Build Your Own Current Limiter for Protection when Repairing and Testing Electronic Equipment

This video describes, in detail, how to construct a Current Limiting Device to protect you, your test equipment, and any electronic gear that you are testing or repairing. When used properly, it prevents short circuits from causing additional damage, particularly to expensive power transformers in amplifiers. It is also quite useful when first powering up previously untested and vintage equipment. It is not suitable for use with medium to large electric motors.
The bulb functions as a high-wattage resistor in which the resistance increases with the increasing heat of the filament. A "cold" bulb may offer only 2 or 3 ohms of resistance (the higher the wattage, the lower the resistance), but this will rapidly rise as the filament heats, particularly in the case of a short circuit.....protecting the circuit and dissipating the resulting filament heat in the form of light (warning you of the existence of the short).
IMPORTANT NOTE: The wattage of the bulb must be tailored to the current draw of the device being protected. The high-wattage bulbs (200W+) recommended in the video are primarily for Tube Amplifiers, which operate at relatively high current levels. With lower-current devices, such as Solid State amps, a lower wattage-higher resistance bulb (even as low as 40W) may be required for circuit protection.
Also, in my wiring diagram within the video, the "hot" AC lead is connected to the threaded socket of the receptacle, which could possibly be touched while changing bulbs. Some viewers have suggested that shock exposure could be reduced if the "hot" lead went to the contact at the bottom (center) of the socket. Please consider this when wiring your own Current Limiter.
The Photoflood Bulb used in this video is available on Amazon. Here is a link: www.amazon.com/...
If you enjoy seeing videos featuring classic vintage jukeboxes and guitar amplifiers, please consider subscribing to my channel, to gain easy access to over 250 videos.....and to be notified when new videos are posted. Thanks for watching !!!

[@UncleDoug]

IMPORTANT NOTE: Before asking any questions, please read the entire video description. It will, most likely, answer them. Thanks.

[@jimlagraff4989]

@Chris Roubis Judging from your question Chris, I wouldn’t attempt to measure your mains current without more instruction or education about electrical work. A direct short with a current meter should scare you. That will only tell you how much current your outlet is capable of providing if it doesn’t blow up your meter in your hands. You just have look at your circuit breaker or fuse to see the current limit for the circuit. A meter is used to check the current used by a device, not the wall. Voltage is checked across the leads, current is checked with the meter in series from the outlet to the appliance. If you’d like to learn more, start by building the limiter in this video. Don’t begin your education by sticking wires in the outlet, I tried that when I was 4 and I still remember it as no fun at all. Be safe!

[@luthiervandros]

Uncle Doug, are you using a light bulb in your current limiter? 😂

[@stevemitchell9848]

Your soo smart...and helpful...and your videos soo fun to watch !

[@davidfoster5906]

Could a GFI outlet be used in the current limiter? Or could a GFI outlet be used as an auxiliary out let behind the amplifier that also powers the amp. Electrician codes require one GFI present on countertops around a sink. That one outlet protects all others connected to it. It may work as a ground fault protector in an amplifier.

[@UncleDoug]

There is no substitute for a Current Limiter when working on electronic gear, David. It, and the GFI serve two different purposes.

[@eixr]

When I was MUCH younger , my father ( who was an electrician) would use this method to find and fix short circuits in old houses with screw in Edison fuses. Remove the blown fuse and screw in a light bulb. When you remove the short, the light goes out. Sure saved on checking with good fuses. We also found lots of pennys under the fuses. Dad always removed them, gave them back to the homeowner and told them "If you use a penny as a fuse, read the warranty on it. In God we trust". Also, Thank You for spreading your knowledge to the amp community. You make the world a smarter place when you do.

[@UncleDoug]

Thanks so much for your very nice comments, E. We really appreciate them......and good electricians, like your dad.

[@johanrynjah8241]

@eixr I learned something new from your comment, thanks for sharing. ..

[@jimlabos]

Great story!

[@PrinceWesterburg]

If you don't build one of these, you can still see a bright light - at the end of a long tunnel.

[@UncleDoug]

Excellent point, Prince. It's best if you never see either light.....but of the two, the latter one is infinitely less desirable :)

[@floatingpoint]

I think it's more for the amp's protection than your own - you get a current across your chest, you're gonna have a shite time

[@UncleDoug]

@@floatingpoint Agreed, FP, but either way you're better off using a CL than not using one.

[@floatingpoint]

@@UncleDoug No question about that!

[@PandemoniumMeltDown]

Underrated comment.

[@DetroitWrecker666]

I am building myself one tomorrow. Inscribed on it will be "Uncle Doug" This is a ABSOLUTE MUST HAVE for anyone who plays around with old or even new amplifiers. Thank you!

[@UncleDoug]

You're welcome, DW. It is indeed a very valuable piece of equipment.

[@Mauitaoist]

Everybody needs one of these Doug, thank you; maybe if you don't have a variac you could have one of these it's better than nothing

[@UncleDoug]

They serve different purposes, Cory. The CL is both cheaper and more useful.

[@GRobLewis]

I don't know if it's a regional variation, but out here on the West Coast we call the white wire "neutral", not "common". It's separate from ground (the green wire) but it is tied to ground back at the electrical panel. So it should normally read no more than a couple of volts different from ground. It's basically a center tap on the 240V power coming to your house, which allows it to be split into two separate 120V circuits.

[@UncleDoug]

Thank you for confirming the benefits of this device, VA. Many people simply rely on the fuse in the amp for protection, but it only functions AFTER the current flow has occurred, and often the fuse has been bypassed or is the wrong current rating. Re my lighting, I use only the overhead fluorescent lights in my shop, and an open door to admit sunlight (and heat and noise, unfortunately). My camera is able to compensate for reduced lighting (i.e. for shots inside the cabinet).

[@thorpejsf]

My 8 year old son and I watched this video together and then made one of these. He had a lot of fun and did his first solder joint! We then used it to test out an antique Marx model train transformer. He thought it was the cat's meow. Thanks again!

[@UncleDoug]

Jason Thorpe You're welcome, Jason. It's great that you're involving your son. Such experiences may inspire him to become an electrical engineer. Regards to you both.

[@Willam_J]

Jason Thorpe - I also applaud you for teaching important skills to your son. I got interested in electronics and started soldering when I was nine. I'm 48 now and I've had a great career as an Electronics Engineer. If I had a nickel for every college-degreed technician or engineer I've worked with that couldn't solder (by their own admission), I'd be a wealthy man. Fortunately, the college I went to frequently met with companies that hire their graduates and modeled the curriculum around their suggestions. UN-fortunately, they came back with egg on their face nearly every time because these companies would say "Your graduates can't solder!!!". Yes, it's THAT important. Take care! :-)

[@THUNDERWORX]

Thank you for posting Doug. It might save someone's life.

[@UncleDoug]

You're welcome, TW. Let's hope :)

[@UncleDoug]

Thanks, BT.....it's definitely no "flash in the pan" :) This jewel has saved me from "smoking" all sorts of components over the years, including some internal organs, and I wanted to be sure that everyone who tinkered with electronic gear knew about this concept. Thanks for watching and responding, I really appreciate it.

