Resistors drifting in value with age?

[john_morris_uk]

Sorry if this is old hat to some, but I have been checking the values of some resistors in the amp I've been repairing. Some are way out of tolerance: e.g. 4k7 now reads over 6k. Is this normal for kit that's over fifty years old?

I'm used to capacitors going leaky, but I'm not used to resistors being out by so much.

[Station X]

It's quite normal, especially where carbon composition resistors are concerned.

[john_morris_uk]

Thank you.

Of course the down-side of this question is that there's a lot more work for me to get this thing back into spec, Like all circuits, some of the values can't be measured without isolating the resistor. Ho hum.

[GrimJosef]

Yes, carbon comps are famous for it. Depending on the manufacturer you might find that the lower value ones (a few kilohms and below) aren't too bad. But things quickly get worse for values above this. It's not really a question of use either, although severe use, corresponding to a lot of heat, can aggravate the problem. Don't bother looking for unused ones in boxes or packets. They can be every bit as bad as the in-circuit ones. Ageing seems to be the primary cause of the trouble.

Cheers,

GJ

[G6Tanuki]

Yes they do drift - badly!

Remember that a lot of the old carbon-comp resistors were toleranced +/- 10% anyway - and now they're half-a-century-or-more old.

Worst ones I've found are the 1-Watt type where the carbon 'stick' is encapsulated in a white ceramic tube with a tan-coloured cement plugging the ends. The high-value ones (250KOhms and upwards, found in places like AGC feeds grid-bias circuits and HT feeds to audio drivers or phase-splitters) seem to drift most of all - presumably there's not much carbon in the 'stick' bit so any chemical changes it does undergo has a major impact on its remaining resistance.

[Station X]

Originally Posted by john_morris_uk

Like all circuits, some of the values can't be measured without isolating the resistor. Ho hum...

Where valve circuits are concerned, resistors aren't generally shunted DC wise. There may be shunt paths via leaky capacitors, but you'll be changing these in any case.

[Radio Wrangler]

20% carbon composition resistors were commonly used in consumer gear. 10% was better quality and 5% were 'Hi-Stab' and got a little gold band.

High values shift the worst, but drift is normal.

The other thing about carbon comp resistors is that they are very noisy. Often noisier than the valves they are used with.

If you want something to look absolutely original, then you don't have much choice, but finding original components in good enough order is a challenge.

Alternatively you replace them as they fail using modern components, or you just whip the whole lot out and head them off at the pass.

Resistors for a given power rating haven't really got much smaller (unlike capacitors) because you need a certain amount of surface area to dissipate a given amount of power without things getting hotter. So the opportunity to hide modern bits in old casings is not so good.

I try to find metal film parts (good reliability, good noise) about the same size body as the original and accept the colour change. The newer parts can stand higher temperatures, so the power rating is usually one step up on old parts of the same size.

David

[G6Tanuki]

Originally Posted by Station X

Where valve circuits are concerned, resistors aren't generally shunted DC wise. There may be shunt paths via leaky capacitors, but you'll be changing these in any case.

It depends... if you're trying to use a modern DMM to measure the resistance of, say, the cathode-bypass resistor of an audio output stage (with a 50uF bypass cap) things can be confusing. Modern electronic ohmmeters apply such a small current through the resistor-under-test that it can take ages for any parallel capacitor to charge up and for the meter reading to become stable.

There's a lot to be said for 'lifting' one end of a resistor if you suspect there may be such a confounding charge-leakage path and want a reliable reading on your meter.

[GrimJosef]

Originally Posted by Station X

Where valve circuits are concerned, resistors aren't generally shunted DC wise ...

There are some cases though. The performance of resistors in audio (and maybe not just audio ?) feedback positions can be critical, and they are routinely shunted by the cathode resistor they're feeding and the output transformer secondary.

Cheers,

GJ

[john_morris_uk]

Some interesting thoughts. Thank you. I've got an AVO 8 which I use for some things. (I actually had it officially serviced and calibrated 35 years ago when I had easy access to such facilities but it might be well off by now?) However the modern DMM gets used rather more often and checks out against the AVO so I'm fairly confident of the readings.

So many components have had to be changed that the only thing that is constant is the layout. It is beginning to sound EXTREMELY nice though as I bring it back into spec.

