Dim Bulb Tester (DBT) Question

jdjohn

Another local hobbyist and friend gifted me one of his home-made dim bulb testers (DBT). This is particularly handy when first firing-up an amp with an unknown history.

I was just using it on a recently-claimed, older Sony 7.1 channel power-amp from my dad's 'collection'. After flipping the power switches, the bulb went bright, but did NOT then go dim. Instead, the amp appeared to go into protection mode (bulb went out), and the cycle repeated - bright bulb, click (and bulb out), rinse and repeat. I'm guessing the amp is going into protection mode, re-booting itself, and then going into protection mode again. Does my interpretation sound correct?

This probably explains why he found it in a thrift store for likely <$20.

I had high hopes for the amp. It is a Sony DA3ES, which does have a rudimentary DAC, and some decent power. I can tell you this sucker is quite heavy with a heavy-duty transformer.

BlueFox

LOL. What the heck is a dim bulb tester? Granted, I feel like a dim bulb for asking that question.

jdjohn

You can read all about it here: https://audiokarma.org/forums/index.php?threads/dim-bulb-tester-build-and-how-to.808399/ It basically tells you if there is a short in the amp being tested. The short could be a transistor, filter cap, etc.

Here's a pic of mine:

BlueFox

Thanks. I think.

jdjohn

I'm wondering if the digital nature of this amp is causing the DBT to act this way. I may get brave and plug the amp straight into the wall.

VR3

Now that is a bright idea....... 😉

daddyjt

Receivers are notoriously difficult to diagnose - however, the fact that it IS a receiver gives a likely idea of what’s wrong. In order of probability (IMHO):

1. Shorted/failed output transistor. These are typically over worked and under speced in AVRs. Check all output transistors for leg-to-ground and leg-to-leg shorts.

2. Failed emitter resistor on one of the output transistors.

3. Something failed in the power supply (although usually the power supply won’t even “boot-up” if it’s bad, which would lead to no light from your dim bulb).

jdjohn

Turns out the receiver/amp is okay after plugging directly into the wall (or power conditioner). I think the protection circuitry built into more modern amps/receivers like this one makes the use of a dim bulb tester unnecessary. My guess is that the current-limiting of the DBT going into the amp caused it to shut down, then cycle back through again. With the full current of a direct plug-in, it's working quite nicely. This Sony is actually not a bad unit for mid-fidelity purposes. It has a built-in tuner, phono input, and DAC...among other things. No wi-fi, BlueTooth, or the like, and no HDMI jacks.

Regarding the DBT, it is really meant for more vintage gear that has no (or limited) protection-mode circuitry. It's kind of like a Variac, but with really just the slight resistance of an incandescent light bulb limiting the current. If a short in the amp is pulling too much current, the DBT limits the current and the bulb shines brightly. That is usually enough to protect other components in the amp from frying, and/or protect the speakers if someone is dumb/brave enough to hook them up before properly testing the amp first. If an amp is working properly, the bulb shines brightly on initial power-up and current draw, but then dims after a second or two when things normalize.

delkal

I am pretty sure with dim bulb testers you want to use a very high wattage incandescent bulb or the voltage drop is too high. One set of plans I saw used a 200 watt bulb! Since it is almost impossible to find even a 100 watt bulb anymore you might want to consider adding a bulb or two.

mhardy6647

The high wattage bulbs with Edison (standard light bulb) bases are readily available still (although who knows for how long?) from farm supply stores like Runnings. We are fortunate enough to have a Runnings in nearby Claremont, NH. Cool store. Imagine a cross between Home Depot, Tractor Supply and Walmart -- they sell pretty much everything -- and it's not all junky stuff, either. E.g., they sell Stihl power equipment & Muck boots. That's half of what you need to get through the year here in Northern New England.

Actually (and FWIW), in the US, the regulations were written specifically to supersede the common (and energy-inefficient) 100W and 60W incandescent bulbs, but to allow more specialized incandescent bulbs (three-ways, low wattage and high wattage 'specialty' bulbs, e.g.) to remain on the market. Maybe a little paradoxical, but true nonetheless.

The DBT is sometimes referred to as a "poor man's Variac" because, with judicious choice of an array of light bulbs it can be used to sequentially deliver increasing voltage to an item under test or rehabilitation. A 200 or 300 W bulb (relatively high filament resistance) will deliver low voltage to the item under test; modern ss stuff with regulated power supplies and/or lots of 'protection' might not be happy at very low line voltage... the primary purpose for a DBT is, e.g., to reform electrolytic capacitors in vintage (tired) vacuum tube equipment. A DBT can also be used to test for shorts or excessive current draw (e.g., problems with powe transformer, rectifier, power supply filter caps, or driver or output transistors) in ss equipment... but ss equipment is so much pickier about input voltages than vacuum tube stuff with big, beefy linear power supplies ever was.

