TAG Lightbulb Current Limiter

[martin manning]

A lightbulb current limiter is an invaluable piece of test gear for first start-up and troubleshooting. It helps prevent damage caused by wiring errors or failed components.

The one described here has a history on TAG. The first version was constructed by member rp and then passed on to me. It progressed from the simple on-off with the bulb in series to the Version III which includes two bulbs, a bypass, and a self contained current and voltage readout. The original thread describing the limiter below, as well as units built by other members, was accidentally lost. Feel free to post or repost your versions.

Notes:
1) Incandescent bulbs are getting hard to find, but they are still being made for specialty applications like oven bulbs.
2) Bulbs with a clear glass envelope are preferred so the filament can be observed at very low brightness, just as it begins to turn orange.
3) Bulb wattage should roughly equal to the amp's power output. The version with two 60W bulbs for a selectable 60 or 120W, has worked well for me.

You tube video of Bulb Limiter III in operation: https://www.youtube.com/shorts/joEk_DKBOK0

A very detailed explanation of lightbulb limiter operation is here: https://www.youtube.com/watch?v=K9_MR7L0hrw

[Stevem]

Thanks!

[sluckey]

This is my simple LBL.

[timrobertson100]

Mine, inspired by Martin’s with a similar current sensor sourced from Amazon. IEC in and out on the back.

IMG_1369.jpeg

[1992guitars]

Finally built this. Thanks for answering my questions several months ago on the now gone thread. It helped a ton! I went a little overboard and added a master switch, breakout box with two gangs, pair of DMMs, one with current meter leads wired permanently in series with the hot so I would never have to probe a live circuit. Then inspired by the advanced transformer test RG Keen wrote about on Geofex years ago, I wanted a way to check transformers (even though I previously had put together an NE2 bulb rig which I used to check all the transformers in the Twin Reverb I'm about to begin rebuilding; the Geofex articles had lingered and I'd wanted a safe(r) way to run these tests since seriously starting to study the site at least 2 or 3 years ago). Transformer leads get secured to the strip with the screws, a simple jumper cord plugs into the DUT receptacle that also gets screwed down to the terminal strip.The low turns side comes from from the receptacle and the high turns side goes to an octal socket so that there no flying leads and the probes for voltage are always taken through an insulated socket on either winding. No slip of the probes arcing across a terminal strip. Of course to keep things as safe as possible ("don't be remembered as a great amp tech back when you were alive" - RG Keen) the strip is never connected unless I deliberately plug in the jumper cable and connect (which I would only rarely ever even need to do, but makes possible).

The bulb limiter is verbatim to Martin's design with a High/Low switch as well as Bypass/Limited selection. Got a stash of 40W incandescent bulbs from Home Depot, had a couple 25W incandescents around, and my Dad had ordered me a box of 75W bulbs a few months ago. After testing all grounds for 0-ohms (or well under 1) from the wall to all metal boxes and being OCD with continuity tests confirming no shorts or fire were about to happen, I got the nerve to plug it in and began testing the breakout box with some low wattage bulbs as my test load, watching the meters in Limited vs. Low vs. High vs. Bypass mode. Finally yesterday I tested out the terminal strip/octal socket/transformer testing side using a disposable transformer I care absolutely nothing about that was removed from a Goodwill cassette deck 18 months ago when I was practicing desoldering/component removal. All is working well.

[martin manning]

Sounds pretty fancy. Can you post a photo?

[1992guitars]

Sounds pretty fancy. Can you post a photo?

Of course. First things first though, upon finishing this I was honestly a little bit embarrassed (which also explains the 10 day lapse responding here). So I probably would not have elected to post photos without having been asked. This took me about 3 weeks and will have to be posted in two replies (provided Amp Garage will let me include a number of photos with each reply). Hopefully I have corrected all typos. So remember, I'm still slightly embarrassed about just how overboard I went.

