UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > General Vintage Technology > Components and Circuits

Notices

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.

Reply
 
Thread Tools
Old 3rd Feb 2019, 1:16 pm   #1
ColinTheAmpMan1
Heptode
 
ColinTheAmpMan1's Avatar
 
Join Date: Jan 2012
Location: Wimbledon, London, UK.
Posts: 867
Default 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.
ColinTheAmpMan1 is offline   Reply With Quote
Old 3rd Feb 2019, 1:41 pm   #2
snowman_al
Hexode
 
snowman_al's Avatar
 
Join Date: Dec 2018
Location: Llandeilo, West Wales, UK.
Posts: 269
Default 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...
__________________
Never Leave Well Enough Alone...
snowman_al is offline   Reply With Quote
Old 3rd Feb 2019, 2:28 pm   #3
Maarten
Nonode
 
Join Date: Dec 2007
Location: Haarlem, Netherlands
Posts: 2,287
Default 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.
Maarten is offline   Reply With Quote
Old 3rd Feb 2019, 3:39 pm   #4
Silicon
Heptode
 
Join Date: Jun 2007
Location: Coulsdon, London, UK.
Posts: 952
Default 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.
Silicon is offline   Reply With Quote
Old 3rd Feb 2019, 4:01 pm   #5
G8HQP Dave
Dekatron
 
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,721
Default 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.
G8HQP Dave is offline   Reply With Quote
Old 6th Feb 2019, 3:44 pm   #6
SteveCG
Nonode
 
Join Date: Jun 2009
Location: Herefordshire, UK.
Posts: 2,049
Default 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.
SteveCG is offline   Reply With Quote
Old 6th Feb 2019, 7:52 pm   #7
BRASSBITS
Pentode
 
Join Date: Dec 2018
Location: Sheffield, South Yorkshire, UK.
Posts: 104
Default Re: Identifying resistor types?

so is carbon comp the same as carbon film
BRASSBITS is offline   Reply With Quote
Old 6th Feb 2019, 8:14 pm   #8
G6Tanuki
Dekatron
 
G6Tanuki's Avatar
 
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 7,113
Default 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.
G6Tanuki is offline   Reply With Quote
Old 6th Feb 2019, 9:32 pm   #9
BRASSBITS
Pentode
 
Join Date: Dec 2018
Location: Sheffield, South Yorkshire, UK.
Posts: 104
Default Re: Identifying resistor types?

Many thanks for that explanation
Mick
BRASSBITS is offline   Reply With Quote
Old 7th Feb 2019, 11:56 am   #10
G8HQP Dave
Dekatron
 
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,721
Default Re: Identifying resistor types?

Quote:
Originally Posted by G6Tanuki
Carbon-film resistors do, however, generally produce less thermal noise which is why they're popular in low-signal-level amplification.
No, carbon film resistors generate exactly the same thermal noise as any other resistor (including CC). What is different is the excess noise. CC are worst, CF better, metal film and wirewound best.

Quote:
Originally Posted by SteveCG
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.
The inductance of a film resistor is rarely a problem below VHF. I could imagine that at 45MHz you might not want to put a film resistor as an unbypassed cathode/emitter resistor as it might increase input impedance but for most purpose they would be fine.
G8HQP Dave is offline   Reply With Quote
Old 7th Feb 2019, 12:25 pm   #11
GrimJosef
Nonode
 
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 2,460
Default Re: Identifying resistor types?

Quote:
Originally Posted by ColinTheAmpMan1 View Post
... 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.
You'll know then that there's often an issue in this respect with carbon composition resistors. The composite material, especially if it's been prepared for relatively high resistivity, can be unstable, changing its resistivity (often to higher values) with time. It seems to be purely an age-related effect, not a use-related one, so unused 'new old stock' components can have drifted to much higher values in just the same way that ones in a working amp have. It doesn't seem to be reversible either, despite what some reports of baking/drying/resealing suggest.