[@g.m.5607]

Uncle Doug could I leave a voltage limiter permanently connected as a warning device , well if I use a 200 watt bulb ?

[@sniperneil53]

Just a thought, Doug. When wiring the current limiter (I use one that I made twenty years ago), I would suggest the hot wire from the mains be wired to the center conductor of the bulb fitting, not the screw base as this makes the base, which is more easily contacted by errant fingers, live (or hot). This way, if the bulb is partially unscrewed, the metal parts of the bulb are not live if it happens to get plugged in in this condition. Great videos. I have an interest in vintage radio restoration in my retirement years and between you and Paul Carlson (Mr Carlson's Lab), I've got valve technology covered.

[@UncleDoug]

Thanks for your nice comments and helpful input, Neil. Your suggestion is a good one.

[@UncleDoug]

working on a chassis, trying out an unfamiliar amp, or even using one of your own (old) amps. In the case of any internal short circuit, the Current Limiter will prevent the flow of current that can damage the components within the amp.....or you. In addition, the bulb will flash brightly to alert you to a hazardous condition. Good luck and thanks again for your input.

[@jimcatanzaro7808]

First you should have a schematic for the amplifier how can you fix something when you have no plans I wouldn’t even bother if my project didn’t have a schematic I sometimes don’t even buy a amplifier if someone has even worked on it before I just love fixing someone’s hack work I usually only work on HH Scott integrated tube amplifiers there every simple if someone hasn’t put there dirtball fingers on it I live in Massachusetts near Cambridge I’m lucky enough to come across many of these amps very cheap

[@UncleDoug]

Elberelrod: Your question was posted without any reply capability, so I'll reply here and hope you see it. Sadly, a lot of organ manuals and schematics aren't available for free. Here is a site that will sell a service manual for the organ: www.organservice.com/lowrey/LowreyManuals.htm But since you are more interested in using the organ parts to build a guitar amplifier (I assume) you should be more concerned with the schematic of the amplifier you are going to build. Watch my videos on power transformers, how amps work, output transformers, etc. to get some hints. Good luck.

[@HazelTheHare]

This is super helpful, though it does make me grateful for British plugs.

[@UncleDoug]

Glad to hear it, M :)

[@TheCrazycatfisher]

I have been taught something new. Great job and great demonstration.

[@UncleDoug]

Thanks, MR :)

[@SaxophoneHitman008]

Hey Doug. Nice job my friend. We deal with lots of tube amps coming into the shop for various problems. We use a 'homemade' current and voltage limiting device. We combine the function of a variac with the good old light bulb method of current limiting, to initially fire up an amp with known problems and to break-in and checkout the repaired items and new builds. They are an integral part of the tube amp test benches, and as you say, use them religiously. We use a slightly lesser wattage bulb in series with the black hot because we can use its 'glowing' intensity as an indicator of the amount of current drawing. However to run the amp up t o spec we do have to bypass the I limiter to ensure unrestricted current supply is able to flow. Again, thank for your public service videos. We know they're not so easy to produce, so we [Greg Guitars] say thank you for your time and talent!

[@UncleDoug]

GREG GUITARS LLC Thanks so much for your kind words and helpful input, GG. Comments like yours make it all worthwhile :)

[@digifortune4062]

Great Video clip! Forgive me for butting in, I would appreciate your opinion. Have you thought about - Saankramer Electronic Magazine System (do a google search)? It is a great exclusive guide for learning how to repair electronic items like a wizard minus the hard work. Ive heard some extraordinary things about it and my cooworker at very last got astronomical results with it.

[@jackallen6261]

Jeff Allen, I guess most of us have stories like this to tell...if we didn't kill ourselves in the process! LOL. Yes the joys of childhood!

[@robertkat]

Where can you find an old tube amp? They dissapeared with the dial phone, teletype, reel to reel tape recorders, cheap sea food.

[@nicholasroos3627]

@@digifortune4062 minus the hard work....lol

[@eddiefitzgerald5198]

Hello Doug, I am a 75 Year old from Ireland and I have got hours of enjoyment and education from watching your videos. Since I retired a number of years ago I have taken a keen interest in the working of old valve equipment even if there is not much of that sort of stuff hanging around here. I decided to build a Current Limiter similar to your one and I have to say I am delighted with my effort. I got my hands on a 1967 Selmer Treble/ Bass MK 111 50W amp and not knowing what the story was with it I had the pleasure of plugging it in to the Current Limiter. Am I glad I did as the bulb lit up like a Christmas Tree and heeding all the advice you have given in your videos regarding safety in dealing with Valve equipment I proceeded with caution. Using another of your home brew aids I discharged all filter caps; At this stage I need help with pin pointing the fault with this amp and I wonder if there is someone out there in YouYube land who would point me in the right direction. I have one other observation on your excellent videos...... That is your interaction with your Dog and Cats. Beautiful to see. Thank you.

[@UncleDoug]

Thanks so much for all your great comments, Eddie. I'm glad the videos have been helpful. Good luck with the repair job.....that's how you learn ;)

[@paulp2033]

Hi Eddie. Did you ever get that amp fixed ? If so, what was the issue with it ?

[@eddiefitzgerald5198]

@@paulp2033 Hello Paul, Thank you for enquiring about my amp. I never got it fixed but did discover that it was the main transformer that was at fault. I did try to find a transformer but so far I've no luck with that.

[@paulp2033]

@@eddiefitzgerald5198 hi again Eddie. I don’t suppose you would consider selling the amp ? If so let me know and we could discuss. Cheers. Paul

[@eddiefitzgerald5198]

@@paulp2033Hi Paul, I'd sell it alright.

[@BurtonBoyz715]

This is the video that made me an Uncle Doug ...Dev-o-tee . A low tech....high concept......elegant solution ....that anyone can replicate.....Wow...Keep-it-simple-stupid.!.....Too Good !!

[@UncleDoug]

Thanks, William. Yes, the mighty Current Limiter is probably my most-used device in the workshop. For a few bucks (to construct) it's the best and cheapest insurance you can buy :)

[@noteconomicalyviable]

We are all Devo

[@johnfeagin3947]

Great video. Thanks for putting it together. Just made mine this weekend!

[@UncleDoug]

Thanks, John. You'll be glad you did :)

[@defendukraine181]

Best explanation on the web...

[@UncleDoug]

Thanks, DU :)

[@jimlafrieda8824]

From a Ph.D in Electrical Engineering, this was an excellent video that showed the value of using a current limiting device when it comes to turning on an amplifier, testing a high pwr tube, etc. in lieu of having a variac where you can bring up the voltage to a device, very slowly.

[@UncleDoug]

Thanks, Jim. I view it as inexpensive protection for the power transformer and other components in the circuit in case of a short circuit.

[@jimmio3727]

This is also super useful in car audio; most amps have huge capacitors in them. Capacitors are dead shorts until they charge, and it's often enough to blow the fuse you just plugged in and/or weld the now blown fuse into the holder. Pass the current through a car lightbulb until it stops glowing, then connect the fuse. Another great video, Doug.

[@UncleDoug]

Thanks for your input Jim :)

[@paulmichaelfreedman8334]

Don't modern amps have inrush current limiters?