After your reply about the LBL 4-6 months ago and studying it a lot, once I'd made sense of it I knew I could build that. However, my Achilles heel is a pattern in life where I get into something and then it just becomes "the thing" to me for awhile, and that extended to what I wanted expanding to include: I went from just building a light bulb limiter to wanting:

(A) Your switchable LBL design, combined with:
(B) A means to run the "Advanced Tests" here that I had become captivated by when first reading a ton of Geofex 2 or 3 years ago (http://www.geofex.com/ampdbug/outtrans.htm)

(Yes I could just run the NE2 bulb test, and also built a much smaller and simpler box for that---but.......remember the Achilles heel mentioned above. I probably get too attached sometimes to old goals and things I thought were cool and wanted to "eventually" do).

I sized the wood base (which I had to hand cut in my garage since 30+ years being a guitar player = not allowed to use power tools) for my specific workbench, so I knew I could get away with a larger footprint before beginning. Obviously the panel meters used in your design are better in a lot of ways (small footprint, all integrated in one panel, less expensive, all of it). But (A) the old Craftsman DMM my guitar player mentor gave me over 20 years ago teaching me about multimeters had gone out of calibration very badly and was toast, (B) the cheaper Extech meter my Dad had given me for Christmas one year was equally toast, (C) With two meters deserving of the trash pile, I didn't I didn't mind spending some money on new multimeters "good enough" for this semi-permanent setup, keeping my cherished Fluke 179 that I bought quite a few years ago now free for when it merits using the Fluke (I've calibrated 1176 builds with it).

(Edit/Update: I want/need to mention at this point that a conscious decision was made to hardwire the current meter, both to eliminate user error, and because it seemed far safer to do it this way than to have two sets of probes that I had to keep track of which goes where. I crimped all connections and must have gone through 100 crimps at least practicing before I was certain I could do this with absolutely no doubt my connections were secure. I practiced crimping on wire and found what pulled out and what did not. So the current meter probe tips were cut off, the other end was crimped to the broken hot between the two gangs, and the meter goes in series. Also, the first photo below does not show the labels that were later placed, but all meters ARE labeled).

Yet......

When "the first draft" I was finished, the exposed terminal strip made me (justifiably) nervous. Even though I'd placed it all the way at the back, I just didn't like it out in the open. In fact, there was a lot I didn't like.

Version-1.jpg

Having watched the LBL video you posted with the demo, I had always wanted (and thought I had planned on) a designer switch would be nicer in the event of having to shut the power rapidly. It's the switch I had always wanted as a master power switch but when I went to fit it to the single gang box (having neglected to check the fit ahead of time, which was a mistake), the ears wouldn't line up. Then I stripped the yoke threads trying. So that one was toast and I needed a new switch.

I had not yet figured out that the designer switch needed the outdoor box. Yet I wasn't ready (yet) to pull the box entirely and redo the wiring, so I thought I was going to just settle on a switch/light combo instead, which would fit fine:

switch-light-version-1.jpg

For something to connect transformers up to, the first idea was to just connect the terminal strip up using a two-conductor lamp cord terminated in spade connectors.

lamp-cord-version-1.jpg

Much more importantly than not liking my master switch and being disappointed the designer switch didn't fit the box I'd committed to, covering the terminal strip took highest priority. Because I'm not normal and disassemble things for fun without throwing parts out, I found a plastic base that had housed a circuit board. The main portion was the perfect length to cover the terminal strip, so I cut off the one ear that was too long with a hacksaw, got some standoffs/coupling nuts from Ace, and drilled the plastic to form a cover:

Adding-Cover-To-Strip.jpg

Then I wasn't about to freely probe a terminal strip and just didn't want to order spade connector attachments for my DMM leads to screw them down to the strip like in the video. So instead, I used an octal socket that didn't fit the Twin Reverb chassis I'm re-building (ordered the wrong parts 18 months ago). Because I had extra sockets, I just took one and mounted it in a single gang box with a blank cover plate. I figured that way I could make the connections, everything is insulated, and the entire procedure is hands-free.

Adding-Octal-Socket.jpg

However, there were still several things I just did not like. I could have only stopped if "done" had been more important than getting what I wanted. Also, I was thinking about the quote from the Guitar Amp Wiring guide (I read all 45 pages!) - "Don't be remembered as a great amp tech back when you were alive." So I was pretty obsessive about safety grounds, doing everything with star washers, ground lug, star washer again, nut tightened down....

