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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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3rd Feb 2019, 1:16 pm | #1 |
Octode
Join Date: Jan 2012
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Identifying resistor types?
I have come by a whole collection of resistors (10 off of values from 10R to 10M in the E12 series), which I am hoping (don't ask why!) are carbon composition. Is there any way of ascertaining the type of a resistor, carbon comp., carbon film, metal film, metal oxide? I realise that all but the carbon comp. types will have some inductance, but is that measurable?
The resistors came in a box marked "Pop's Electronical Shop" in Croydon. The 'phone number of this establishment is given with an 01- prefix, which indicates to me an approximate date of packing, at least. The resistors are rated at 1/4W and 5%. They are bandolier mounted and about the same size as a modern metal film type (6mm x 2mm). Since they are all bagged and boxed together, they are probably all the same type. Any ideas? TIA, Colin. P.S. I am interested in carbon comp. resistors because I repair, make and generally mess about with valve amplifiers, etc. and like to keep them as original as I can. |
3rd Feb 2019, 1:41 pm | #2 |
Octode
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Re: Identifying resistor types?
What colour are they?
Metal films are mostly blue. Carbons are often brown. Next try destruction testing next. Scratch through the insulation (paint) and see if there is a spiral of 'material' underneath. If it is a spiral it will likely be a film type. Side cutters and split one in two. If there is material in the core it will be a comp type of some sort. All a bit of a guess then...
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3rd Feb 2019, 2:28 pm | #3 |
Dekatron
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Re: Identifying resistor types?
A picture would also make it easier to identify.
Also, don't assume that resistors that are boxed together are of the same type. Of course if they look identical, chances are already much higher. |
3rd Feb 2019, 3:39 pm | #4 |
Nonode
Join Date: Jun 2007
Location: Coulsdon, London, UK.
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Re: Identifying resistor types?
The resistance of a carbon composition resistor varies depending on the voltage across it.
It is slightly non-linear. I have read about it in books but not actually tried to measure the effect myself. If I was going to investigate this effect I would make a bridge circuit with one carbon and three metal film resistors. I would probably need a wirewound variable resistor to get the initial balance, and a DVM. |
3rd Feb 2019, 4:01 pm | #5 |
Rest in Peace
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Re: Identifying resistor types?
Carbon comp are nonlinear, have more noise, less inductance, and can cope with termporary overloads better than other resistors. Any of these features could be tested for, if you have known samples to compare with. CC also have much worse long term drift in value - so if lots of the resistors measure different from their marked value then they are probably CC, but then you might not want to use them. Most people would not want to use them anyway, except perhaps in guitar amps.
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6th Feb 2019, 3:44 pm | #6 |
Nonode
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Re: Identifying resistor types?
IF stages, centred on 45 Mc/s, used in Radio Astronomy in the 1970s always used CC resistors for just the low inductance point G8HQP mentioned.
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6th Feb 2019, 7:52 pm | #7 |
Hexode
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Location: Sheffield, South Yorkshire, UK.
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Re: Identifying resistor types?
so is carbon comp the same as carbon film
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6th Feb 2019, 8:14 pm | #8 |
Dekatron
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Re: Identifying resistor types?
No; carbon-composition resistors are made with a solid rod of resistance-material, think of it like the graphite 'lead' in a pencil. The electrons flow through the entre thickness of the rod, so its inductive properties are essentially those of a short rod.
Carbon-film resistors use an insulating ceramic former that is coated with a carbon film deposit (hence the name) and then in many instances a spiral is cut round the body, through the film to lengthen the path so that the current flows spirally round the former. This spiral construction means that carbon-film resistors are slightly inductive. Carbon-film resistors do, however, generally produce less thermal noise which is why they're popular in low-signal-level amplification. |
6th Feb 2019, 9:32 pm | #9 |
Hexode
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Re: Identifying resistor types?
Many thanks for that explanation
Mick |
7th Feb 2019, 11:56 am | #10 | ||
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Re: Identifying resistor types?
Quote:
Quote:
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7th Feb 2019, 12:25 pm | #11 | |
Dekatron
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Re: Identifying resistor types?
Quote:
Given that few carbon composition resistors are now in production, the amp restorer who wants to keep things 'original' is faced with a dilemma. He can stick with old carbon comps, in which case the original look-and-feel, noise, nonlinearity etc of these resistors will be preserved. But the original resistance and therefore the original circuit behaviour will have been lost. Or he can go for more modern resistors and keep the second type of originality but lose the first. There are a couple of possible compromises. If, say, a 100kohm resistor is needed then the restorer can track down an old 82kohm one. By now it may well actually be 100kohm and so should allow the circuit to work as intended. The only question then is whether the changes which have altered the resistance will also have altered the other imperfections. I once came across a drifted carbon comp which was very nonlinear, adding a good deal of distortion to the amp in which it was used. The manufacturers had a reputation for careful testing of their products, so it's hard to believe that the amp had left the factory working that badly. There is also the small matter of the look of the circuit. If anyone should have to work on it in the future they might be confused to discover a grey-red-orange where they were expecting a black-brown-yellow. Another approach is to leave the original drifted resistors in situ and discreetly wire a modern resistor in parallel to 'pad' the drifted one back down to its correct value. Now (say) 80% of the current will flow through the carbon comp, with all its eccentricities, and the 20% that flows through the padding resistor will be pure and clean, just adding a splash of distilled water, if you like, to the carbon comp's single malt (or lemon barley water, if you prefer ). Cheers, GJ
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7th Feb 2019, 1:12 pm | #12 |
Hexode
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Re: Identifying resistor types?