[@willie_mccoy]

I've always thought the outlet looks like that because that's what your face will look like if you get zapped!

[@Rainbow__cookie]

lol

[@Greg-lo1tl]

I've been zapped. It does.

[@HatlessChimp1]

This light bulb video is awesome!

[@UncleDoug]

Thanks, HC.

[@PrinceWesterburg]

Don't forget, if you're in the UK you have 240v one the mains and not 230v - When we came into line with Europe, rather than waste billions replacing substation transformers, it was easier to relabel everything. This is why US amps run higher HT voltages than they should in the UK - worth testing those HT smoothing caps. Fender amps run 400v caps at 380 but in the UK you can find 420v HT, not funny!

[@UncleDoug]

Most of the vintage Fender amps I have owned and worked on use filter capacitors with 450V to 500V working voltage ratings, Prince....but I agree, since the wall receptacle voltages are higher now in both the US and UK than they were when these amps were designed and built, we must be vigilant to ensure that our filter caps are up to the challenge.

[@PrinceWesterburg]

Yeah, I have a load of gear from the US marked at 110v to 117v. The assumption is (for others reading) that a mains transformer has two 115v primary coils. In the states these run in parallel for 115v, in UK & Europe they run in series for 230v. Great, right? No! If you have a 380v HT rail and 400v caps, then the mains is actually 240v but that day its a bit higher, say 245v, then your HT rail is now 405v. Add fluctuations in the circuit and thats really hammering the circuit. Add to this CAD designed modern amps that run a 1w resistor at 0.9w and its not a long term career move! LOL Great videos Doug, I'm sure you shall continue for years to come! :o)

[@UncleDoug]

Thanks for your excellent analysis and input, Prince. We really appreciate it :)

[@joecarson8281]

Doug, I followed your directions but always wondered if the thing actually worked because it never lit up. Well, last night while troubleshooting no sound, I made a change and got what sounds like both 120 and 60 cycle hum. More importantly, I looked out of the corner of my eye and saw 300 watts of "Shut her down now!" Thanks for the safety device instructions. Back to the drawing board.

[@UncleDoug]

Greetings, Joe. It's always good to hear from you. I'm glad the CL worked, and probably saved your power transformer. Best of luck with your repair (of what sounds like a dead short in the power supply).

[@joecarson8281]

Thanks Doug. Means a lot. It was an input issue and that little dotted line at the bottom of the Vibrato part of the circuit on the layout. Found it on the schematic, blends in with yellow highlighter. Now I just have to troubleshoot the low volume, but I've seen that one before. The vibrato sounds great even at low volume. Thank you for the time you put into your explanations. It makes this road less bumpy.

[@UncleDoug]

You're welcome, and congratulations on solving the problem, Joe. It sounds like your amp project is really working out well. I hope other viewers read your first comment and become "true believers" in the value of the Current Limiter. Thanks for your input :)

[@joecarson8281]

I leaned a couple of lessons on this one. Always use the current limiter, always read the schematic twice before you take the amp apart 3 times, and make sure the number of grounds on the schematic matches the number of grounds on the amp. I might want to include, the GZ34 is a great rectifier. I appreciate the lessons.

[@UncleDoug]

Greetings, Joe, and thanks for confirming the value of several very good practices when working on electronic circuits....and that the GZ34 is a great rectifier. I'm glad the videos were helpful :)

[@stevebarrett_WF3E]

Use these in the r/c hobby, we call them smoke stoppers. Keeps you from releasing the magic smoke...

[@UncleDoug]

They are excellent for that purpose, Steve.

[@hydorah]

I love the way American power outlets look like a shocked and dismayed face

[@whatyoumakeofit6635]

Yes.......It was designed that way to attract small children. Causing them to take interest and then trying too shove onjects in the holes. Now, while I'm neing sarcastic, it is still true that small children look at our outlets ( in the U.S.A.) and see a face. Just as you mentioned you see a face in the outlets. And due to that they then stick thi gs in the holes. Its a very ignorant design in my opinion. But, difficult at this point too change. Some electricians I know, purposely install outlets upside. In the hope that a child wont come along and see a face and try sticking something in the holes. Have a good one and take it easy !!!!

[@wesrichards6168]

@@whatyoumakeofit6635 Not all kids shove stuff in electrical sockets. The sockets installed upside down has nothing to do with a "face".

[@5400bowen]

Can you protect Lithium Iron Phosphate batteries hooked to a 3500 watt solar inverter from high current surges at startup/restart connection by using this device with a 3500 watt bulb? It would have to handle the continuous flow of 3500 watts with up to 4k-6k surges. And if not, can you use any other simple device that can take continuous 3500 watt flow from batteries to an inverter and limit current surges without it cutting the circuit during momentary effectively short circuit surges? The batteries are 24 volts.Only one solar equipment manufacturer makes a device to do this and they are $280 plus $55 shipping to Hawaii.

[@UncleDoug]

No......twice.

[@steelcantuna]

Wow!!!!!!!!!!!!!! I tested mine for the first time tonight employing your screwdriver set up technique !!!!! I was ascared... but it worked great!!!!. Thanks to your video, I'm on my way to being the next Thomas Eddy's son!!!

[@scottmcmurtrie]

I always test my 3 wire plugs rather than trust they are wired right. I bought some awhile back, and they were switched. Probably the reason I got a good deal on them.

[@UncleDoug]

I haven't encountered this problem, Scott, but will begin checking them prior to installation. Thanks !!

[@Willam_J]

Uncle Doug - I sometimes use computer power supply cords in situations like this. It seems that some foreign manufacturers use, what seem to be, random colors for the wires in these cords. Definitely a best practice is to verify the wire colors before using them. I've been using one of these (along with variacs, isolation transformers and GFCI outlets) for years. Can't be too safe. Also, if at all possible, have someone nearby (or check on you frequently) when you're working with high voltage. It's also worth mentioning that when you say that this device can protect a person that you don't mean it will protect them from electrocution. This device can still supply 2 amperes of current and it only takes as little as .065 amperes (65ma) of current to be fatal. I humbly submit that changing those regular outlets with GFCI outlets would be a great upgrade to that device. Be safe, everybody! :-)

[@goodun6081]

William J., those modular computer type power cords are typically green brown and blue, green is the ground of course, and if I remember correctly blue is usually the hot, but I always check with a multimeter. By the way, a GFCI electrical outlet will often trip and save you from a shock if your vintage amplifier has an internal short to the chassis or an electrically leaky "death cap" from the power cord to the chassis, as is almost always seen on vintage tube amps whether or not they have a "ground polarity" switch.

[@CarRadiosMichigan48439]

I have seen the same concept applied to speakers on a radio repair bench. If the radios output had a high current short the correct size lamp should minimize damage to the speaker. We would use 12V brake lamps in series with the speakers.

[@UncleDoug]

Greetings, KK, and thanks for your input :)

[@Willam_J]

Kim Kusluski - That's a good idea. Some companies also use lamps in series with tweeters and horn drivers in their speaker cabinets to prevent damage from customers applying to much power to them. (Bose home speakers and JBL PA speakers, to name a few.) Despite the protection lamps, people still manage to roast the tweeters and horn drivers. That's OK with me. I do a lot of side work on live sound equipment and speaker repair pays well. :-)

[@wansolve2036]

Cool! Vintage Yaesu Ham Radio transceivers used a small light bulb in series with the antenna input as a lightning/static discharge protector.

