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/Eiko-Supreme-Ph...
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!

[@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! :-)

[@UncleDoug]

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

[@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.

[@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.

[@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.

[@onemancarnage]

Can't believe I haven't watched this video sooner. I can easily see how important this limiter is to working with electronics . Thanks so much for posting this . :)

[@UncleDoug]

You're welcome, Bryan. It has saved me from damaging components (and myself) more times than I care to admit, over many years. You really need to build one and use it consistently.

[@covingtonpeoples]

Hey Uncle Doug! Thank you for everything you and your various assistants have done for the guitar/amp/electronics/hot rod communities - your contributions are priceless. I just finished my first current limiter, following your video. Keep up the good work!

[@UncleDoug]

Thanks so much, CP. Will do :)

[@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 ?

[@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).

[@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.

[@td7456]

Uncle Doug, thanks for all of your wonderful, informative videos! Appreciate your style! I tinker with guitar amps & old AA5 transformerless radios and think this will be a great addition to the workbench along with my isolation transformer & variac! Thanks again for all you do with the vintage electronics videos!

[@UncleDoug]

You're quite welcome, Tim. I'm glad to hear that the videos are informative and helpful. Best of luck with all your vintage electronics projects :)

[@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]

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

[@fyrcrack1]

Thank you Uncle Doug. I have been using one of these for years.

[@stephenbunton2159]

Another one of your videos that has inspired me to make something. I have had the parts for one for quite a while but with my year and a half old daughter keeping me busy most of the time, I have finally gotten around to making one. Thank you for all you do for the tube amp world! You are truly an inspiration to thousands of people including myself. I want to be just like you when I grow up.

[@UncleDoug]

You're quite welcome, Stephen. Thanks for your very nice comments :)

[@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.

[@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?

[@toysmitty]

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

[@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 :)

[@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.

[@treadmillrepair754]

Excellent way to protect the electronics under test. I use this every day for test amplifiers, power inverters and other things. Cheers.

[@UncleDoug]

You're right, Ricardo. It's one of the most useful of all tools for amp test and repair.

[@evahle]

Built one of those many years ago. It works great! Thanks for the refresher. Nice video!

[@UncleDoug]

You're welcome.....Glad you liked the video :)

[@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 :)

[@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!!!

[@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..........

[@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

[@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.

[@wesrichards6168]

Excellent Doug! Thank you.

[@mickgentry8128]

Great videos Doug! I enjoy learning from them.I reckon this is a must have in any electronics workshop!Thank you for all your hard work, all the best Mick

[@UncleDoug]

You're welcome, Mick. Yes, the current limiter is indeed an essential tool in any electronic workshop.

[@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.

[@NETBotic]

nice!

[@UncleDoug]

Thanks, Jon.

[@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.

[@johnmarkelewicz5495]

Hi Doug, thanks for the video. I was so impressed, and not to mention how valuable this device is that I built one for myself. Such a great simple unit to make yet so necessary when working with electronic equipment. I look forward for more of your videos. God Bless!

[@UncleDoug]

You're welcome, John. Yes, it is a critical piece of equipment.....simple, inexpensive, and absolutely essential. Likewise on the blessing.

[@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.

[@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.

[@60retired]

Uncle Doug, thank you for all these great informative videos you have done. I have learned so much from you and enjoy doing my own servicing on my vintage Fender amps. Thanks

[@UncleDoug]

You're welcome, 60R. We're really glad that you are utilizing what you gained from the videos in a creative way.

[@klauspopp3788]

Thank you Uncle Doug and Jack for this perfect manual reg. safety ! Especially thank you for the additional note ref.users of 220 Volt, as we get supplied in Germany.

[@UncleDoug]

You're quite welcome, Klaus. I'm glad the video was helpful.

[@josipzlk8019]

or use more blubs in paralel

[@UncleDoug]

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

[@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".

[@TrackingWidgets]

Going to build one this week! Fabulous videos Doug!!

[@horsthartung4682]

Just finished building your current limiter using a 250watt bulb and tested it using a shorted plug end and it works great. Thanks again for your wonderful videos and knowledge.....always a real pleasure watching and learning....

[@UncleDoug]

Congratulations on a successful project, Horst. You're welcome :)

[@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 :-)

[@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!]

[@mpccenturion]

Thank you! UD. I have some ancient equipment at the summer place. Radios and amps of 70 yrs ago. Cheers to you!

[@UncleDoug]

Tell me where the summer place is and I'll relieve you of the responsibility of ownership, CW ;) Likewise to you all.

