Model T coil rebuilders will soon be seeing different replacement capacitors sold by some Model T parts vendors.
Before explaining this change I would like to clarify some commonly misunderstood technical terms and how they relate to selection of the correct replacement capacitors used in Model T ignition coils.
The two most common capacitor characteristics that apply to the coil circuit are capacitance, which is expressed in Farad’s (or a portion thereof) and Working Voltage, which is the maximum steady state DC voltage rating of the component. These two ratings are commonly marked on the body of the capacitor in one form or another. For example: “.47K” and “400V”.
There are many other technical characteristics of capacitors. These characteristics are of interest to circuit designers who need to ensure the selected component meets all the requirements of the circuit. I will not try to discuss all these technical characteristics here, but focus upon one characteristic which is of significant importance in the Model T ignition coil circuit.
During normal operation the Model T ignition coil has narrow voltage spikes that occur in the circuit. The capability of a capacitor to withstand these spikes is referred to as the dV/dT rating and is commonly expressed in volts (V) per microsecond (uSec). The letter ”d” in the equation is rate of change, the letter “V” is voltage and the letter “T” is time. As you can see from this equation, dV/dT rating is a specification of rate of voltage change over rate of time change. Many have confused Working Voltage (WV) with dV/dT voltage ratings and, as you can see, these are two significantly different capacitor characteristics.
The Model T coil circuit requires a capacitor of .47uF, 400WV Film Foil type internal construction with a sufficiently high dV/dT rating to withstand the voltage spikes that normally occur. The capacitor we have been recommending till now (“A” in the photo) has a dV/dT rating of 700V/uSec. In over twenty years of coil rebuilding I have never seen one of these capacitors fail in service.
Now about the new capacitor: (“B” in the photo). A source has been located for a .47uF, 400WV film foil type construction capacitor with a dV/dT rating of 1700v/uSec, it is somewhat less expensive and it has the added benefit of being manufactured in a pressed format which makes it easier to install. Last year, samples of the new capacitors were used to build a set of coils and loaned to a fellow who drives his Model T a lot. The car was driven normally for several months. These Beta test coils worked exactly as anticipated. After this test a closer examination of the capacitors revealed no measurable deterioration.
Most of you have seen the effect of using commonly available metalized Mylar type construction capacitors (“C” in the photo) in Model T coils and how they last a very short period of time. These capacitors have a WV rating of 630WV, but a very low (30-60volts) dV/dT rating. Because this rating is exceeded during normal Model T ignition coil operation it ultimately destroys the capacitor’s dielectric and the coil very quickly ceases to function correctly.
Capacitor “A” in the photo is commonly called an “Orange Drop”, but that term is commonly misunderstood. “Orange Drop” is a trade name and not an electrical characteristic. In fact the company who uses that trade name makes many different types of “Orange Drop” capacitors each with characteristics suitable for different circuit applications but only one suitable for application to the Model T coil.
The new capacitor we are recommending (“B” in the photos) is now available from Lang’s Old Car Parts and FunProjects.
Ron the Coilman
Ron: Thanks for the update on the capacitor change and availability. I had to dust of long ago forgotten knowledge, learned as a much younger lad doing ham radio work, to follow the physics. JP
Ron, could you translate that to medical or Latin so I can understand it? Once again, thank you for your assistance on keeping our 90 plus year old cars running. Since the last 8 coils I've gotten from you are still working well, I won't need another set until I acquire another T, or make one from my parts stash!
Thanks, for the update Ron. I've followed your coil building advice for several years with good success. In fact, after over 1100 units rebuilt for customers, I've only had one capacitor fail. That was on the one coil that I rebuilt with a capacitor like the one shown in the upper right of your photo. I was rebuilding a batch and had one more coil than orange drop capacitors on hand. But, I had a bag of those crummy ones sitting on my shelf that had come from Mac's before I learned not to order supplies from them, so I used one. About a two years later, I had to warranty that coil as the capacitor had failed. So, there you have it 1100 orange drops in service without a failure compared to one failure for one use with the brand X capacitors. It pays to use the right parts.