[@josipzlk8019]

or use more blubs in paralel

[@UncleDoug]

Yes, this is an excellent way to tailor the bulb wattage to your amp.

[@riomar9]

Couple of Questions Uncle Doug: 1. Can your INCREDIBLY AWESOME Current Limiter setup be used to operate something like a Weed Eater - or some other tools with brush motors - whereas, if the weed eater uses 4.5 Amps, would the bulb-resistor therefore have to be something like a 500 or 600 watt bulb? 2. Also, Can I connect your Current Limiter directly to the OUTPUT of a 120, 2000 Watt voltage controller to lower the SPEED of the Weed Eater so that the Weed Eater does not continue drawing the 4.5 Amps after reducing the voltage with my Voltage Controller, because, I already burnt out one weed eater! According to something I read, I believe the Wd Etr was still drawing the 4.5 Amps despite me having lowered the voltage with my controller? It operated fine for a few minutes but, after I increased the speed, it completely gave out and stopped working! Hope you understand my questions? Thank you so much for this extremely informative video and REALLY COOL, SIMPLE project...have to use it somewhere and hope it can do what I'm asking!!!

[@UncleDoug]

Sadly, the current limiter does not work well with motors, Rey, and definitely not with those drawing so much current.

[@riomar9]

@@UncleDoug THANK YOU VERY MUCH!! How about this: What if I use a 5 Amp FUSE instead of a light bulb, and wouldn't that also do the same job on what you use your current limiter for? In my case, as soon as the motor would begin to pull the current above the motor's rating - 4.5 amps - wouldn't the fuse blow as soon as it reaches its 5 amp rating so the motor won't burn, and wouldn't that also be the case in what you use your current limiter for by using a FUSE, instead of a light bulb? Just wondering if that would work? Thanks again brother and God Bless!!!

[@stephenmstephenson4001]

UD-- I'll give it a shot , I have my own 24 track recording studio so I am versed in proper mike and setup ,like you said the SM-57 is a better choice to use for guitar/drums/keyboards etc. in certain case's it sometimes helps to use a compressor and noise limiter to cut down on the background noise and tightens the sound somewhat (keeps the amp speaker from reflecting from the back wall and returning to the mic in my opinion , I noticed on one of your vids you where using a recorder ,have you considered a small mixing board along with that ? it opens up a lot more venues like EQs/white noise setup for the room your in , some times the camera mike does not work to good when you move away, I wonder if a person could take a drone camera and place it on a head ban and wi-fi it to the camera wha-la hands free to unplug cords/ do vids/ play guitar/shop work/hot rod vids etc. etc. etc. without hold-n the camera + lets you place the camera on a stand when your do-n your Jack and Cassey sound checks here I go again make-n a long story longer, I should become a novel writer

[@UncleDoug]

Thanks for your helpful suggestions, Stephen. I'll keep them in mind.

[@LFOVCF]

11 people pissed that you didn't post it sooner, and blew up their amps! lol

[@UncleDoug]

Better late than never, I guess :)

[@computerlen]

Hi. This is my first time viewing any of your videos. It is one of the best videos explaining the subject but I must disagree with one minor thought of yours. I use a 60-100 watt bulb because I want to limit the current to any device connect so that there will be no damage to newly-installed devices such as transistors,etc. Does this make sense? A high wattage bulb would let too much current go through the deice under test. I have no need to operate the electrical device being repaired at almost full voltage and current. I understand that some electronics devices 'act strange' when the full potential/current is applied but most items that I repair work OK with the 100 watt bulb in series. This series light wiring is excellent for circuit tracing intermittent shorts because of the current limiting function of the series lamp. One more thing- I have successfully used a lower wattage bulb such as 60 watts to see the dim glow of the bulb when an amplifier is running in series and I am able to adjust the small biasing controls. When I adjust the bias control and the current through the output transistor circuit is increasing the limiting bulb glows brighter. A higher wattage series bulb such as 250-300 watts would not let me see its glow. Len

[@UncleDoug]

The video was intended to guide viewers who work on tube circuits, Len, not transistorized circuits. The device can be used on transistorized circuits, but since their current demands are so much lower than with tube circuits, the bulb wattage does indeed have to be lowered dramatically. I believe this is explained in the video description.

[@computerlen]

NOW I disagree with you about using a 250 watt bulb. A 100 watt bulb or 60 watt bulb is what I have been using for decades and they work fine. I test large and small transistorized amps and large and small tube amps. I NEED to see the the amount of glow the 100 watt bulb shows me. Initially turning on the device the bulb glows and then settles down to a dull glow without any audio signal in the amp. Of course putting a signal into the amp makes the bulb glow brighter according to the level of the signal. If there is an intermittent short somewhere in the device under test the LOWER WATTAGE bulb will suddenly glow brighter informing me of the short BUT it will not allow any damage caused by the less than 1 amp of current going into the circuit because of this lower wattage bulb. A larger amount of current will go through a 250 watt bulb. This has helped me out countless times after I had installed new transistors and resistors, for example. If I were to use the 250 watt bulb and there was still a short somewhere in the amp or I had installed the transistors incorrectly the high wattage bulb might let enough current into the amp to blow the new transistors. I know that you will not agree with this because the higher wattage bulbs work for you but the techs staring out are highly advised to use the lower wattage bulb. Another advantage with the use of a lower wattage series bulb is that I can safely adjust the bias of an amplifier when I initially adjust the bias control one way or the other. The bulb glows brighter when the bias voltage is too low and moving the bias controls to a more negative level decreases the amount of glow to the small wattage bub. Very handy. You cannot see a glow [as you stated] in the 250 watt bulb. [phew!]

[@thomascoughran1374]

Back in the TV shop, we called that a suicide cord... :-))

[@masonclark4951]

Thomas Coughran

[@Willam_J]

Thomas Coughran - Back in the hot-chassis days, when power cords where clipped to the fiberboard back of TV's and radios to disconnect power when the back was removed, we used to swipe the cords off of junk sets so that we could plug sets in and work on them live. We also called those "suicide cords". It's amazing that we're still alive to talk about some of these things. LOL :-)

[@MichaelStoneham]

Great job. But no good to me NZ/AUS use different plugs.

[@UncleDoug]

Thanks. Then modify what I suggested to suit your own locale.

[@christheother9088]

Nice demonstration. Would a GFI outlet work acceptably in this arrangement? I would add that twice in my life I have seen light bulbs burn out and short internally, causing circuit breakers to pop...probably the filament flopping around in the bulb. Exceedingly unlikely, but something to think about.

[@UncleDoug]

+Chris Gonzales A GFI might protect you from being shocked, but it would not offer the visible (glowing bulb) evidence of increased current flow, or the ability to estimate the amount of (or changes in) current flow from the brightness of the illumination.

[@christheother9088]

Actually I meant the inclusion of a GFI outlet on the glowing bulb board...in conjunction with it.