[@mrGoesto11]

I've also heard this called a "Dim bulb tester' by the old timers. Great explanation, clear, sharp video. Thanks.

[@UncleDoug]

You're welcome, Mr. G. I'm glad you liked the video :)

[@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.

[@LFOVCF]

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

[@UncleDoug]

Better late than never, I guess :)

[@dogbarley7178]

Hi Uncle Doug, I finally got around to making one of these. Works great! Thank you again for all your excellent teaching on amps, etc. Sincerely, DB

[@UncleDoug]

You're welcome, DB. It will become one of your most useful tools.

[@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.

[@vincet68]

What a great tutorial, Doug. Thank you for the continuing education!

[@UncleDoug]

You're welcome, Vincent :)

[@godfreydesilva5222]

Some of the technics are known to us, but they go ignored and face risk and Damage to equipment. This video opens the mind and shows how simple to understand and build your own. Great Job Doug. ( I am from the UK and much similar, except the working mains Voltage.)

[@UncleDoug]

Thanks, Godfrey. I'm glad the video was informative.

[@donaldthompson4857]

didn't realize how important this was, till I made one.Great video uncle Doug

[@UncleDoug]

Donald Thompson Thanks, Donald, and may your bulb never glow brightly :)

[@stevenwozniak8365]

Thanks again for sharing your knowledge. You do a great job explaining the circuits and electrical diagrams in your videos. Your efforts serve as an easy to follow introduction into electrical engineering - much appreciated!

[@UncleDoug]

You're welcome, Steve. We're glad the videos are helpful.

[@MrJoeydano]

What a great but simple way to protect your equipment 👍 Thanks Uncle Doug !

[@UncleDoug]

You are very welcome, Doug.

[@donaldwallace7934]

I just made one of these. I did add a switch in order to turn it on and off. A very simple, and great piece of safety equipment!

[@UncleDoug]

I'm glad you made one, Donald, but wonder why you added the switch. It's a continuous monitoring device and a switch sort of defeats its purpose......but no matter what, I think you'll find it to be a very useful device.

[@BoydWaters]

Such a great walk-through of a simple and vital piece of test equipment. Thanks!

[@UncleDoug]

I'm glad it was helpful, Boyd.

[@DawidCiecierski]

Excellent explanation that is so much more useful than dozens of other videos that only show you how to make one. Thank you for taking the time to prepare, record and share.

[@UncleDoug]

You're welcome, Dawid. Thanks for your nice comments.

[@georgechambless2719]

Good old 1930s technology. So many of these old safety methods still work on modern equipment and they don't have to be fancy or expensive.Thank you for sharing this!

[@UncleDoug]

George Chambless You're welcome, George. Sometimes you just can't beat good, old-fashioned, inexpensive techniques :)

[@aftershock2222]

I’m getting into vintage tube radio repair. Thanks for the great vid.

[@UncleDoug]

You're welcome, Bennie. Good luck with your new career.

[@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 :)

[@THUNDERWORX]

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

[@UncleDoug]

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

[@Finom1]

Your a Genius Uncle Doug!!! Thank you for teaching me.

[@UncleDoug]

+Finom1 You're welcome, Finom......and you're too kind. It's actually Rusty who writes all the videos :)

[@johngeddes7894]

Uncle Doug- It’s amazing how deep some will dive when a discussion of building a simple test tool is shown. Very inspiring to get the concept and its application. As a byproduct, I was forced to learn with the aid of yt videos how to solder by learning what makes a good solder joint. That and having taken my backyard welding up a few notches in industrial machine repair. All I need is a lathe, a surface grinder, Bridgeport milling machine and a 6X48 belt sander to round out shop equipment! Or maybe just bend sheetmetal with angle iron and C clamps!

[@UncleDoug]

As Archimedes once said, "Give me some angle iron and C-clamps, and I can make an amp chassis".

[@ALtheDoctorWho]

Great video Doug You were very thorough.

[@Valerifon1]

This is outstanding! I have gained such an education watching your videos! Thank you so much! I'm definitely putting one of these on my bench! 😉👍

[@UncleDoug]

You're welcome, V. It's my most valuable diagnostic device.....simply because it protects each circuit from self-destruction during the repair process.

[@guitarmdpittsburgh7139]

One of the first tricks I learned for tube amp servicing. I loved to show this gadget to my clients, and watch their jaws drop. It's so low tech, and it really works.

[@UncleDoug]

Yes, it does, GMD. Thanks for your input.

