Light Bulb Haul

[WA1GFZ]

Bought about 50 light bulbs last night at Home Depot. I notice the size of the light bulb display changing to include a lot more RF generators. Also the price was right. The only expensive ones were the 15 watters I use in the closets.
4 for $3. 60 watters were just over a buck for 6.

[k4kyv]

Time to stock up.

[WA1QHQ]

Frank is building a dummy load for the big rig.

[KA1ZGC]

Light bulbs make horrible dummy loads. I'd like to know who spread the common misconception that they're any good for that purpose. A lot of people seem to have been told that over the years, and it's just plain wrong.

The resistance of a light bulb changes dramatically according to filament temperature. That's why they're used in series with the power supply leads of homebrew electro-machining setups. If the bit gets stuck, the bulb gets hot, the resistance shoots way up, and the current gets choked off.

You want a dummy load that presents the same resistance no matter the input power or amount of time that power is applied. That's physically impossible with an incandescent bulb.

Don't know who started that particular wive's tale, but I sure hope they never worked on any of my rigs.

--Thom
Killer Appetizer One Zesty German Chocolate

[k4kyv]

Back when most hams built their own tube type transmitters and could afford to run about 100 watts output absolute maximum, it was very common practice to use a light bulb dummy load.  That was the first kind I ever used.  You didn't worry about the swr being exactly 1:1, in fact, you probably didn't have a clue what the swr was.  You just wanted to load  the transmitter up to maximum input power to check modulation and to make sure it was really transmitting a clean signal with reasonable power output, before you put it on the air.  You were perfectly aware that you would have to  re-tune the rig when you put the actual antenna on.

You estimated what power level you should be getting out and bought a light bulb to match, whether 15w, 25w, 50, 60, 75 or 100w.  You judged output power by how close to normal brilliance the bulb was, and guesstimated efficiency by comparing that with the DC input to the final.

I have a 1938-era Gates AM broadcast peak limiter that uses a resistive bridge network that includes two light bulbs, something on the order of #47 pilot lamps, as part of a passive compression system that takes advantage of the non-linear resistance characteristic of a light bulb filament.

I recall a Popular Electronics construction article from the late 50's that used the  light bulbs for the opposite purpose, as a volume expander.

[WA1GFZ]

Don,
My first two rigs a 6V6 and 6L6 both drove the light bulb...then I got a license learned about SWR and got a dummy load.

[Opcom]

In the same line as Mr. Don says, the venerable light bulb is the cheapest possible and most readily available power indicator. In the ancient times, perhaps RF power meters and RF ammeters were very costly.

[K9ACT]

Light bulbs make horrible dummy loads.

>You want a dummy load that presents the same resistance no matter the input power or amount of time that power is applied. That's physically impossible with an incandescent bulb.

Not necessarily. 

I tried to order a dummy from MFJ but they were waiting for the boat from China and I didn't want to wait.

I needed a load to do modulation and audio testing on a 300 W transmitter so I used a 500W photoflood and had all sorts of problems loading it.

People also claim that you should match the power out with the lamp wattage but this is nonsense in most cases.

Take 100W for example....  E squared over 100 gives a full power resistance of 144 ohms.  I have no idea what the resistance of my 500W bulb is at 300W but it would be 28.8 ohms at 500W, again a lousy load.

However, when I did the math on a 150W lamp I got 96 ohms.  With wo of these  in parallel, it comes out to 48 ohms; a nearly perfect load for the transmitter.

It works like a charm and it is re-assuring to see the light brighten with modulation.

I haven't the brains to determine whether 300 watts is a unique case but you could not find a better dummy load at that level.

Jack K9ACT






 

[KA1ZGC]

Relative power indicator, yes. Stable dummy load, no.

If all you're looking for is "when I tweak this coil/cap, does it make more smoke" and you don't give a damn whether the load even remotely resembles a constant real-life on-air load, the light bulb works just fine.

That has often lead to the misconception that a light bulb makes a good general-purpose dummy load with a built-in wattmeter. It doesn't. I've heard many hams say this on the air, so I know I'm not imagining that misunderstanding.

A light bulb presents a load, yes. A light bulb changes brilliance based upon the amount of excitation it is provided, yes again.

A light bulb also rapidly changes its load charactaristic (both resistive and reactive) based upon the temperature of the filament at that moment, which in this case would mean the amount of power being delivered at that same moment. Just modulating a carrier could easily double or halve your resistive and/or reactive load (vs. an unmodulated carrier), depending of course on the modulating waveform. Your finals aren't going to think too highly of you if you make a habit of giving them that unstable a load.

If you want some light-emitting device that gives you some relative indication of the amount of smoke your transmitter is ejaculating, run a balanced feed near opposite ends of a PW flourescent bulb. You get the exact same effect without presenting a constantly-shifting load resistance/reactance to your finals (which, when you think about it, also translates to a more accurate indication).

The days of needing to cook your finals in the name of saving a dime or two on a real dummy load (penny-wise and pound-foolish) are long since dead, if they ever existed at all.

This, of course, has nothing to do with the price of light bulbs. I'm just throwing this out as food for thought.

Either way, they're an endangered species that will soon be more expensive to buy (or illegal to own) than saddle hackle.

--Thom
King Abraham One Zebraham George Charlie
The anglers in the crowd will get the "saddle hackle" reference. The rest of you can ask your friendly local angler.