Regarding valve equipment and measuring values, many of the components I'm anxious about are in the feedback and equalisation circuits of this Hi Fi amplifier and so are shunted by various combinations of other resistors. Many have high values and are shunted by lower value resistors with electrolytics or capacitors etc to the point that lifting one end is the only sure fire way of telling if they are at their design resistance.

Some seem less critical and I'm not sure you notice when listening but I might check the frequency response when my new Sig Gen arrives and see how flat (or more likely not so flat) it really is...

[David G4EBT]

Originally Posted by G6Tanuki

Yes they do drift - badly!

Worst ones I've found are the 1-Watt type where the carbon 'stick' is encapsulated in a white ceramic tube with a tan-coloured cement plugging the ends. The high-value ones (250KOhms and upwards, found in places like AGC feeds grid-bias.

I can certainly endorse that comment - in my de-cluttering mission I've recently binned about 2,000 NOS ones which I bought at a bargain price at a radio rally some years ago and had put up in the loft.

They looked pristine - just as when they rolled off the production line, but the values had drifted so high that they bore no relationship to the contents. About as useful as NOS AFxxx transistors! Basically, just landfill. Thankfully, new 1, 2 & 3 Watt metal oxide film resistors, which look the part physically as replacements for anything from 1/2 Watt to 3 Watt carbon composition resistors, are plentiful and cheap from the likes of Spiratronics - for now, at least.

Let's just be grateful that valves soldier on and don't generally suffer the same fate as do carbon composition resistors.

[john_morris_uk]

Well I've just ordered a stock of 1 watt metal foil to have some to hand in my repairs so hopefully I will sort it all out and it will last for a few more years. The variety of manufactured types of resistor in this HIFI amp is remarkable. I pretty confident that they are original by the style of construction and soldering.

One can also spot the VERY botched repairs that had taken place over the years. For example one smoothing capacitor had been replaced with a metal can stuck on the top of the chassis with some sort of window sealant and with flying leads through a spare hole to the circuit below. It was silly as the EXACT size and value to fit the clamp under the chassis is freely and easily available.

[Radio Wrangler]

Back in the day, most of the manufacturers would have been glad to have come across a batch of surplus components at a bargain price and would have just used them. Or if the rep from one resistor company or capacitor company came in and offered a good deal.... and then the other guy came in a couple of weeks later when they were needing to order....

They didn't have the benefit of hindsight which we have. We know which components lasted and which failed.

On the other hand, they didn't have a market which believed in directional wire hokum and fairies dancing at the bottom of the garden (except in Cottingley) So if they made an amplifier and it worked well and sounded good, people generally agreed, bought them, and got on with listening to music.

David

[G6Tanuki]

Originally Posted by john_morris_uk

Well I've just ordered a stock of 1 watt metal foil to have some to hand in my repairs so hopefully I will sort it all out and it will last for a few more years. The variety of manufactured types of resistor in this HIFI amp is remarkable. I pretty confident that they are original by the style of construction and soldering.

Modern metal-film resistors are invariably much better than your by now well-matured 'carbon stick' ones ever were when new!

[I remember my first introduction to metal-film resistors used in telephone-exchange electronics in the mid-1970s: they were Welwyn ones, painted salmon-pink all over, had the value printed on them and were 2% tolerance - a degree of precision previously undreamed-of!]

[ricard]

Originally Posted by Radio Wrangler

Back in the day, most of the manufacturers would have been glad to have come across a batch of surplus components at a bargain price and would have just used them. Or if the rep from one resistor company or capacitor company came in and offered a good deal.... and then the other guy came in a couple of weeks later when they were needing to order.... They didn't have the benefit of hindsight which we have. ...

On the other hand, the equipment in which these components were mounted is now being used well passed its designed end-of-life date. The fact that lots of components have drifted out of tolerance after 50+ years is not really surprising, given that the average customer would have replaced his gear every 10 years or so, and the circuitry designed accordingly.

The fact that carbon composites were noisier than film resistors was well known at least in the 60's and I suspect much earlier too, where some manufacturers used metal film was used for anode resistors in input stages, and composites in the later stages.