[@viol999]

I found your channel about a year ago and immediately subscribed even though I posses only a very basic level of electronics. Anyhow, it's given me the bug and I just picked up my first vintage amp. Actually It's a circa 1960 console radio with a turntable. It has a Sherwood radio and (mono I think)s-1000 amp combo with a Thorens td-124 turntable which is what I was really after. I will use this video of yours to build a current limiter to power it up when I am ready to test it. Thank you so much for your wonderful video's over the past year. You are a true maestro when it comes to explaining electronics to a noob like me!

[@UncleDoug]

You're welcome, Viol, and best of luck with your radio amp conversion. Be sure to let us know how it turns out.

[@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.

[@doctorbo5810]

Hi Uncle Doug based on your video I just built my own current limiter for my hobby lab. A few weeks ago I had damaged an older amplifier due to a short circuit caused by me. With the current limiter the damage would have been less likely :) Great thing!!!

[@UncleDoug]

Congratulations on a successful build, Dr. B. Your amp circuits will be safe now :)

[@filmmac3]

I don't know what I'd do without your videos Uncle Doug

[@UncleDoug]

We feel the same about our viewers, F3 :)

[@SaccoBelmonte]

I'm not an amp technician and I think that is a darn good tip! Thank you!

[@UncleDoug]

You're welcome, Sacco :)

[@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.

[@Elecifun]

I remember on my work experience from school at an electrical repair shop, there was a lamp with the exact same purpose. I use one in my workshop still today they are great.

[@UncleDoug]

MonkeyMagic Elecifun Thanks for your input, MM. Once a person starts using a current limiter, they'll quickly see just how helpful it can be.

[@boonedockjourneyman7979]

This such an important topic that it is really good to see many different people from various parts of the EE world do videos. Different communities need different presentation. Great job.

[@UncleDoug]

Thanks so much, BJ.

[@jegr3398]

I just built one of these a couple days ago to fire up the Marshall build that I've been putting together. It worked like a charm. Thanks again UD.

[@UncleDoug]

Glad to hear it, JG :)

[@richardjeffery5594]

Simple genius. Thank you.

[@mickjagger8904]

Thank you for this video Uncle Doug! I'm considering building an amp kit, and knew from your videos that a current limiter is a necessity.

[@UncleDoug]

It is indeed, Mick.....absolutely essential. Good luck with your project.

[@larrygenedavis]

I plan on using this along with an isolation transformer. I like the fact that a light goes on to warn me the moment a short is happening. Also I remember , years ago , a mechanic using something like this to find shorts in a starter armature . Good video . Already subscribed .

[@UncleDoug]

Thanks, Larry. It's a wonderful, inexpensive, foolproof, reliable tool. Best of luck with it and your projects.

[@SteverRob]

Excellent video. I've built one of these several years back and used it along w/a Variac to slowly pull up an old ham radio rig (Collins). Very useful tool, and I'd never bring up anything old that's not been powered up in years without a DBT. Thanks again Oh yeah, my Dad (now 81) taught me how to solder my first project when I was 8. And he always told me to keep one hand in my pocket when working around lethal voltages. :) An old-school habit.

[@UncleDoug]

Thanks, SR. It sounds like your dad was a good source for electronics advice :)

[@billyorston1]

Just wanted to say thanks for this, not only helped me while building first set amp, also referred an online friend that was having issues, anything new or new to me goes through this first!

[@UncleDoug]

You're welcome, William. Some day, you will be very glad you have it, when it protects a circuit from harm.

[@billyorston1]

@@UncleDoug Using it religiously with new builds, even sent a friend that was having issues with a new build to your link and we got it sorted out, again many Thanks! Do you have a shop in El Paso? I go there lots, moved from there to Ruidoso but would love to meet you in person some day.

[@joeyboyd5860]

I am a 62 yr old man and have always been into collecting and playing stringed music instruments, and just about anything having to do with amps, stereos, and basic electronics, but I could only do small basic repairs, honestly I was scared to death to work on anything that produced high voltage. Since discovering your channel and a few others on you tube, it has helped me to understand so many things I never knew before. I have several old amps that I have wanted to fix, but was never comfortable till now. One of my amps is a Gibson GA35 RVT that one day just fizzled out and stopped working. It's a shame but I really was afraid to work on it other than checking fuses and tubes, so it's been in one of my closets since about 1979. Thanks to your excellent and down to earth teaching techniques, I feel more confident and somewhat more knowledgeable now (thank you) so I'm gonna attempt to troubleshoot it and hopefully get it working again, and maybe even attempt to do some tone mods also.

[@UncleDoug]

We're glad to hear that the videos are helpful and informative, ir. Best of luck with your repairs.

[@silasfatchett7380]

Don't forget that a light bulb limiter protects the device you're working on, IT DOESN'T PROTECT YOU!