Although I could barely follow what you were saying, thank you for keeping us up to date. I'm pretty sure I'll never be rebuilding any coils myself. That's why it's so nice to have someone like you we can depend on to keep up with improvements and keep our coils in tip-top shape. Thank you again for what you do for our hobby!
Bob (still running Coilman coils!)
Thank you so much for all your hard work at keeping the coils on our cars working as originally designed. And for giving us a heads up that the "B" style capacitor is also correct and will be showing up soon.
Hap l9l5 cut off
Just to be clear.
This has been a two year project to define requirements, identify a suitable source, test samples to finally make these new capacitors available.
Several people who participate on this forum have contributed significantly to this project.
I am just lucky to also have been the scribe.
Ron the Coilman
Polypropylene capacitors are the best you can get for the Model T coil application. While a DV/DT of 1700 is more than adequate and more than the old orange drop capacitors our capacitor is a polypropylene 0.47uf, 400vdc with DV/DT of 6000. We are looking for a replacement as these are no longer available. However, we were smart enough to buy 1000 pce and still have a large stock on hand. Price $2.50 ea.
There doesn't seem to be a reason to pay a premium for a capacitor with capabilities not needed or required.
Why kill cockroaches with a gold plated sledge hammer?
A gold plated hammer will dazzle the roach,freezeing it in it's tracks and make it easyer to hit! :>)
Thanks for the information Ron.
I use the orange capacitors in old radios and ofter wondered if they would work in coils,but never tried 1 and glad now I didnt.
I took 1 apart 1 time before I learned,just send them to you,and it had a briggs and straton capacitor in it from a 5s or model 8 , or similar vintage engine.
I have often wondered,if a T coil had been put in service in 1920 say,and was used regularly,how long would it hold up without a capacitor being replaced? The tech back then was primitive to what it is now.
Antique Electronic Supply is where i git my capacitors for old radios and tubes.They cater more to the music amplifier crowd but sell parts that fit old radios.
I think it is Www.tubesandmore.com or something similar.
they have a good selection of capacitors of different grades and prices.
Thanks Ron for the update! Nice that we can still get capacitors of the right capacity and capability :-)
Any europeans listning to this channel I will just (mis)use my bandwith here to inform that I do rebuilding of T-coils within the EU using the capacitors recommended by Ron.
Mack, Old time capacitors and other electrical radio components were manufactured with superior standards than todays products. Most of the old radio components other than tubes never failed. The model T capacitor was a metal foil capacitor with high quality insulation. They seldom ever fail except for a mechanical failure where the end terminations become loose due to years of vibration in a Model T Ford. I have never seen one that was shorted. Todays polypropolyene capacitors are a good substitute. Both the one we sell and the one Ron uses are more than adequate for the application and both are competativelly priced.
To me, electricity and magic are the same thing. Everyone agrees that one of the things that enabled the Model T to survive so well was its simplicity of repair. My Flivver has taught me to perform maintenance such as I'd never thought I could tackle, yet there remains a world of arcane skills like pouring babbitt and building ignition coils which remains the exclusive domain of wizards. I sure am grateful there are learned fellows out there who can read the theoretical hieroglyphics and ship Tesla's physics in a cardboard FedEx box so I can slit the bubble-wrap, slide the part into the slot and—wonder of wonders; it works!
Michael: One European listening here! However I am known to preform the same service as you do from time to time. That beeng whenever I have time away from work, house building, wife and 3 kids!
Thanks for the update! I will start ordering the new style when my stock is consumed. A narrower fit will reduce the digging work witch is fine by me. We appreciate the reserch you have done and the desire to share the correct information with ous!
Arriving in Houston tomorrow, going home on Friday!
I agree, I think the flattened type will fit much better. Looking forward to seeing these.
My previous post + your post = "Lightbulb"
Thankyou very much,
Aside from being less tar to dig out, I like the idea that the tar digging tools will be just that much further away from accidentally stabbing the outer part of the coil winding. I am sure many a coil rebuilder has known the heartache of having a coil chamber almost cleaned out for the new capacitor only to have an errant move of the tar removing tool hit the outer part of the coil winding and all is then lost. The less tar to be removed the safer it is for the health of the coil windings.