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
__________________
http://www.ampregen.com
GrimJosef is offline   Reply With Quote
Old 7th Feb 2019, 1:12 pm   #12
RF Burn
Pentode
 
RF Burn's Avatar
 
Join Date: Apr 2008
Location: Ramsgate, Kent, UK.
Posts: 208
Default 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.
RF Burn is offline   Reply With Quote
Old 7th Feb 2019, 1:48 pm   #13
The Philpott
Octode
 
Join Date: Aug 2016
Location: Colchester, Essex, UK.
Posts: 1,701
Default 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
The Philpott is offline   Reply With Quote
Old 7th Feb 2019, 2:22 pm   #14
ColinTheAmpMan1
Heptode
 
ColinTheAmpMan1's Avatar
 
Join Date: Jan 2012
Location: Wimbledon, London, UK.
Posts: 867
Default 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.
Attached Thumbnails
Click image for larger version

Name:	10M.jpg
Views:	67
Size:	164.3 KB
ID:	177934   Click image for larger version

Name:	47R.jpg
Views:	66
Size:	161.1 KB
ID:	177935  
ColinTheAmpMan1 is offline   Reply With Quote
Old 7th Feb 2019, 2:53 pm   #15
turretslug
Nonode
 
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 2,693
Default Re: Identifying resistor types?

Quote:
Originally Posted by The Philpott View Post
Q. How do Erie Hi-Stab resistors (with the pink band) manage to be so stable long term? Which type of construction are they?
These are often described as "cracked carbon"- if so, this was a process developed in Germany in the late '20s (the Germans have long been pioneers in sophisticated industrial chemistry) whereby hydrocarbon vapour is catalytically decomposed (cracked) at refractory temperature in order to deposit a fine film of very pure and homogenous carbon on a ceramic former. This was a process way ahead of others at the time- people restoring German pre-war and wartime electronics often comment on just how close to the original value cracked carbon resistors turn out to be after many decades.

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!
turretslug is offline   Reply With Quote
Old 7th Feb 2019, 3:12 pm   #16
Silicon
Heptode
 
Join Date: Jun 2007
Location: Coulsdon, London, UK.
Posts: 952
Default 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.
Silicon is offline   Reply With Quote
Old 7th Feb 2019, 8:35 pm   #17
G6Tanuki
Dekatron
 
G6Tanuki's Avatar
 
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 7,113
Default Re: Identifying resistor types?

Quote:
Originally Posted by G8HQP Dave View Post
Quote:
Originally Posted by G6Tanuki
Carbon-film resistors do, however, generally produce less thermal noise which is why they're popular in low-signal-level amplification.
No, carbon film resistors generate exactly the same thermal noise as any other resistor (including CC). What is different is the excess noise. CC are worst, CF better, metal film and wirewound best.
Yes, you're right: "Thermal noise" is a function of the ohmic resistance, temperature-above-absolute-zero, and a derivative function involving Boltzmann's Constant.

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.
G6Tanuki is offline   Reply With Quote
Old 7th Feb 2019, 9:25 pm   #18
Paul JD
Tetrode
 
Join Date: Oct 2018
Location: Coventry, West Midlands, UK.
Posts: 96
Default 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.
Attached Thumbnails
Click image for larger version

Name:	s-l500.jpg
Views:	48
Size:	6.8 KB
ID:	177968  
Paul JD is offline   Reply With Quote
Old 12th Feb 2019, 12:57 pm   #19
ColinTheAmpMan1
Heptode
 
ColinTheAmpMan1's Avatar
 
Join Date: Jan 2012
Location: Wimbledon, London, UK.
Posts: 867
Default Re: Identifying resistor types?

Quote:
Originally Posted by Paul JD View Post
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.
I think that you are right. I am sure that the blue ones are metal film, but the brown ones had me thinking. I have some modern carbon comps purchased from Rapid and made by Arcol which look exactly like your photo. I am sure that when I was soldering new carbon comps into circuits (over 50 yrs ago?), there were several physical designs. There were the tubular types of your photo, similar ones with metal caps on the ends for the leads and those that look more like modern dog-bone resistors in that they are paint-coated overall with a very pale beige colour, but are somewhat larger physically at ~9mm x ~3mm.

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.
ColinTheAmpMan1 is offline   Reply With Quote
Old 12th Feb 2019, 5:38 pm   #20
ColinTheAmpMan1
Heptode
 
ColinTheAmpMan1's Avatar
 
Join Date: Jan 2012
Location: Wimbledon, London, UK.
Posts: 867
Default Re: Identifying resistor types?

Quote:
Originally Posted by Silicon View Post
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.
That is interesting! Did you note what kind of resistors were sold there? In the 80s, I would guess carbon film and metal film.

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.
ColinTheAmpMan1 is offline   Reply With Quote
Reply

Thread Tools



All times are GMT +1. The time now is 5:01 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Copyright ©2002 - 2019, Paul Stenning.