It seems that Carbon Composition resistors are somewhat popular again as several manufacturers are currently producing them. From the point of view of resistance value drift, looking at the data sheets there does not seem to be any info on long term stability, so it is not certain that they are any better than their predecessors such as Morganite Crucible.
Ohmite Little Demon are available from RS Components Neohm/TE Connectivity CBT are available from Farnell Arcol RCC are available from Rapid Electronics (cheapest) Looking at carbon composition resistors they all seem to physically a straight tube with square ends (the leads are welded to the ends of the carbon rod, whereas the film type (and indeed most modern types) all have bulges at the ends where the end-caps are fitted over the ceramic rod on which the resistance material is affixed. The much older Eerie carbon comps where slightly different in that the carbon was packed inside a ceramic tube with the leads fixed into the ends with a orangey-brown cement, as a result these were comparatively large for their power rating. |
7th Feb 2019, 1:48 pm | #13 |
Dekatron
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Re: Identifying resistor types?
Q. How do Erie Hi-Stab resistors (with the pink band) manage to be so stable long term? Which type of construction are they?
(Carbon film..?) Dave |
7th Feb 2019, 2:22 pm | #14 |
Octode
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Re: Identifying resistor types?
I have now fully investigated the contents of the envelopes and it seems as though this was someone's resistor stock. Some of the envelopes are empty, but judging by the numbers in most of the envelopes, there were originally 10 of each value. Most of the resistors have a pale brown background colour, but where a re-stock of commonly-used values has been done, the background colour is blue - rather similar to metal-film ones. I have attached photos of the brown 10M ones and of the blue 47R ones.
I also checked the resistances of the ten 10M ones using my Fluke 175.The lowest value is 10M20 and the highest 10M97. With the stated 5% tolerance, this indicates to me that the 10M have all drifted high. I don't suppose anyone has ever bought any thing from Pop's Electronical Shop, or has any knowledge of it, but you never know.... Colin. |
7th Feb 2019, 2:53 pm | #15 | |
Dekatron
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Re: Identifying resistor types?
Quote:
I'd be interested to know of any background to the Jugoslav-made "Iskra" carbon film resistors that were so widespread and popular for several decades- there was a great deal of "expedited acquisition" of German industrial plant and process by both UK/US and Soviet forces and it may be that Iskra resistors were a spin-off from this. It wasn't just exciting things like rocket motors and jet engines that were sought after. The good long-term stability certainly shames those ordinary white ceramic-sleeved carbon comps that plagued even expensive UK military and professional kit post-war! |
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7th Feb 2019, 3:12 pm | #16 |
Nonode
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Re: Identifying resistor types?
I remember a few visits to Pop's Electronical shop, probably in the 1980's.
At one time a young lady was working behind the counter. That 10M20 resistor is only 2% away from the nominal value. |
7th Feb 2019, 8:35 pm | #17 | ||
Dekatron
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Re: Identifying resistor types?
Quote:
What I meant to say was that "carbon composition" resistors introduce additional noise because of their construction which gives multiple current-paths through the carbon 'stick' and this granular nature causes noise to be generated because these diverse paths interchange in a random fashion. It's a 'micro' version of the old telephone-handset carbon-microphones where the random shuffling and contact-variation of the carbon granules caused a continuous background 'hiss/crackle' effect. |
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7th Feb 2019, 9:25 pm | #18 |
Pentode
Join Date: Oct 2018
Location: Coventry, West Midlands, UK.
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Re: Identifying resistor types?
To answer the original question the resistors shown in the pictures in post #14 are not carbon composite, the brown ones appear to be carbon film and the blue ones are likely to be metal film.
Carbon composite resistors typically look like this. |
12th Feb 2019, 12:57 pm | #19 | |
Octode
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Re: Identifying resistor types?
Quote:
I made the silly error some years ago of "refurbishing" my Vox AC30 with new components. At the time, I didn't realise that I was sacrificing the re-sale value and probably messing with the classic tone. I don't intend to sell it, but I would like to have it sound as close to as it did originally. Hence the interest in carbon-comp resistors. I wouldn't dream of using carbon-comps in other circumstances, though there are those situations where a corbon-comp should be used because of the lower inductance. I am thinking of some places in oscilloscopes, typically. Thanks for all the help and advice, guys. Colin. |
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12th Feb 2019, 5:38 pm | #20 | |
Octode
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Re: Identifying resistor types?
Quote:
I take your point about the 10M20 resistor still being in spec., but my point was that all of them had drifted up, there were none that were below 10M, as you might expect with zero drift. Colin. |
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