Hard to tell from the picture, but is there any chance it will fit without removing the glass?
In most cases, YES.
"Old time capacitors and other electrical radio components were manufactured with superior standards than todays products"
That's certainly not what I've found after restoring vintage radio and television sets for the last 30 years. Carbon resistors of old were very unstable and most of them go high in value. This has thankfully been eliminated with metal film types, and as for old paper capacitors, the reliability is absolutely appalling. Try as they might, with all kinds of sealing methods, the fact is the internals were always organic and always absorbed moisture. Even mica capacitors are starting to be a problem 50 years on.
It's the resistors and capacitors that are the problem, not the vacuum tubes.
I am glad I am not the only one who also questioned that info. I got my ham license at age 15 and began fixing radio's and then TV's for years after that. I also built a fair number of transmitters and receivers and ran my own evening TV repair shop for a number of years when I got married and money was short. I grew up with tubes and the fact that tubes were in sockets made them easy to test and helped debug things but as John H points out - the other components were way worse than what we have to choose from today. Today the biggest issue is picking the correct technology to go with for a given design. For instance today we have thick film resistors, carbon film resistors, metal film resistors and wire wound resistors too but we can even buy off the shelf extremely low value precision resistors or laser trimmed precision resistors with .1% accuracy and not overly expensive. The standard resistor in the old days was a carbon composition resistor with a 10% tolerance. Tight tolerance parts like 1% resistors were expensive. Today in all of my designs I routinely use 1% resistors and rarely use even 5% resistors. Capacitors are wonderful by comparison and there are some extremely large values that can be readily designed in. LED's are way more reliable than old pilot bulbs.
The problems come in when somebody uses the wrong part in a repair and does not understand the needs of the circuit. Best example is the Model T coil itself. If one thinks the old capacitors are all just fine then clearly the leakage hasn't been tested on very many old coils. Tony Cimorelli once did a seminar with his T chapter and they tested everybody's coils starting with capacitor leakage tests. There were some long faces in that crowd that night as Tony tells it. The old capacitors were wound up and installed in the T coil and moisture and tar can easily migrate into the thing since they were not conformally coated. There is no comparison between those old capacitors and any of the new epoxy dipped and coated modern caps which just don't leak unless you take a hammer to them before you put them in there (don't laugh I know a guy who put some caps in a vice to flatten them). The only problem is if somebody assumes all of the epoxy dipped parts are the same and tries to use a metallized polyester part or some such part with a low dV/dT rating. Those don't last very long but that is not the fault of the capacitor but rather the application it was designed for is something else that does not require the high dV/dT rating. I wish the internet had been available when I was actively designing products since it is just wonderful to have such an array of fine parts out there to choose from and information so readily at hand. As for components - these are the good old days.
Back in the day (about 1963) I was building up a piece of electronic equipment and needed a filter capacitor for the power supply. Found one in my parts box and installed it. Plugged everything into the 110 ac and a short time later was jolted by a very large bang. Yep. Forgot to check the working voltage of the cap. It was about 35 vdc.
Talking about tubes made me think about when I was a kid. When the TV would go on the blink, Daddy would take all the tubes out of it and take them down to the drug store where they had a tube testing machine. He would find out which one(s) was bad and buy another. They were stored inside the cabinet of the test machine. Sometimes that wouldn't be the problem and he'd have to call the repair man. Today, you just throw it away and buy a new one.
Thank you for exposing the nonsense.
Ron the Coilman
Hal, I still have my Dad's tubes and tube testing equipment. I think some of those tubes are older than I am.
When I was a kid, I was his little tube checker. First thing I learned to do was test tubes.
I have the top section of 1 of those tube testers,like a dummy gave away the cabinet.Didnt have room for it all.No book was with it so it is useless to a degree.
I use a Heathkit tube tester and a cap tester that was hand built sometime years ago.
Works ok for what times I use it.If working on a old radio,I useally just change them all.