[Jim, W5JO]

In the 40s and 50s no one worried about reactance and couldn't measure it.   The only way to measure SWR was with two back to back neon bulbs across the commonly available 300 ohm TV lead to your windom or dipole.  Things have come a long way for you guys.  We used light bulbs for an indication, not some accurate device. 

First load into the bulb and about 99 percent of the people ran only 100 watts or less.  Then hook to the antenna and dip and load until the florescent bulb suspended near your antenna was the brightest is would get.

[k4kyv]

And if you were fortunate enough, you might own a thermocouple ammeter to put in the feedline.  Just tune for maximum deflection.

The only instrument that I have that anywhere near accurately measures output power is my 2 kw continuous duty, convection-cooled 50-ohm resistive dummy load, and several thermocouple rf ammeters.  With every measurement I take, regardless of which rf ammeter I use, the power output calculates substantially lower than I would have guessed it to be. 

I think most hams have an inflated opinion of how much juice their transmitter really puts out.

I have an old Mirage swr/wattmeter (typical Hammie Hambone meter).  It gives accurate enough swr readings, but even into a 1:1 swr, the thing indicates more rf output watts than I have DC input to the final.  Go figure.

But what Tom says is true.  A light bulb's resistance increases with filament temperature, so with a significant increase in average output power under modulation, the loading placed on the final will actually decrease with modulation.  With AM, that would result in a negative carrier shift, reducing the apparent sideband power.  But with normal voice modulated AM, the increase in average (mean) output power when the transmitter is modulated 100% on voice peaks, is not enough to significantly change the light bulb resistance and thus the load seen by the transmitter, so a light bulb load should be satisfactory for testing an AM transmitter if the transmitter can be loaded up to normal operating power working into a bulb running at near normal brilliance.

A light bulb is somewhat of a self-regulating load.  Increasing the source voltage causes the bulb to pull more power and the filament temperature to rise, which causes the filament resistance to rise, which causes the bulb to pull less power from the source.  That's fundamentally the reason why the Gates passive audio compressor works.

A light bulb would make an extremely poor dummy load for a slopbucket transmitter.

[WA1GFZ]

True but a bulb would make a perfect antenna for a slop bucket

[WA1QHQ]

Geeeze it looks like I opened Padora's box with that comment, it was just a freakin joke, I think we all know a light bulb makes a lousy dummy load, unless your a JN getting all your knowledge from 1950s ARRL publications.

[Jim, W5JO]

Some of us were remembering, so don't freak out.  It is fun to look back.  I worked about 38 states with 15 watts into a long wire antenna and used an ARC type of receiver.

Today I use a digital vector wattmeter that is very accurate and measure my antenna with a bridge.  I have 2 dummy loads each within 2.5% of 50 ohms.  But in the real world you design, build, then set about making it work with whatever it takes.

[KC4ALF]

I still use em in a pinch. And don't forget "tune to maximum brightness"

[WA1GFZ]

Hey Mark we need to have some fun...

[K3ZS]

They make good RF attenuators too.    Not like the fancy fixed impedance lab types.   I use a series light bulb between my Elmac AF-67 and Gonset GSB-101 linear amp.   You have to experiment with the wattage.   I can tune up the AF-67 to full power and have the right drive for the linear by using a series 200W bulb on 40M and a 60W on 75M.   You get a quick surge of power as the bulb heats up otherwise all units run at their AM specs.    Not suitable for fixed impedance rice boxes though.

[K6JEK]

I found a $119 60W bulb.   Actually it's an LED bulb that produces the same light as a 60 watt incandescent while consuming only 8 watts and your wallet.

http://www.ccrane.com/geobulb/index.aspx

People buy these?

Jon

[Pete, WA2CWA]

They make good RF attenuators too.    Not like the fancy fixed impedance lab types.   I use a series light bulb between my Elmac AF-67 and Gonset GSB-101 linear amp.   You have to experiment with the wattage.   I can tune up the AF-67 to full power and have the right drive for the linear by using a series 200W bulb on 40M and a 60W on 75M.   You get a quick surge of power as the bulb heats up otherwise all units run at their AM specs.    Not suitable for fixed impedance rice boxes though.

If you're running QRP on CW or even AM, you can use a lamp (I've used those 7.5 W Xmas type lamps or nite-light type lamps) as a "TR" switch especially if receiver uses an input antenna coil with the cold end to ground. Just connect one side of the lamp to the receiver input lead and the other side of the lamp to the transmitter output connection.

Or, if you want to scare the heck out of your neighbors, tie a fluorescent tube (the older types) or a couple of neon lamps at one end of your dipole. Looks eerie in the night sky pulsating to your modulation or your CW sending. 

[WA1GFZ]

Bought another 42 last night. Home depot display is shrinking fast and glo lamp emi generators taking over.

[W1RKW]

Had a compact flourescent go up in smoke last night while watching the tube.  It started flickering  then went out completely.  The living room was dark except for the TV glow.  Seconds later the thing   goes into a hiss mode with smoke pouring out.  Plastic and resistor smell in the house.  Glad it didn't flame out.  Replace it with a safe and good ole incandescent.  The CFL was a well known brand name.  My cheapos have yet to exhibit this type of catastrophic failure.  It went right to the trash bin after it cooled down.

[WA1GFZ]

Well the dispaly size was almost cut in half in two weeks time. I have about 100 different bulbs. We are trying to determine how many bulbs we use in a year. Since last fall one in the shack, one near the washing machine and one in the livingroom. I figure 6 to 8 a year. One more haul and I should be good until they replace CFLs with LEDS.