[@UncleDoug]

It would probably interfere with the bulb's function....opening the circuit rather than letting the bulb limit the current. You could try it and see, however.

[@silasfatchett7380]

@@UncleDoug Even if the bulb illuminates to indicate a dead short, the GCFI would not trip. It's function is to detect a difference in the currents in the live and neutral wires, indicating a leakage to ground.

[@UncleDoug]

Thanks for your input, Silas.@@silasfatchett7380

[@NETBotic]

nice!

[@UncleDoug]

Thanks, Jon.

[@Whitewolf4869]

Just built this because i have an older Yorkville solid state mixer amp with a short. I could only find a 200 watt bulb. Tested it on the bad amp and the bulb glowed brightly. Disconnected transformer from the board and the bulb glowed dimly. Tested limiter on an identical good amp mixer and the bulb glowed dimly. i assume its because i need the 250 watt bulb. thanks

[@UncleDoug]

You're welcome, Lorne. Good luck with your repair.

[@saymyname103]

Uncle Doug smart man u r cheers silvio from australia...

[@UncleDoug]

Thanks, Silvio :)

[@Bill_N_ATX]

What are we going to use when there aren’t any incandescent bulbs left on the market?

[@UncleDoug]

Thank our intrusive government(s) for dictating the details of our personal needs. Meanwhile, hoard several bulbs while you can.

[@dbingamon]

I built one of these this weekend and used it to test a recapped 1950 Emerson 503 AM Radio. I wished I would have done this a long time ago.

[@UncleDoug]

It really is a useful, practical device, isn't it, Daniel? I have used mine on every single amp repair job I have ever done..

[@jwren51]

Hi Doug, I saw your very useful device and thought I would build one however in the UK the availability of high wattage bulbs is restricted and expensive. 100w bulbs are available on eBay so I coupled 2 in parallel to acheive almost the same result. I enjoy your videos immensely, very informative and a voice that's easy on the ear. Keep it up.

[@UncleDoug]

Greetings, JW, and thanks for your nice comments. Since your voltage in the UK is double the 110VAC we have here, your bulb wattage should be about half what we use here......so the two 100W's would be about right, or even two 75W's in parallel.

[@UncleDoug]

Thanks for your kind words, TG, I really appreciate it. I don't think you need an isolation transformer unless you have an amp that has no power transformer and uses the tube filaments (50L6, 35Z5) in series to provide the voltage drop. Also, I think the concern over "death caps" is unnecessary as long as the amp is properly fused. All of my amps that came with them still have them and I have never had an issue in 35 years. I would recommend a Current Limiter, especially when you are..........

[@gillespiewrga]

Question about impact of the current limiter. Testing a functioning Silvertone 1484, powered through a 300 watt bulb which glows very dimly when the 1484 is on. Measure Voltage on 6L6 plate at 430 VDC. Then I plugged the 1484 directly into the wall socket and measured the 6L6 plate at 479 VDC. What am I to learn from this 49 volt variance? 122.6 VAC at the extra CL out when amp is off, after amp warms up the extra CL outlet drops to 113.1 VAC. So the bulb is sucking up 9.6 VAC when the amp is on. Measuring A/C current by patching between the on/off switch connectors: 645 mA with the CL; 768 mA powered directly from the wall outlet. So I declare my Silvertone is fine. But taking measurements with the CL "engaged" is now questionable. Advice?

[@UncleDoug]

On higher-wattage amps that draw a lot of current and illuminate the bulb, the CL does significantly reduce the PV and PC within the tubes. On lower-wattage amps, which barely even warm the bulb, the reduction is negligible. It's best to perform the final bias adjustment without a CL.

[@LockRocker]

I think im going to get on of those outlets that has just one outlet and a switch in the second spot to kill it.

[@UncleDoug]

By all means, improve the circuit to suit your needs, LR.

[@toysmitty]

Thanks as always Doug! I’m making one of these today!

[@imark7777777]

The diagram is weird hot lamp socket as in the incoming hotline should be going to the bottom of the Lamp socket and the outer ring should not be immediately hot because it's easier to come in contact with. The Brits have figured this out and grounded their side ring. After all the polarization (wide pin, small pin, white wire, black wire) does matter You can also trust the color of the wires to be correct as long as you're cord didn't come from China.

[@UncleDoug]

Thanks for your input, Imark.

[@RulgertGhostalker]

Excellent ... looking over all your stay safe gizmos ... my new guitar amp proto is mostly getting built in the virtual realm, but when it comes to the real world proto, i have to cover my bases... it is going to have a circuit board, but all through hole discrete, and some hand wiring... ( no ICs at all anymore, i got it worked out on discretes )

[@UncleDoug]

Thanks, Eric.

[@RulgertGhostalker]

@@UncleDoug actually on my low voltage circuits this winter ....and one IC did find it's way into the amp's bias circuit... hope you get some good rainfall down there this winter, i gotta get stocked and bundled up.

[@UncleDoug]

@@RulgertGhostalker Thanks. We've had a lot of rain so far this year.

[@andrewjmcgee]

excellent!

[@UncleDoug]

Thanks, Andy.

[@kipb2078]

After 904 comments my answer is probably buried here somewhere, but haven't seen it yet. I in the U.S, have a 220V 75W bulb, will that work? Im thinking higher voltage=carry more wattage. Best video I've found, thanks

[@UncleDoug]

You're welcome, Kip. I would advise against it. You can generally find 200W bulbs at the Home Depot or even higher wattage on Amazon.

[@TexasRebellion]

I love your very informative videos and especially Rusty the Wonder Dog! I understand very little about electronics end of amps but am loving the schooling I'm getting. keep em coming!

[@UncleDoug]

Thanks, TR. Rusty and I are glad our videos are helpful :)

[@nonsuch]

Hey UD, I just picked up a stereo tube amp from an old jukebox. I've ran it through your circuit and it plays both channels but has a bad hum. The bulb is on dimly but came up gradually, not right away. Do you think this could be an issue with one of the tubes? Thanks in advance!

[@UncleDoug]

+Rob Cosentino Rob, you need to watch my video on types of hum and their causes. It may be a tube, but more likely the filter capacitors. Good luck.

[@ronbercan1000]

It does help for mishaps and shorted equipment. But I dont think you should say that it protects us ( us meaning are body). It helps to not damage even more all ready faulty equipment that you plugin. (IMPORTENT NOTE: a 250w lightbulb does let 2 amps of curent go through your body.

[@UncleDoug]

I agree, Ronald. The device is primarily to protect the circuit being tested.

[@zulumax1]

As a general rule I use the same wattage of tungsten light bulbs as the devices rated wattage. I have 3 lamp sockets wired in parallel, in series with the black wire. For example, for an all american five radio I would use a 40 watt bulb, whereas a vintage tv would need up to 300 to 500 watts of bulbs. A variac and a light bulb together is what I normally use. You are a good and patient instructor, you can never take safety for granted.

[@UncleDoug]

Thanks for sharing this with us, Z1. I appreciate your kind comments.

[@handwoundpickups6555]

Brilliant as ever👍

[@UncleDoug]

Thanks so much, HW.

