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We recap, why not re-sist?

No doubt I’ll be displaying my utter ignorance of electronics with this question; why are capacitors recommended to be replaced, but never the resistors? There’s always talk about recapping, but never a mention about 're-sisting'.

What am I missing?
parabolic
12 responses Add your response
erik_squires
16,056 posts
 
Hi Parabolic,

I re-resist. :) But depends. For resistors there are in fact many boutique or highly sought after brands for electronics as well as speakers.

The big issue in tube gear though is the caps. They are complicated to manufacture, sound quite different and almost always will be in the signal path. I wrote this elsewhere as a joke:

"A good tube preamp is a very nice set of capacitors surrounded by tubes and other crap."

Outside of tubes, transistor gear can be built without a cap in the signal path. Caps are also found in speaker crossovers, especially on the way to the tweeter.

For resistors, IMHO the biggest issue after noise is thermal effects, which cause the resistance to change as it warms up, so the resistors most immune to this command the big bucks.  However, some like a certain vintage sound and seek out resistors just for this.  If you were building a guitar amp, with tubes and wanted it to sound as close to something from the 60's as you could this is the way to go.

Best,


Erik

parabolic OP
106 posts
 
I’m asking because I have a Krell KRC-3 preamp on the way. Assuming it was made in the first year of its release (1995) that makes it 21 years old by now. It was recommended to recap the unit, but it got me to wondering about the rest of it.  However, the thought of trying such a feat on a piece of equipment like this scares the bejeezus out of me.

So theoretically, any piece of audio (or other electronics equipment for that matter) has a definite lifespan then, correct? Or is this a case where the caps - because they’re in the signal path - ought to be replaced but everything surrounding them is less critical and can function for decades or longer because it doesn’t affect the audio characteristics?

Just curious about how it all works.
rodman99999
6,130 posts
 
There are two main reasons people recap electronics.  One, regarding electrolytics(specifically), is that they dry out over time(increasing their internal resistance or ESR) or can leak their electrolyte.   The other is presentation and that some caps(ie: film/foil) can sound so much more transparent than aluminum and paper.   Some prefer the presentation of film and oil caps(different strokes/tastes).    Individual components all generate a certain amount of thermal(Johnson–Nyquist) noise and some resistors are noisier than others.   Carbon composition resistors are usually considered the noisiest, metal-films pretty quiet and wire-wounds the quietest.  One can also tweak the sound of their gear with resistor swaps in certain circuits.  ie: Carbon resistors will impart are warmer presentation than metal films, in most circuits.    One of the reasons older tube gear has a tendency to maintain a, "warm and tubey" sound(regardless of tubes rolled and chosen) is the proliferation of carbon comp resistors, in those units.    I've brought numerous pieces up to today's cleaner/more accurate presentation, with the judicious application of Vishay's metal-film resistors(for instance).    Then there were the popular Sprague Black Beauty caps............
erik_squires
16,056 posts
 
Hi Parabolic,

Aging and quality are the two reasons why you may recap. Electrolitics have a life span measured in decades, while for filmcaps it's in hundreds of years.

Almost all electronics have electrolytics in the power supply. I'd leave them alone for another decade or two unless there  is a problem. Often these caps leak, or bulge or show signs of burning so problem caps are easier to spot.

As for any film caps, the last 20 years have seen some great improvements, so preamps made before the 21st century may benefit immensely from replacing coupling caps, but again,this tends to be in the tube preamps much more than solid state.  Many high-end solid state pre-s use no coupling caps, so your benefit, if any, is less clear.

Best,


Erik
erik_squires
16,056 posts
 
You might want to check out this video. There's even a parts list at the end.

https://www.youtube.com/watch?v=G9vtk4CONiU

I'm not sure when the Panasonic FC and FM caps were introduced, but they are very nice upgrades.  It seems all the caps replaced are power supply related, so they should be relatively inexpensive.
bombaywalla's avatar
bombaywalla
1,958 posts
 
rodman99999 brings up some of the key reasons to re-cap older audio gear.
the thing to understand is that re-capping is directed mostly to electrolytic caps which are mostly in the power supply. Boutique electrolytic caps (paper in oil, silver/gold foil in oil, etc) are also in the signal path but they encounter much smaller signal amplitudes hence their degradation over time is much less.
So another reason to re-cap is that over time electrolytic capacitors in the power supply are heat cycled & heat is one of the worse enemies of caps. As rodman99999 wrote these caps’ ESR increases & the power supply becomes grungier. Additionally, power supply caps (due to heat cycling) can show bulging on the top. This is a sign of internal mechanical failure & the capacitor can explode while operating thereby destroying a lot of other things inside the amp. You don’t want this to happen - repairs can be very expensive.
So, if the audio gear is 10-15-20 yrs old, re-cap the power supply electrolytics.... Do not re-sist! ;-)
erik_squires
16,056 posts
 
I should also say, going back to your original question, unless the resistors are thermally stressed, they will probably last thousands of years. :) They are incredibly stable over time, while electrolytics go bad just sitting on a shelf.

That's another reason why re-resisting is not something you should consider in that unit, especially if you like the sound of it. :)

Best,

Erik

minorl
1,016 posts
 

The responses related the electrolytic capacitors re-capping is accurate.  However, it isn't just the power supply caps.  All electrolytic caps should be replaced when recapping.  In my opinion.

Power supply cycling add stress to capacitors. This is mainly true for equipment that do not have standby circuitry or slow start circuits.  For standby circuitry, the caps are pretty much charged all the time or in a lower state and as with  slow start circuits, current and voltage is slowly applied (relatively speaking).  Therefore, there is much less stress on the capacitors. So, to me this is a non issue in modern electronics.

Heat is another issue for failure of electronics.  properly designed and constructed electronics with proper air flow/cooling helps prevent heat related failures dramatically.  Again, a non-issue if this is employed.

I would be more concerned with Class A solid state amps, where the bias current is such that the amp's output transistors are fully "turned on".  in other words biased such that the transistors are operating at or near full output current with no input signal.  this means that components are getting a lot of heat, which significantly lessen component life. 

Another good thing to do with solid state amps that one wants to keep is to replace or upgrade the pre-drivers and output transistors with exact replacements or upgraded transistors. Not only are the newer transistors more linear than the older units, they (in my opinion) sound much better.

I've upgraded many older amplifiers this way.  The transistors must fall within the same general specifications as the older one. such as voltage ratings, current ratings, hfe, etc.

Also, equally important is to at least remove and replace the old transistor insulators with new insulators and silicon thermo insulating compound. I've found that the compound dries out over time and the insulators dry and crack and cause shorts between the transistors and the heat sinks.  This, to me is more important than re-capping.

I wouldn't worry about the replacing resistors unless I see an obvious problem.

enjoy

parabolic OP
106 posts
 
Thanks to all.  My question was first and foremost about the resistors, and that's been answered clearly.  Good info - AGON knocks it out of the park again!
Post removed 
parabolic OP
106 posts
 
I didn't miss a thing at all.
ebm
9,311 posts
 
Lets all re-sist.
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