As an aside, using a light bulb filament as a cheap, reliable current limiting device is
an old trick. There have been myriad commercial loudspeakers that used light bulbs in their crossovers to help protect drivers! It can be quite disconcerting to see light flickering via the bass reflex port in a loudspeaker so-equipped when it's being driven hard.

EDIT: Heck, my local Home Despot claims to have 300-watters in stock!

https://www.homedepot.com/p/Sylvania-300-Watt-PS30-Incandescent-Light-Bulb-19011/303761900

mhardy6647

sorry, I'm bein' obsessive. I get like that sometimes!
Here's an current, off-the-shelf "pro sound" XO that is equipped with tungsten lamp(s) for driver protection



https://www.parts-express.com/eminence-pxb2-1k6-2-way-speaker-crossover-board-1600-hz--290-634

The high-pass protection uses custom-built aerospace lamps. The tungsten filaments effectively track the program material, dynamically maintaining a safe maximum current level to the high frequency driver without introducing distortion. This circuitry provides smooth 3:1 analog compression during input overload conditions over 250 watts RMS.

whoo-hoo! Not just aerospace technology... custom-built aerospace technology!

jdjohn

I was counting on @mhardy6647 to chime-in on this I knew he could explain things more thoroughly than me. Along those lines, and to be sure, @delkal 's post confused me a bit, suggesting that the voltage drop is higher with a weaker bulb? Isn't it the other way around? A 60-watt bulb allows more voltage to pass through than a 100-watt...right?

mhardy6647

EDIT: derp. Let's try this again. The bulb's wired in series with the load. A high wattage bulb will draw very little current relative to the load (and not light up much), a low wattage bulb will draw too little (and light up brightly), all else being equal.

https://antiqueradio.org/dimbulb.htm

here's a good rule of thumb from that link.

Your radio should play normally without fully lighting a bulb that is roughly 1.5 to 2 times the radio's [or whatever electrical gizmo is under test ] stated wattage.

[note my added comment]

"Stated wattage", of course, refers to the nominal power consumption of the radio/gizmo under test... this is fine for something like a "AA5" radio or a single-ended triode hifi amp... not so much with a solid state class AB amplifier (which draws very little current at idle).

jdjohn

Ok, I think I've got it now. A lower-wattage bulb(*) has a higher resistance than a higher-wattage bulb, so there is indeed a bigger voltage drop in a low-watt bulb. As such, using a lower-wattage bulb in a DBT could drop wattage so much that a typical A, A/B amp won't be able to 'boot-up', and/or if it does, it probably won't have full functionality or sound output. Using a higher-wattage bulb will allow more functionality, but still provide some protection from a straight short.

As an aside, the higher resistance of a low-wattage bulb does not allow as many electrons to pass through as a lower resistance, high-wattage bulb. So...
- High resistance, low-wattage, less electrons = lower lumens/brightness
- Low resistance, high-wattage, more electrons = higher lumens/brightness

Sorry, but I needed to dumb-it-down for myself

Thanks @delkal and @mhardy6647


* Traditional incandescent

BlueFox

To make it even simpliar, don't use that rediculous tool.

jdjohn

Haha...tripping circuit breakers can be fun!

mhardy6647

There is a place for a DBT -- but, personally, I prefer a Variac and an ammeter. Truth be told, though I usually cheat and just use a "Kill-A-Watt" to measure power consumption. Works great except that, at low voltages, the Kill-A-Watt itself doesn't turn on!

delkal

I believe higher wattage bulbs give less of a current drop (but of course I may have gotten things mixed up). The bottom line is the DBT plans I saw used a 200 watt bulb. So putting in a modern "100 watt equivalent" that actually draws 60 watts could give you problems.

The confusing factor is that for incandescent bulbs resistance goes way up when it gets hot and glows. Just guessing here but the way I see it if you have a 200 watt bulb and you are only drawing 20 watts the bulb is not going to glow and not heat up. So its resistance and voltage drop stays low. If you use a 60 watt bulb and its drawing 20 watts it might start to glow, then resistance goes way up, then you get a big current drop.

But this is from memory and my memory could be wrong. If anyone could confirm one way or the other it would be appreciated!

jdjohn

I think you are spot-on that the higher wattage bulb has less of a current drop. Compact fluorescent bulbs need not apply. A DBT really requires an incandescent bulb...which are getting harder to find as Mark has mentioned.

A bulb's reaction to the current draw is dependent on two things: 1) the bulb's wattage, and 2) the draw from the amp being tested. A typical vintage A/B power amp of 100-200WPC output will draw about 35-50W at idle, so I'd say a bulb of at least 100W is necessary for a good test. A lower-wattage receiver may be fine with a 60W bulb. These are rough estimates.

For a properly-working test candidate, we'd be looking for an initial bulb brightness on power-up, and then a fade to 'dim' If the bulb stays bright, it would be an indication of excessive current draw, and a possible short somewhere in the unit.

tonyb

Dim Bulb tester ?? People still use those things ? I just happen to have the best one...

lightman1

You killin me, Tony!