Dim Bulb Tester - What Watt Bulb?

arossphoto

Active Member
I'm planning to do some work on a Sansui AU-D11 integrated amp and getting all my tools together first. I've been reading about Dim Bulb Testers, but I'm confused about what wattage bulb to use. I've read that you should use a bulb that is 1.5-2 times the wattage of the device being tested, but are they referring to the power output or the power consumption?

The output on my amp is 120wpc and the power consumption is 600 watts, so what wattage bulb should I be using? Is 200watts going to be enough?

Thanks.
 
I think you will find 200w is too much - you need to have a selection (say 40-100w to cater for different wattage devices under test).

I find the one in the middle - 60w is about right for the integrated amplifiers I have tested, with 40w or lower for tuners, CDP's and the like. I use 100w if I want reduced voltage applied to an amp - but hope to get protection relay activation for the next level of testing if needed.

However, the mains voltage here is 240v -soooo YMMV if using other mains voltages.
 
Last edited:
Thanks for the quick reply. The voltage here is 120, and I didn't think 100 watt bulb would be enough because I kept reading you should work your way up to a bulb that is a higher wattage than the device.
 
Quite right - you should work your way up - it is difficult to know the power consumption of a device when connected to the mains like this.
 
The main purpose of the DBT is to confirm that there is nothing so amiss in the unit under test that it is drawing large amounts of current. Too small a bulb will simply glow with nearly full illumination. Too large and the ability to determine if there is indeed a fault becomes difficult. It's a fairly quick and simple 'pass or fail' test (but can also be used to adjust the bias current to a guaranteed minimum value before powering up directly from the AC line). With that in mind, I'm wondering what the purpose of 'working your way up' might be?
 
I know pretty well what it is used for...I asked what they were using it for, as I can't see any point in swapping in a bunch of different bulbs.
 
I know you know EW ;), let me refocus that: I think the idea is that a fault in the amp might be more likely to cause additional damage if you allow for more current draw than is necessary to find to the fault. It seems to me that any damage likely to occur in the seconds you have it on would have already occurred before the test. Is that your thought as well, or something else?
 
I've used a DBT with 100 watt bulb on a multitude of SS amps and receivers and it has quickly told me if said unit is drawing excess current and unsafe to power up direct. Per Glen's suggestion years ago, I have also used it to carefully set bias on repaired amps without danger of going too high due to the current limitation of the DBT @ 100W. Never found the need to go with a lower watt bulb.
 
I use a 60W on odd occasions for small amps, and a 200W for big brutes like the Sansui BA-5000, but the 100W is the standby for most everything.
 
I probably haven't used a DBT enough times, with enough basket case amplifiers, to have found out what EW and others with their experience have arrived at, DBT bulb-wise.

I just think it is good practise to tread softly when applying power to a device in an unknown condition - hence the 'work your way up' (bulb wattage) suggestion. As an extreme example, it is unlikely to reveal anything useful using a 40w bulb on a big amp for example - knowing that comes with experience as is already clear.

Personally I would rather try a 60w bulb and hopefully find out what I want to know about an amp before using a 100w bulb.
 
Last edited:
I can't remember the exact wattage of mine (I built it into a wood box that also contains a Variac and a receptacle on the front panel), but I'm sure it's no less than 60 nor more than 100. I use the one bulb for everything, with no apparent need to match the bulb to the amp under test.
 
If I am understanding EchoWar, the use of dim bulb testing for SS amps/equipment is different than that for tube equipment. Can someone please tell us how one uses a dim bulb tester to determine if too much current is being drawn, also, how does a dim bulb tester help safely adjust bias?

TIA!
 
If I am understanding EchoWar, the use of dim bulb testing for SS amps/equipment is different than that for tube equipment. Can someone please tell us how one uses a dim bulb tester to determine if too much current is being drawn, also, how does a dim bulb tester help safely adjust bias?

TIA!
The DBT is in series with the mains input of what ever gear you choose to power. If the device being powered has a fault or a short, the bulb will take the current and burn brightly. If the device is in good operating condition the bulb will brighten, initially and then burn dimly.

You can use the DBT to initially set bias by adjusting the trim pots until the bulb is bright. This would indicate that the trimmers are set to their lowest setting and hence, not passing current to the circuit.

Hope that helps.
 
100W is a good bulb to try. You don't wan't to size the bulb to provide enough current to power test any amps.

When amps (not Class-A) come on, they use a fraction of their max current spec to power the bias circuit and lamps, sytem control, etc. You want the lamp to let that current get through. If there's any large pulls of current, say from an open bias string or shorted outputs, more current will go through the lamp, heating up it's filiment and making it a higher resistance. Now the ratio of voltage across the lamp vs the DUT is changed, preventing the DUT from having a China Syndrome. You size the bulb to achieve this balance. There is no one size fits all.

Get in there and do a few times and develop some experience. It's really the only way. 100W is a good start.
 
Westend, yes that helps a lot. Thank you!

I just built a DBT this weekend and wanted to know how to properly use it.
 
This has probably been asked somewhere before, but ... Here in Ontario, our misguided government has basically 'outlawed' incandescent light bulbs in favour of Compact Fluorescent bulbs. I'm assuming a CF bulb won't do, will it?

I guess I should stockpile a few incandenscent bulbs before they become unobtainium!
 
Back
Top Bottom