Where do the carbon resistors matter most?

Discussion in 'Amp Tech Center' started by jonrpick, Apr 13, 2014.

  1. jonrpick

    jonrpick Friend of Leo's

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    As I go through replacing components on my Vibro Champ, it keeps sounding better.

    I've read countless times about how much different carbon composition resistors sound vs metal film, though the latter apparently has much less noise. My research is telling me that the metal film is the way to go in the power section, but that carbon is preferable in the "tone" parts of the amp.

    I'm just curious, which specific resistors would benefit from the "mojo" ( :rolleyes: ) provided by the carbon composition??
     
  2. printer2

    printer2 Poster Extraordinaire

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    The plate resistors, although I doubt they make all that much of a difference.
     
  3. Cleeve

    Cleeve Tele-Holic

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    Try to avoid using carbon resistors in any part of the circuit that would put audio and DC across the resistor at the same time. Such as plate resistors, cathode resistors and grid resistors. In a passive tone circuit with decoupling capacitors they would be ok.
    The most obvious place to avoid carbon is in the first stage preamp plate circuits.
     
  4. jonrpick

    jonrpick Friend of Leo's

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    I read that putting them up front is a bad idea as the noise just gets amplified. I'm still not sold on the idea that they're beneficial.
     
  5. 6942

    6942 Poster Extraordinaire

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    Carbon comps are the first resistors I use, after the input jacks.
     
  6. jonrpick

    jonrpick Friend of Leo's

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    ..... which directly contradicts what I read. And I'm certainly not implying that you're wrong or unwise to do so.

    It does however reinforce the idea that it's all opinion and personal preference, including whether or not to even bother using them.

    I did read a scientific perspective on how and why they affect tone, but it looks like it's not a slam dunk, "you have to use these or your tone will suck" kind of deal.
     
  7. Tatercaster

    Tatercaster Friend of Leo's

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    How are cathode, plate, and grid bias resistors signal path resistors? They each have one end connected to ground (the plate resistor grounds through the power supply). So, DC crosses these resistors, but any signal across these goes to ground, or am I misunderstanding?
     
  8. Cleeve

    Cleeve Tele-Holic

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    All resistors make noise just because they are made of matter. When no current is being carried by the resistor, only the resistor value and temperature matter noise-wise. So for places in an audio circuit that have only audio signal, like perhaps in a tone circuit that is AC coupled by a capacitor blocking the DC, tens of dollars could be saved on a production run of amps by using cheaper carbon! The construction of the resistor starts to matter big time when DC is there. The gist is, wirewound resistors are the quietest, but they also have inductance, so the meme got out that you'll have a silent amp that can't do high frequencies if you use wirewound resistors. Some audiophile lore got going amoungst the guys on the old usenet forums connecting the dots that if treble is sucked by those newfangled wirewound resistors vs the noisy but bright carbon comps that grammaw used to make, then all newer resistor types must also be bad, because that is easy to remember- carbon good, wire bad! tortches, pitchforks. Soon a belief was formed, and once a belief was established amoungst those who break in speaker wires and listen to equipment rather than music, the carbon good, everything else position got perpetuated.
    Plate resistors are hard to make with a wirewound resistor because of the value, often 100k ohm, thst's alotta wire- think a guitar pickup is usually only 5k! So well, here's a link, I don't have my glasses on and I must be misspelling loads! The inductance problem is solved with foil and film.. Carbon anywhere is fine except plates on the presmp tubes, because they are noisy in thst applicstion. And yes, sometimes wire bad, carbon comps dont have any real inductance problems, just crazy current noise. http://www.eetimes.com/document.asp?doc_id=1272283
     
  9. Tatercaster

    Tatercaster Friend of Leo's

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    Metal film resistors = good
     
  10. Cleeve

    Cleeve Tele-Holic

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    One end of each of the above mentioned resistors is at signal ground, (the plate resistor is at B+ but is at AC ground via the filter cap), but the other end of all three resistors has the audio signal on it, except the cathode if the cathode resistor has a bypass capacitor.
    The grid resistor has only the slight grid leakage current on it's non grounded end that is connected to the grid, but it's value is high, usually 1meg ohm, so any current at all makes some pretty good noise there.
    The main culprit is the plate resistor, often 100k ohms or more, with the neighborhood of 60 to 100 volts across it plus the signal at the plate end.
    The plate end of the plate resistor is not in the signal path I guess you could say, rather it is the signal.
     