And when I had installed that octal socket I thought, great, now if a live wire touches the metal it's live.

But preferring for a transformer to die instead of me if the unthinkable were to occur, my initial solution was too hokey and clumsy too live with for more than a couple days. I (temporarily) installed a 3-prong cable to the box with the hot and neutral entirely insulated away using heatshrink and flush cutters, only using the safety ground. The extra cord was big and bulky. Everything felt like a hack job. I learned that all the planning I thought I had done hadn't been complete enough because I was constantly making changes and more or less feeling this out as I went along. I was about to redo everything.

[1992guitars]

First, I ran a dedicated ground from the octal box over to the main earth ground at the entry point.

Octal-Socket-Cover-on-V.-1.jpg

This might have been a logical time to stop but instead it became a gateway to just get what I wanted from the start. I still wanted that nice designer switch, and as I looked at the lamp cord and setup I had, I thought I could do better. So the project expanded yet again. My goals now extended to:

(C) Add a switch to turn the terminal strip on/off. Get rid of the lamp cord, which didn't seem elegant or even as safe as having everything permanently wired and switchable so that "off" is truly off. (I also got a switch guard not pictured. Terminal strip covered, insulated, switch off, guard on -- now I was feeling better about things).

(E) Add another meter to monitor transformer secondary voltages in real-time alongside the primary voltage and current measurements.


So I pulled everything apart again, got an outdoor box, confirmed it would fit the designer switch, and got going.

new-box-for-designer-switch-redo-grounds.jpg

Confirmed the fit and that my plan was going to work this time:

new-switch-installed-(master).jpg

Added a single gang box for a Double Pole switch that would switch the hot and neutral to the terminal strip:

adding-box-for-Double-Pole-switch.jpg

Made sure the wiring into the octal box had an insulated strain relief

strain-relief-into-octal-box.jpg

Finally finished everything up about 3 weeks after beginning:

final-version.jpg

Tested the bulb limiter with various other bulbs used as a dummy load in the DUT socket. Nothing blew up and no meters were killed. I also placed the literal words "No!" with adhesive over the banana connections into the DMMs making it as impossible as possible to connect to the wrong jacks, and the current meter (again) was hardwired into the receptacle box, with the hot going in series through the meter. I thought a long time about hardwiring the voltmeter but decided against it once and for all realizing that if I then went to change out a battery and reconnected cables, then I'd have too many leads creating the very real and present risk of plugging one into the wrong spot and blowing something up. Keeping the only one hardwired as the current meter meant that's the only one that ever goes to the current meter, which was labeled. And again "No!" is taped over the jacks to never connect up to.

Used a power transformer from a consumer grade cheapo cassette deck I disassembled at least a full year ago since if something bad happened, it was a disposable transformer I didn't care about to begin with. All worked well though and the bulb limiter was there when I needed it (experimenting with 7.5 watt night light bulbs up to 15W since I knew it wasn't going to draw much current. Obviously for amps I'll use the 40W - 60W bulbs in parallel).

(distinction: in this cheapo cassette deck transformer case for a test subject alone, I knew it was a 120 - 30v 4:1 step down. Because of that I gave the low turns 30v and looked for 120v on the primary. Mentioning that so the picture does not trigger any alarm. Obviously with an output transformer I would follow Mr. Keen's instructions to the letter and limit the voltage to 3.15vAC).

testing-cassette-deck-transformer.jpg

So that's what I did this September. Again, total and complete overkill, but it was fun at least.

[Jennings]

Talking of overkill, here's mine. I made myself a blub limiter, but got bored of having to hook up other things, like an isolation transformer or variac etc. So this thing has all those things together, and selectable via the switches. Old direct to mains valve radio you're testing after a cap job? No problem...select isolation transformer, bulb limiter and variac to slowly bring up whilst checking for excess current draw whilst separated from mains supply. And so on. Use it all the time...weights a bit though, with the two transformers and heavy duty case, thus the handles.