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Old 3rd June 2012, 09:46 AM   #1
tomst is offline tomst  United Kingdom
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Default metal film resistors

Hi I'm working on a 70's amp and most of the resistors are well out of spec. so i will replace them.
After some research it seams the best thing to do is use metal film and then use carbon type in the later part of the signal path to warm the sound up and/or soften it up if needs be. is this correct?
Also the general feeling seems to be Vishay dale rn and CFM are really good but they are not that redly available in the uk
My main question is what do people think the welwyn MFR, MFP, PR5Y and seres.
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Old 3rd June 2012, 11:37 AM   #2
Bonsai is offline Bonsai  Taiwan
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Wow, it's it all that normal for resistors to go out of spec, unless they are over stressed. Are you sure your meter is not the problem eg flat battery?

I wouldn't worry about resistor sound. The Vishay Dales are good resistors
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Old 3rd June 2012, 11:50 AM   #3
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Quote:
Originally Posted by tomst View Post
use carbon type in the later part of the signal path to warm the sound up
If the sound needs to be warmed up at the end of the signal path, there's probably a lack of heat insulation in the input stages - don't let it cool down too much there! Resistor sleeves handknitted from wool will probably help!

*scnr*

Andreas

PS. What I want an amplifier to do is to amplify, as linearly and as distortion-free as possible. If you want something to chance, warm-up, cool-down, harden, soften, whatever, ... the sound, you should buy an effect device.
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Old 3rd June 2012, 11:56 AM   #4
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In musical instrument amps like guitar amplifiers, carbon resistor are still used today to get a vintage sound. They can add some noise and they are known for sounding a bit warm. For HiFi amps metal film is better.
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Old 3rd June 2012, 01:11 PM   #5
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... didn't realize it was an instrument amp...

Greetings,
Andreas
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Old 3rd June 2012, 01:26 PM   #6
tomst is offline tomst  United Kingdom
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It is not an instrument amp.
I will try the battery for the multi meter but am sure it is fine. but the resistors are all carbon composite and some are well out of spec (showing high resistance) and very unbalanced. I have read that sometimes if you use metal film the sound is a lot clearer but can harsh an to clear. so adding carbon in the later stages can can soften up the sound. I agree the a good amplifier should amplify not add anything and or take anything away and a good amplifier design can be let dow by poor components. Again as i said I'm new to this and am still learning.
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Old 3rd June 2012, 01:37 PM   #7
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The various audible sound differences between different kind of caps at different places are for me good known.

But I was not able to distinguish by a listening test the sonic differences between a modern carbon film resistor and a metall film resistor (the evaluation was performed as input resistor, NFB resistor and the resistor in the collector line of a LTP input stage).

Clearly audible differences I note rather by different wattages of the NFB resistors in audio amplifiers. The bigger (i. e. more watts) the better.

A good choice are the MG785 from follow pdf:
http://www.caddock.com/Online_catalo...Lit/TypeMG.pdf

The most cost effective solution is the use of a lot of parallel switched devices, no matters whether carbon or metal film resistors.
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Old 3rd June 2012, 02:59 PM   #8
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Originally Posted by tiefbassuebertr View Post
.....The most cost effective solution is the use of a lot of parallel switched devices, no matters whether carbon or metal film resistors.
I guess you are referring to potentiometers such as volume control etc?
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Old 3rd June 2012, 03:06 PM   #9
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If one is trying to save every penny, carbon film resistors may have a place. I've also seen them used as fuses. Other than that, metal films are superior in every way. I'd never put a carbon film resistor in a circuit on purpose.

Carbon composition resistors are notorious for increasing in value with age, sometimes by quite a lot. There was also an old thread here where someone who knew their stuff made a good technical case for using carbon composition resistors in one particular place in a tube amp. Sometimes they make sense in RF applications, but other than those limited applications I'd never voluntarily use one in a modern circuit. If you're doing a restoration and want to keep things original, that's another story.

I'd stick with metal films entirely and do any sonic tuning in other ways, rather than using inferior parts, regardless of whether it's a hifi or instrument amp.
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Old 3rd June 2012, 03:09 PM   #10
Bonsai is offline Bonsai  Taiwan
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CC's are sometimes recommended for grid stoppers beg pause they have lower inductance than MF types.
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