[@robertcabral1366]

Great video. I am going to make one of these for myself. I test older electronic equipment, and sometimes the capacitors decide to short. This will help me to see the capacitor short, before I get capacitors blowing up on the equipment. Thanks

[@UncleDoug]

Thanks, Robert. That's exacty the purpose this device is intended for. Best of luck with it.

[@spelunkerd]

We use the same principle in automotive diagnostics, the test light protects sensitive electronics in computers from dangerous surges of current. Just as importantly, a properly chosen incandescent test light draws enough current to help identify occult circuit resistance that would be completely invisible to simple voltage checks with a multimeter. It is interesting to see how you're using it here for a diagnostic AC circuit power supply.

[@UncleDoug]

That's interesting, S. Thanks for your input.

[@Dinco422]

AC polarized ? What ? lol.. not in eastern EU they're not...

[@UncleDoug]

Thanks for sharing, Dinco.

[@dannylee4377]

Great Video! Can a GFCI receptor be used for protection instead of building a current limiter? Or should a GFCI receptor be used Plus this current limiter?

[@UncleDoug]

Thanks, Danny. They serve two different purposes. The GFI is mainly for your safety, while the current limiter protects the circuit from damage while offering a visible indication of normal and excess current flow. I guess you could use both.....but I only use the CL.

[@dannylee4377]

Thanks for your reply! I will add both to my setup. FYI, I have been getting a lot less sleep after finding your videos - they are so much fun and educational to watch! Your teaching skills are very apparent.

[@UncleDoug]

You're welcome, Danny. We're glad you enjoy our videos :)

[@quinnjin2]

Warning! Wiring description relative to US only !

[@seanrosedotcom]

Note to builders: You should reverse the black wires on the bulb in the schematic. That way if the bulb filament blows, the base is not energized when you start to unscrew it plugged in. Light bulb and fuse 101. No nasty bites (aka shocks) that way. PS, just unplug it.

[@UncleDoug]

Thanks for the helpful input, Sean. Hopefully none of the viewers is foolish enough to unscrew a large bulb by trying to grip the nearly hidden brass base.....but I guess you never know.

Doug, I don't understand the negative votes on your videos. There appears to be a concerted effort of possibly a competitor goofing around. Not that it reflects poorly on anything you do, just irks me .. maybe just jealous people who have no time for knowledge or just cat haters. LOL I wonder if you have examined your youtube 'stats' to find they all come at once (bought for) or if they are scattered dislikes. Just in the 10 videos or so I have viewed of yours, you share such details that are priceless from both standpoints of your collective knowledge and rarity of these electronics. I have already plugged some huge gaps I was searching for answers over the years meddling around in tube gear from your words and demonstrations. You are most certainly one of the very few who truly deserves the accolades and gifts from viewers / supporters !!! You go from videos like this one, a ground floor introduction for people's safety when working on old gear , to extremely complex breakdowns of rare spring reverbs. Just the top notch Uncle Doug :)

[@UncleDoug]

Thanks so much for your very nice comments, NI, they more than make up for the few slobbering fools who try to blemish the work of those who work hard to post helpful, informative videos. Everyone gets a few of these idiotic negative votes......even Eric Clapton's live version of Layla got 12,000 thumbs down. Obviously such votes....and voters......are meaningless.

[@wallyjohn7260]

Sorry but I just can't get why some people put thumbs down. Thanks Doug for making useful and easy to understand videos. Keep the good work! Charles from Canada

[@UncleDoug]

I view those losers with the same disdain as dog poop on my shoe sole, Wally. Thanks so much for your kind words :)

[@UncleDoug]

Thanks for the kind appraisal, V. I never really thought of using a current limiter in that way, but it would reduce the voltage to the electronic device, so I guess it would behave somewhat like a Variac. You could build one and test the voltage drop with various bulbs.

[@elluisito000]

Hello uncle Doug! I am about to start building a tube amp, and wanted to build a current limiter like the one you used. I have a 150 W halogen bulb with its socket, do you think I can use that instead of a incandescent? Thanks!

[@UncleDoug]

I wouldn't trust it, Luis. You can get incandescent bulbs at most building supply stores as well as Amazon and probably Ebay.

[@alessandrolandi77]

Dear Uncle Doug, I've discovered your videos 1 day ago, and I thank you for making then, they are very well made and give a lot of fantastic informations for who, like me, is not an expert. I have a question for you. I live in Italy and my outlets AC Voltage is 220V. Making some simple (I hope right...) calculations, I should use an 850W bulb to mantain the internal resistance comparable (i.e. low) to the 250W (on 120VAC) of the bulb that you use. Given that this assumptions are right, in your opinion could I use a parallel of four halogen lamps of 230W each to meet the goal? (incandescents bulbs are not in commerce anymore...). Thank you for your attention and again my most sincere compliments for your videos!!!

[@UncleDoug]

Greetings, Alessandro. Actually, it's the other way around. Your voltage is twice as high, but your current is half as much as ours.....so you need a bulb in the 100W to 150W range for most small to medium amp circuits. Use only incandescent bulbs.....LED will not work, and I doubt that halogen will work either. Good luck.

[@alessandrolandi77]

You’re so right Doug, being a current limiter if you double the voltage you have to double the resistance too.....what a noob I am....Thank you for your quick answer!

[@ElliotGKnapp]

Uncle Doug, any tips on mounting the outlet to the board safely? The porcelain base seems straightforward, but I'm curious if you have insulation around the outlet base, and if it's just sitting on the single center screw or if there's more support (sorry if you've already answered this, I tried to search the comments and didn't see anything). I'm wrapping up my 8th tube amp build and it's time I stop rolling the dice on my high level of care and attention to detail and build one of these for safety!

[@UncleDoug]

The porcelain base I used had two screw slots, about half way toward the outer perimeter, Elliot. It was firmly attached to the base board.

[@GUILLERMOJGARZON]

I want to build one for checking 24-28 volts AC. If there is a shortage in somewhere .What incandescent bulb should I get? There is a formula to calculate the watts for the bulb? Thanks

[@UncleDoug]

You would have to find a bulb that operates on 24 to 28 Volts, Guillermo. Good luck.

[@UncleDoug]

Gary: It sounds like a great project.....especially with hand-cut dovetails !!! Be sure to post a detailed video of the Pro Reverb when you are done.

[@dougsharrott]

Just ordered the bulb to build this. I overhauled my first customer amp and he's thrilled (Ampeg B15/Want one!!!). Thanks for the knowledge and confidence!

[@UncleDoug]

Congratulations on your first repair for a customer, Doug, and best wishes for a great future. You're welcome.

[@gtrjoe1901]

Wonderful video - I'm learning alot from your wonderful channel & I know what it means to get feedback ~ So thank you for all the time and effort ~ Very Much Appreciated ~ My Best ~ Joe

[@UncleDoug]

You're welcome, Joe. Thanks for your very nice comments.

[@Gsilvio]

I find your videos to be so insightful, I really appreciate the skill you have as a teacher, if I could I would spend a whole month hanging around your shop hoping to gain all that I could of your wonderful enthusiasm for vacuum amps. Thank you for all the lessons, we need more teachers such as yourself!

[@UncleDoug]

Thanks so much, GS. I really appreciate your kind and supportive comments.

