protection".
Say the careless builder creates a ZERO-Ohm short. Plug in turn on, "infinite" current flows from the wall. Heat, flames, molten metal.
Now add in series any household size lamp. Even a 1,000 Watt beast. The dead-short build will at worst light the light, at a fraction of the safe limit of household wiring. I would have 8 Amps on a 20A line and breaker. In Luzern it would be 4.2A on a 13A breaker and line. Yes, the 1000W lamp would pass enough power to blow-up a 10W toy, but at least the house does not burn down.
If things are ok the lamp will flash briefly at power up then go out or very dim.
The flash at power up is the capacitors charging up and taking extra current.
The flash at power up is the capacitors charging up and taking extra current.
Sorry, shoud have suggested some actual values.
Yes, 40W is right for a wide range of home or guitar amps, from, say, 20+20W Hi fi to 100/120W MI or PA which covers *a lot*.
60W would be right for 100 to 250W stuff, remembering we test in principle at idle and so power consumption is very low.
25W only for very low powered stuff, say a 15 to 25W (total) amplifier, or some preamp level stuff such as preamps, tuners, CD or Digital players, etc.
If used with a larger one, just transformer magnetization current (with even no amp board connected go figure) will make it noticeably orange, and some amps will not even *start* , or "start stupid" and latch output against some rail.
100W in my book (others may differ of course) is already too large, problem being that many parts will happily cook , say an overbiased transistor, a driver trying to straight drive a load (because of final transistor failure or misconnection), etc.
In case I do not make myself clear: a 100W bulb will easily let through, say, 25W with ease without showing too much brightness so not alerting Tech and that can quickly cook many parts.
The dreaded: "amp turned on, apparently fine ... and 2 or 3 minutes later a resistor started to smoke - a capacitor popped - a transistor became incredibly hot - a transistor/chip exploded or cracked - etc."
Notice nothing I mention here is related to power amp output, by any means, but internal parts dissipation/overheating.
I assume we are dealing with a *sick* amp to begin with, the idea is first to avoid (if possible) making it sicker.
One problem with my "old style" approach is that with dissapeareance of "old style" tungsten filament bulbs, the classic 25 - 40 - 60 - 75 - 100W series of bulbs is extinct, nowadays the relatively easy to find and general purpose bulb is the "75W equivalent" , actual 50-55W internal quartz bulb one so in many cases we have to make good of it, or else.
Yes, you can mail order a wider variety, but thinking at local corner shop or nearby supermarket, that´s the easier to find one.
And even so .....
Yes, 40W is right for a wide range of home or guitar amps, from, say, 20+20W Hi fi to 100/120W MI or PA which covers *a lot*.
60W would be right for 100 to 250W stuff, remembering we test in principle at idle and so power consumption is very low.
25W only for very low powered stuff, say a 15 to 25W (total) amplifier, or some preamp level stuff such as preamps, tuners, CD or Digital players, etc.
If used with a larger one, just transformer magnetization current (with even no amp board connected go figure) will make it noticeably orange, and some amps will not even *start* , or "start stupid" and latch output against some rail.
100W in my book (others may differ of course) is already too large, problem being that many parts will happily cook , say an overbiased transistor, a driver trying to straight drive a load (because of final transistor failure or misconnection), etc.
In case I do not make myself clear: a 100W bulb will easily let through, say, 25W with ease without showing too much brightness so not alerting Tech and that can quickly cook many parts.
The dreaded: "amp turned on, apparently fine ... and 2 or 3 minutes later a resistor started to smoke - a capacitor popped - a transistor became incredibly hot - a transistor/chip exploded or cracked - etc."
Notice nothing I mention here is related to power amp output, by any means, but internal parts dissipation/overheating.
I assume we are dealing with a *sick* amp to begin with, the idea is first to avoid (if possible) making it sicker.
One problem with my "old style" approach is that with dissapeareance of "old style" tungsten filament bulbs, the classic 25 - 40 - 60 - 75 - 100W series of bulbs is extinct, nowadays the relatively easy to find and general purpose bulb is the "75W equivalent" , actual 50-55W internal quartz bulb one so in many cases we have to make good of it, or else.
Yes, you can mail order a wider variety, but thinking at local corner shop or nearby supermarket, that´s the easier to find one.
And even so .....