[@charlesgwyn2115]

Thanks for the video - built one last week and it works great!

[@UncleDoug]

That's good news Charles.....it's good to know that both you and the amp will be safer during repairs, and good to see that you have turned an abstract video into a concrete (and very practical) piece of equipment. Best of luck !!!

[@4hammonds672]

thanks uncle Doug i am currently watching multiple times your other set of videos on how to bias a single ended and double ended amplifiers. they are very interesting, thanks for sharing your knowledge.

[@UncleDoug]

You're welcome, 4H :)

[@trinita.62]

Hello, I'm Italian and I'm part of Rome dell'A.I.R.E. (Italian Association for Radio valves) and I have to get you many compliments !!! would be nice if even here ', there were competent people like you !!!!

[@UncleDoug]

Wow, thanks so much GF. Rusty and I really appreciate your very nice comments :)

[@WastelandSeven]

Very cool piece of equipment! Thanks for sharing!

[@UncleDoug]

***** You're welcome, W7. Glad it was helpful.

[@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..

[@danielgoodson703]

Nice to spend time on KZbin that enriches and interests. Thank you for excellent video to watch.

[@UncleDoug]

Glad you enjoyed it, Daniel.

[@ozewiespekel]

Hey Uncle Doug. I've been learning so much from your amp video's. Proves even more that (no offence), you have a lifetime of experience that cannot be described in a book. I like your clear explanation in the video's and it all truelly makes sense. Just as a little addition to this video of the current limiting device. In Europe the "hot" wire, also known as Line (L) or Fase is BROWN. The common wire, also known as Zero or "0" is blue. And our ground wire is Green/Yellow. Just to make sure all the European fans of your video's can keep watching them. Kind regards from the Netherlands (ofcourse, also to the misses and pets).

[@UncleDoug]

Thanks so much, Erik. We appreciate your nice comments and your helpful input :)

[@stephenyoud6125]

in the UK, we call the blue Neutral, and i think the brown is called LIVE, and now i also find out the blue neutral is what the COMMON is in the US. the jigsaw puzzle becomes more complete

[@cristianosar]

I already have make mine before building my amp. Thanks for the precious info.

[@UncleDoug]

You're welcome, Cris. Use it in good health :)

[@anthonyvee]

Thank you Doug. As always, beautifully explained. Stay well. I will make one.

[@UncleDoug]

You'll be glad you did, Anthony. You're welcome :)

[@anthonyvee]

@@UncleDoug yes sir. How’s the work with the vehicles going? Primarily that manifold/ carb swap?

[@jukeboxheros7021]

I like this, I'm going to build one to plug in my 63 Princeton amp we've been talking about. Thanks

[@UncleDoug]

You'll be glad you did, JBH. It will protect the circuit from self-destruction due to internal short circuits.

[@rogeliooviedo5985]

One of the most stunning and incredible tools that I have seen in my life! Muchas gracias Uncle Doug. greetings from 🇲🇽 city

[@UncleDoug]

You're quite welcome, MO......I hope it serves you well.

[@luthiervandros]

Don’t let you kid brag about his joints in front of mom.

[@spazimdam]

Great idea. Cheap too. All these years I have worked on tube amps with no such current limiting device. Been lucky so far I guess! But now I will build one of these. Thanks!

[@UncleDoug]

You're welcome, Monte. It will prove to be a very valuable safety feature for your workbench.

[@donaldfilbert4832]

Thanks a ton for the great explanation Doug !! Funny - I never even considered that the resistance of a light bulb changes with heat. And - as many videos and articles I've reviewed about the dim bulb tester - none have ever mentioned this fact; even tho the actual workability of the dim bulb tester is completely dependent upon this fact! Just when it needs it (during an equipment failure) - the bulb resistance increases dramatically and automatically to limit current and protect the equipment. Now I know !!!

[@UncleDoug]

You're welcome, Donald. You're right about the general omission of exactly how the bulb current limiter works. I will add a note to the video explaining this phenomenon. Thanks for bringing this matter to "light" :))

[@alvagoldbook2]

Another great video, and a great safety video to boot. I really need to make myself one of these.

[@UncleDoug]

If you are going to work on virtually any electrical device, except certain motors (which won't work well with a current limiter), it can be a real life-saver. If you have trouble finding a high-wattage incandescent bulb, they are available on Amazon. Good luck, Alva.

[@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 :)

[@eddypagan8675]

Outstanding, All technicians that make repair for a living "should" have this in there shop

[@UncleDoug]

Agreed, Eddy. Thanks !!!