[@whatyoumakeofit6635]

Uncle Doug, woill this still work with led bulbs ? I know ive been hearing for several years that they stopped making incandescent bulbs. I went to the store the other day, no incandescent bulbs on the shelves. So, I went to another store. No incandescent bulbs. Oy ! I dont like the like you get from led bulbs. I wanted to build one of these so I came back to your video to check it out. Then I thought, gee I cant even get a incandescent bulb. So if you know if a lef bulb will work the same id appreciate it if you share your knowledge. Thanks a million !!!

[@UncleDoug]

No, LED bulbs will not work at all. Check on Amazon......they still offer high-wattage incandescent bulbs.

[@DarkLinkAD]

Home depot, lowes, walmart, Tractor supply etc Red Heat lamp bulb.

[@UncleDoug]

Thanks so much, JZ. I really appreciate your input and consideration, and I hope you too have a great day.

[@sktechnology8411]

good

[@DanielsGameVault]

This is very handy for when you're fixing SMPSs and you're not sure whether you've got a shorted semiconductor somewhere. Recently though I've noticed that with SMPSs that have PFC on them (most notably TV PSUs), the bulb stays on all the time, indicating a very low resistance through the SMPS ! Of course, I had already replaced any shorted MOSFETs and diodes, which would account for the bulb being bright, but it still stays like that, so I'm guessing there's something else still shorted that I missed, or it's because this PFC circuit that tries to reach the 400V "bar", but can't because of the resistance of the bulb, thus keeping the MOSFETs in their "on" state and supersaturating the PFC transformer, causing the high current demand through the bulb and keeping it bright.....damn, that was a mouthful, I know :)) Has anyone noticed this before ? Did I even make sense ? :))

[@UncleDoug]

I have very limited experience with semiconductor circuits, Dan, so it's difficult for me to provide any meaningful input on this subject. Hopefully, another more knowledgeable viewer can help.

[@Sloxx701]

Too low of a wattage bulb (high resistance) will probably cause the PFC voltage to fluctuate and the SMPS/inverter would probably behave erratically. The bulb will be lit but the PFC may be unable to start (it doesn't necessarily mean there's a short). Anything less than ~370 volts from the PFC will cause issues. Some TVs may require using 300 - 500 watts worth of bulbs in order to make use of a dim bulb tester. A variac is probably a better idea.

[@unhinged17]

Hi Doug, thank you for all your great videos - I'm learning more from them than I have in the past 20+ years on my own and through books. They are an invaluable resource! I built a current limiter as you describe here, and it works great with amps and other electronics I've plugged into it - no bulb glowing yet. However, I do have one question - I plugged my electric drill and then my heat gun into it, and surprisingly both illuminated the bulb. It was fairly dim at first but started glowing brightly before I quickly turned them off and unplugged them. Does this indicate my drill and heat gun have a partial or dead short? Is this something I should be concerned about, and should I take them apart to figure out what's up or stop using them? Thanks again!

[@UncleDoug]

You're welcome, Jason. Your drill and heat gun are working properly, they simply draw more current that the bulb will allow to pass. I have a sign on my own current limiter that says "Not for motors", for just the reasons you discovered. The heat gun probably produces 1500W of power (heat). According to Ohm's Law, I = P/V, so the current flow would be 1500/120 or 12.5 Amps, which is about 6 times more than the bulb will allow to pass.

[@UOttawaScotty]

Would you suggest using the current limiter bulb device in series with an isolation transformer for added protection ?

[@UncleDoug]

For routine circuit repair, I use only a current limiter. You are welcome to use both if it provides a sense of safety, but simply obeying the rules for safe handling of high voltage, i.e. "one hand in your pocket", should provide adequate protection. Bear in mind that the CL protects the circuit......not necessarily the tech.

[@UOttawaScotty]

@@UncleDoug thanks for the response Uncle Doug, love the videos.

[@UncleDoug]

You're welcome, Scotty :)@@UOttawaScotty

[@hugeshows]

I reckon if you took a candelabra or other multi-bulb fixture you could "dial in" whatever current you needed by adding or removing bulbs. Maybe even a string or two of those old 7w 120v xmas lights would work.

[@UncleDoug]

You're right, HS......either switch bulbs in and out of a single fixture, or have multiple fixtures in parallel.

[@craigkeller]

Doug! That receptacle should be tamper resistant (for dopes like me) and water resistant (beer). Just saying.

[@UncleDoug]

My workshop is a child-free zone, Craig, so I feel reasonably safe with it as-is......except for the beer part ;)

[@donaldfilbert4832]

Uncle Doug. I am confused a little by your Ohms rating of 1.6 Ohms. Please help me with my math. If V=IR and Wattage = IV. Then it seems that the current of a 1.6 Ohm load at 110 volts during a short situation (or normal light operation) would be 110 / 1.6 = 68 amps. Of course - that's higher than the circuit breaker current - and can't be right. I calculate that the resistance of a 250 watt resistive load (the bulb) at 110 VAC would be as follows: 250 Watt = 110 Volts X current. Current = I = V/R. so 250 = 110 X 110 / R. So R = 110 X 110 / 250 === > 48.4 Ohms !! Is there something wrong with my calculations ?? I do not really know how much AC current an amp draws - but if for an example - it drew 1 amp - then with the Current limiter in place - the limiter would use up 48.4 X 1 = 48.4 Volts; leaving only 110-48.4 = 61.4 volts to power the amp ??

[@UncleDoug]

Donald, here is my understanding of how the current limiting bulb works: 1.) The resistance of the cold filament is indeed 1.6 ohms but (and this is important) this resistance increases significantly as the filament heats up. 2.) The Photoflood bulb is rated at 250W, which means that the filament does not get hot enough to fully glow until it reaches its power rating of 250W / 120VAC = about 2.1A current flowing through it. At this temperature, the effective resistance of the filament is 120V / 2.1A = 57 Ohms. So the filament resistance varies from 1.6 Ohms cold up to 57 Ohms hot. As more current flows, the filament gets hotter and its resistance increases. This is why the bulb will only draw a maximum of 2.1A even though it is plugged into a receptacle that is capable of passing 30A or more, and this is why the bulb is considered to be a "current limiting" device. 3.) I selected a 250W bulb because virtually all of amps I deal with draw less current than the bulb at full brightness. For example, a Fender Deluxe Reverb draws about 1.5A @ 120VAC = 180W vs 2.1A and 250W for the bulb. The bulb may get a little warm but it should not glow when the DR draws its 1.5A through it. If it does glow, then the amp is drawing too much current and the glowing bulb not only warns me to unplug the amp, but its increasing filament resistance and the fact that the bulb will not pass more than 2.1A (instead of 15A to 30A if the amp were plugged directly into the AC outlet) protects the amp circuit from harm. I hope this makes sense and clarifies the issue. Thanks for your excellent question.

[@daredeviltm159]

Meaning to say when light didnt get bright when i plug the unit is ok?

[@UncleDoug]

Yes, the bulb should be very dim or not lit at all, which indicates the proper current flow.