I don't claim to be a coil expert but I have rebuilt my share with proven success. And, I won't brag about my electrical engineering experience but I was into it a long time before transistors. In the early days wire wound resistors were the most accurate and durable. Today metal film resistors fill that need. Sure, carbon resistors had 5 or 10 percent tolerance but tolerance only affects absolute value not performance. Most early electrical component designs were overkill. A resistor rated at one watt was really a two watt resistor. Transformers, coils and potentiomoters were made to last a long time and their power ratings were well over the requirement specs. With the exception of electrolytic capacitors , capacitors like Henry's coil capacitor were well designed. As I said earlier, you seldom find a bad coil capacitor unless the end terminations have been shaken loose. You seldom find a leaky or shorted capacitor and this is after 80 or more years of service. There is little if any moisture that penetrates a coil unless it is submerged in water and Henry encapsulated it in tar to protect it. I know Ron routinly replaces all original capacitors and that is fine. They are easy to replace and are inexpensive so why not? All I am saying is Most original coils still have good capacitors and can be easily rebuilt. Henry's coils were well built and have withstood the test of time and that in itself is amazing.
Just remember guys...lol...all of the Project Mercury space flight items that took us into space where THIS OLD STOCK...lol...with resitors and the like hand soldered, and wire ties were waxed cord with surgeons knots...what is it they say today? "It ain't rocket science it's only a T?" Somehow seems fitting...and thanks for the updates on whats what as there has been a 'lot' of confusion in the past. I gave up and just buy new ones...lol
Hal, you missed out...like Doug, it was SOP in our house when a tube was out that Dad just gave you a box with questionable ones and sent you to the store...I thought it was great but always wondered about getting zapped...lol...yes...sometimes there was a charred resistor so some of the above comments are correct, and that was fun too...Dad would just dump the box of resistors out on the floor (and sometimes I think he did hide the right color sequenced ones), fire up the soldering iron and then tell me the find the right one either by seeing through the char or 'reading' that glued on schematic of the old 1948 Admiral chassis, but when there were none in the pile had to then learn 'math' on how to make 'clusters' and still get all the solder to be good and tight. Then...other than adjusting horizontal and vertical once turned back on the TV always worked! My younger brother didn't learn too good...when he was about 7 he decided to hook up the Lionel Transformer from the 1936 dated set and run wires to the 'fine tuning knobs' (they were never knobs, just knurled shafts...and then use the variable point lever settings on the transformer for 'remote control' (My grandfather had a toggle switch mounted to his chair to turn off sound during commercials at his house).
That took Dad and me a whole Saturday to put the Admiral back together and I think we managed to keep it going somewhere in his house for another 25 years. My brother? He wound up being an electronics tech at Plattsburg AFB. Amazing those B-52 stayed up, eh?
Your wrong about the original Model T coil capacitor being "encapsulated" in tar. Only one side and two ends are encapsulated. When manufactured one side of the capacitor was placed directly against the wood case inside the coilbox. The original capacitor has a wax paper dielectric and is paper wrapped. Moisture moves through the wood side and into this paper causing it to leak lowering it's DC resistance. I have never seen a Model T that does not have a leaky capacitor. Because of this most original coils need the capacitor replaced.
I will cede the point that in some cases the leakage is low enough to allow the coil to work OK without replacing the capacitor But if your going to rebuild the coil why not do it right and use n appropriate replacement part?
Ron the Coilman
Hi Ron, Hope you enjoyed the Ruckstell book. You are right the two ends of the capacitor are encapsolated but that is where the terminations are, so the terminations are shunted with tar. Every capacitor, includng the new ones have some leakage. Just check them with an ohmmeter. The higher the capacitance the higher the leakage. The shunting tar will have some conductivity or leakage also that might add to the total leakage measured. I don't believe that moisture is a major factor or anything to worry about. Moisture in the coils would be a problem but highly unlikely to occur. After all is said and done the coils have withstood the test of time remarkably well and I do agree that replacing the capacitor is a good idea even if it is good.
What kind of leakage resistances are being obtained with the new capacitors? I am curious how 100+ megohms can be detected with an ordinary ohm meter testing at not more than 9V.
It is true that large value electrolytic capacitors will show detectable leakage, but we're talking plastic dielectric here.