As for replacements, although it's nice to be able to restore a unit to its original state, at the same time it also makes sense to repair it in such a way that it will last for many years to come, and also that it's reasonable to make repairs in a similar manner to which said equipment would have been repaired back then.

One alternative if one wants to stick with composites would be to measure the potential replacement and mount one which has the correct measured (but not marked) value. I suspect though that the drift in value comes with an increase in noise, and of course the drift will continue as the device gets older which would seem to put such a repair in the same league as the apparent shortsightedness of the original designer.

[turretslug]

There are probably almost as many shades of opinion on component originality as forum members and most of us would adjust the criteria on equipment rarity and value anyway. I say, now why do I like this old stuff, what makes it good? My answer is that, very broadly, the thermionic bits and the wound components (IFTs, coilpacks, output transformers, etc.) were developed to high degrees of refinement in their day and still cut the mustard. Also, things like complex metalwork and switching banks just don't exist anymore, so they get to stay. The everyday passive components, i.e. the Cs and Rs haven't fared nearly so well but the great thing is that vastly better equivalent components are still available and even made at a pretty favourable price. (Though OEM PCB and latterly SMD construction is inexorably diminishing popularity of long-leaded axial components).To me, one of the classic valve amps whose tag-boards have been neatly repopulated with metal film resistors and plastic film capacitors looks lean, fit and ready for the 21st century and is likely to measure well and sound good. Others differ hugely here and will put a lot of effort and money into acquiring original-state kit with drifted, noisy carbon-comps and leaky old output-valve-destroying paper capacitors. This is where the definition of "got at" gets very elastic...

When I had the coil-boxes and screening of a AR88 dismantled (not something you want to do very often, so you change anything suspect) for overhaul, I replaced the LO's 10k 0.5W carbon-comp anode load with a 10k 3W metal film (still small enough to fit neatly across the IO base tags) in the hope that not only would it be more stable in value but that less wide-band noise would end up in the mixer and consequent less inter-mod mush- possibly straw-clutching or even irrelevant but very cheap to do anyway.

[60 oldjohn]

Originally Posted by ricard

One alternative if one wants to stick with composites would be to measure the potential replacement and mount one which has the correct measured (but not marked) value.

I would not recommend one to do this, I did it on a Philips TV while I had some resistors on order. What a job I had identifying what the correct value should be when the new resistors arrived. Also the old resistors will continue to drift and in a few years time will be out of tolerance once again.

John.

[GrimJosef]

I have a pair of Quad IIs which came to me a few years ago with the original carbon comp resistors in them, many of which had drifted high. I measured them when they arrived and I repeated the measurements a few years later. The sample was small and the results were a bit noisy, so the stats were far from great, but for what it's worth the 'current drift rate' as measured over the recent few years more-or-less matched the 'drift rate from new' as calculated from my initial measurements and an assumption that the resistors were close to their nominal values when new.

Long story short - the drifting doesn't seem to get substantially better or worse with time. It just continues, at least over 50 years or so, at a fairly constant rate.

Cheers,

GJ

[bikerhifinut]

Interesting stuff.
Theres a chapter in one of the Morgan jones books about values drifting, and although not specifically about carbon compo resistors , He describes a technique of basically "cooking" Wirewound High power resistors to "age" them before selecting on measurement.
I'll be wary now of using any of the old carbon comps still in the toybox. I held on to them as useful for grid stoppers, where the absolute value isn't that important. I havent noticed any noise problems as such, but I haven't used them in any areas forward of a power amp input.
But the information given here does explain my findings that just about all of them have drifted way beyond even 20% tolerance.

The comments about restoration of old gear are good too. I am firmly in the camp of using modern film resistors, preferably 1% metal films, and decently constructed plastic dielectric coupling capacitors. Please note that by "decently constructed" I am not advocating special "boutique" components assembled by exotic maidens with papyrus and earwax dielectrics! Electrolytics also and I tend to use ones a bit higher in working voltage with as high a working temperature as possible in Valve gear. I am firmly of the belief that a lot of the old Amplifier designs sound better than they did when new (or at least a couple of years old) when all the old passives are replaced and the amps are probably working exactly as designed for the first time in their lives?

Andy.

[HamishBoxer]

That is how i restore stuff Andy and I am sure they do sound better than new. Possibly not as many issuses either with the make of ECC83's.