  11. printer2

    printer2 Poster Extraordinaire

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    Carbon resistors are noisier than other types. Some people like the hiss that they produce as it makes you think there is more highs. A resistor that hisses less will sound duller. Just tweak your tone control and you will be better ahead with non-carbon resistors.

    The place carbon resistors can have a effect on your signal, other than noise, is in the plate circuit. The resistance is said to change a little by the varying voltage across it when your tube is amplifying your signal. Can't remember which way and could figure it out if I thought of it but I would not use them if I did not have to so I am not going to bother. The effect is not all that great and you need a lot of voltage across the resistor (oh yeah, it causes compression, I know which way they change) so they have no effect (other than to add noise) in the grid or cathode positions.
     
  12. mfowler314

    mfowler314 Tele-Meister

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    Here is an article by R. G. Keen about the nonlinearity of carbon comp resistors when they have a large signal swing across them. http://www.geofex.com/article_folders/carbon_comp/carboncomp.htm

    This says that there *might* be some advantage to using them in your plate circuits to get that nonlinear "mojo" at the expense of some noise, though since the signal tends to be large at the plate the effect of the noise is minimized. I've heard that not all CCs are created equal and the recommendation I've seen is to get "good quality" ones like Allen-Bradley.

    I tried this on a recent 5e3 build and it was not noticeably hissier than similar build I've done with metal film or carbon film... Perhaps just a bit hissier. Could I notice any obvious extra mojo? No... Not really! As I expected. But now I have an amp that I can tell others has some extra mojo built in ;-)
     
  13. jonrpick

    jonrpick Friend of Leo's

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    I had found that article after posting the thread. Very informative reading.

    I think I'll dispense with the "mojo" and just buy reliable, high-tolerance components...

    :cool:
     
  14. muchxs

    muchxs Doctor of Teleocity

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    Aw. Check this out:

    http://www.tdpri.com/forum/amp-cent...everb-build-ptp-conversion-progress-pics.html

    Carbon everywhere! There's only one carbon film by the trem bug. The OP is a little sensitive so he may not understand how visually stimulating his conversion is. He's done an excellent job of turning back the clock to make his reissue more like a real '65.
     
  15. printer2

    printer2 Poster Extraordinaire

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    That carbon film sure does stick out.
     
  16. CoolBlueGlow

    CoolBlueGlow Tele-Afflicted

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  17. GuitarJonz

    GuitarJonz Poster Extraordinaire

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    In a van, down by the river
     
  18. CoolBlueGlow

    CoolBlueGlow Tele-Afflicted

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    Ha! True LoL :)
     
  19. Middleman

    Middleman Friend of Leo's

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    I had a conversation with Dave Pearlman of Pearlman microphones and he was adamant that carbon resistors provided more sonic mojo than metal resistors. So that said they are better placed in the tone section of the amp.

    Carbon resistors have been used for over 60 years in electronics. I think they've earned their place.
     
  20. CoolBlueGlow

    CoolBlueGlow Tele-Afflicted

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    I use CC all the time, because I am fortunate enough to have a huge supply of vintage CC. I also use common sense MOX on the things Aiken outlines in his white paper. The science is compelling, and Aiken's explanation is clear and easy to follow.
    I think the most obvious and yet most overlooked noise maker is the 1m grid resistor at the jack and the 1m carbon track volume pot everybody seems to use without thinking, along with the 3m carbon track reverb knob. Big noise sources - and few ever complain about carbon track volume/reverb potentiometer noise. :)

    Compared to those huge sources of noise, measly 68k grid stoppers and 100k plate resistors are very tiny sources of resistor noise. As I said earlier, I use 2w CC plate resistors all the time.

    I think I can hear the difference, and I'm skeptic enough that I don't believe in UFO, the grassy knoll, or lost Inca gold. :)

    I would never use carbon film resistors for any audio circuit application.
     
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