[@davidfalconer8913]

I like your ( USA ! ) porcelain bulbholder , this ( shape ) model is not available in the UK , but we have brass and plastic types .... stage lighting uses 500 and 1000 Watt bulbs but you need to buy an E40 ( giant ) Edison screw holder ... this device is also useful as a VITAL input ballast to EHT transformers running many types of gas discharge tubes ( like big Neons / mercury vapour tubes etc ... ) ... in addition this device can be used to ( safely ! ) determine the windings of an UNMARKED transforner , accidental connection to a 12 Volt winding just leads to a bright bulb ( as you describe ! ) , I have made lots of these ( 230 Volts types ) over the years , and I hope the two extra uses are of help to you and your viewers ..... ( tried - n - tested ) ...............

[@UncleDoug]

Thanks for your helpful input, David.

[@patprop74]

its that simple? Wow, thank you! your knowledge is a gift that keeps on giving! as far as old device repairing entertainment goes! you and Mr. Carlson are right up at the top!

[@UncleDoug]

Thanks, Pat :)

[@salvadorrodenas3071]

Hi! Greetings from abroad my uncle! This is Sal, I hope you are great! 10 minutes ago I powered my Sony TA-n80es but the amp doesn't turn on. Another amp I have, not under repair, makes the same, nada. I suspect that the bulb light limits too much the voltage across. I used two 40 Watts on each AC mains cables, seeing this didn't work, I removed one and proceeded with one only of 40 Watts. This also didn't work. Maybe if I used a bigger bulb, maybe 80 or 100 Watts it would work. I accept your suggestions...

[@UncleDoug]

Did the bulbs illuminate to full brightness? If so, try a 100W and see how it works.

[@youarethecreator1]

Can anyone help. I was going to buy a variac to bring up a couple vintage guitar tube amps I've had for years. The idea was to try and warm up/re-form the caps etc. But will the variac protect me and the amp the same way as this setup? I'm almost thinking this is better because if I see trouble I can cut the power and go straight into testing the caps and/or replacing them. Also, one of the amps is a 300 Watt bass head. I saw a 500 Watt flood light at home depot. Is that the right kind of bulb? Thanks in advance.

[@GRobLewis]

A head that puts out a legit 300W is going to draw at least 400W from the line (unless it's a newer super-efficient switching mode amplifier). Under those conditions a 500W bulb will use up almost half the line voltage. You could try a 1000W bulb but that's a beast. I'd go with the variac and start low and slow and increase gradually. I'm not an expert on re-forming caps but that sounds like a good idea.

[@steel6322]

Doug, thank you for this video. It is helping me nearly a decade after posting. I have read the description and I still have a few questions about the watt value of the lightbulb. 1. Is a 250 W bulb the recommendation for a low-wattage tube amp (say, 5W amp) as well as a high-wattage tube amp (such as 100W)? 2. Is there a way to estimate or calculate the “ideal” wattage for the lightbulb for any given tube amp? Thanks again.

[@UncleDoug]

I have found that a 250W bulb works fine up to around 50W of amp output power, but for higher-powered amps, you may need additional bulb wattage......i.e. two 200's in parallel. There probably is a mathematical relationship between bulb and amp wattage, but I have not taken the time to calculate one. It's easier to simply use trial and error to find a bulb wattage that allows for proper amp function without noticeable illumination.

[@MrBeblis]

Your knowledge in electronics especially tube amplifiers is outstanding. I wish I could be your assistant you can pay me with knowledge,

[@UncleDoug]

Thanks so much, PB.

[@jamescallaghan1183]

Could I use 2 or 3 100w bulbs in series???...I ask because I don't know where I would get a 250w bulb... Could I use a 250w flood lamp bulb?...it's the only thing I can find...

[@UncleDoug]

Yes, but you must use them in parallel to achieve the lower net resistance found with higher wattage bulbs, like the 250W. Without specs for the flood lamp bulb, I can't judge its applicability.

[@monto39]

Hey Doug - as a former cannabis grower I have an abundance of HID bulbs and receptacles (400w and 1000w). Do you consider 250w to be the ideal for Fender amps (I have a couple ranging 20w up to a Twin Reverb). I didn't really realize these lightbulb concoctions were so useful

[@UncleDoug]

If the bulbs are incandescent, the 250W would be ideal for most medium powered amps. A pair of them in parallel would be good for 100W monster amps.

[@StupidEarthlings]

I see at 2:18 ish youre calling the green (ground) "common".. then proceed to call the white wire "common" again like the beginning.. (just fyi, for those being confused). Green = Ground, White= Common.

[@UncleDoug]

If I did, then it was indeed an error, SE. Thanks for clarifying this.

[@richiemiller5691]

I built the current limiter as Uncle Doug described. It works perfectly. I'm working on fixing a Korg SDD 2000 Sampling Digital Delay. When plugged into a regular outlet, the unit LEDs light up for a split second then go dark. But when I plugged it into the Uncle Doug Current Limiter, the unti worked! The bulb had just a tiny almost imperceptible glow. (It did not glow bright, then dim.) Does anyone have a clue what this means? I mean...the current limiter ITSELF fixed the problem...whatever it is/was. Rather than finding the resistor or ? that is faulty in this pretty complex Korg, I am thinking...why not just keep the current limiter connected to the Korg full time? Would be interested in any thoughts on this matter.

[@UncleDoug]

The current limiter reduces the wall voltage and current a bit.....around 8%. Perhaps the AC voltage entering the Korg is now ideal for its circuit.

[@silasfatchett7380]

Re halogen bulb limiter. I just checked some old voltage charts for a stand-alone preamp. Without limiter B+ was 353V, and without it was 350V. Not much of a voltage drop.

[@UncleDoug]

Surely you realize that voltage drop is directly related to current flow, and the current draw of a stand-alone preamp is but a tiny fraction of that encountered with a full amplifier circuit. I have personally investigated the use of halogen bulbs in current limiters: With a low to medium-power Fender amp, i.e. Princeton Reverb, a 100W halogen bulb can cause -56V of drop, and two 100W bulbs in parallel can cause -18V drop. By comparison a single 200W incandescent bulb caused only -2V of drop. A single 100W halogen is not even usable for a Fender Champ.

[@dale1956ties]

So, with high-wattage incandescent bulbs quickly going the way of the dinosaur I'm wondering if one of those "Y" adapters that turn a single bulb socket into a 2-bulb socket would work if you need a 200-watt bulb but can only put your hands on 100-watt bulbs. The workaround then is to install the "Y" adapter and screw a 100-watt bulb into each arm of the "Y" giving you a total of 200-watts. Will it work? Or did I just come up with an even better way to blow myself up?

[@UncleDoug]

As long as the bulbs are in parallel, which seems quite likely, it would work just fine, DE. You could check by seeing if the overall filament resistance dropped to 1/2 when the second bulb was screwed in. This would make the wattage capacity of your CL adjustable.......good idea :)

[@anthonysilva5312]

If you can’t build this, you shouldn’t be messing around in a tube amp!!

[@UncleDoug]

Agreed, Anthony. Well said.

[@velimattiollilainen]

What about if you connect this current limitert to an isolation transformer which output has only live and neutral (common) wires

[@UncleDoug]

Instead, connect the IT to the CL......problem solved :)