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Carbon comp vs Carbon film dropping resistors

Discussion in 'Let's Talk Vintage' started by boola1, Jun 21, 2021.

  1. boola1

    boola1 Well-Known Member

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    Sorry it's another one on my 71 50W... I've just replaced the original carbon comp dropping resistors in this amp.

    The old resistors have drifted up to 10k and 11k. Now I know why this amp had so much distortion on tap.

    First I put in some 8k2s. The amp didn't have enough distortion so I put in some 10ks. It was better but still not enough.

    So I put a .68uf on the 1K of V2a. Now the amp is where I'd like it distortion-wise but it just lacks some mojo.

    What I've found is that there is a big difference in tone between the carbon comp. resistors and the carbon film.

    Yes, the carbon comps are very hissy but they seem to add a nice 'brown' compression to the tone as well as helping with harmonic bloom.

    I'd describe the carbon film as more modern sounding. They make the amp MUCH brighter and clearer. They could well make the amp more present in a mix but they lack some of the mojo that I'm used to.

    So I need to find some 'new' carbon comps. I didn't realise they were so hard to come by :(

    These are the before and after shots

    [​IMG]

    [​IMG]
     
  2. Pete Farrington

    Pete Farrington Well-Known Member

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    So you’re finding is there’s a difference in tone in regard of the resistor type used for HT dropper applications?
    That seems strange, as they should be fully decoupled back to 0V common.
    Or were you thinking of the anode load resistor type?
     
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  3. boola1

    boola1 Well-Known Member

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    You're more technical than me. Have a look at the 2 pics I posted, you can see the change from 2 8k2 (or used to be!) to 2 10k Pihers.

    The change in tone is big, surely much bigger than can be attributed to the 21k total resistance of the old resistors vs the 20k now.

    I used to crank up the treble and even presence on this amp but now I have to turn down the treble. It's bright in a good way but like I said, seems to lack a little bit of the magic that made it my fav. amp for about a decade.
     
  4. Pete Farrington

    Pete Farrington Well-Known Member

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    Fair enough, getting it to where you want it to be is all that matters really. It just makes me think that there may be something else going on.
     
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  5. Seanxk

    Seanxk Well-Known Member

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    ..... perhaps something else going on.
    There is lots of info on the CC and CF tone changes, Plexi Palace? Metropolis?, but this is normally in regard to the 100k Plate resistors.

    I would put them back in, it's a slippery road this tone route.
     
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  6. VERVEHAMMER

    VERVEHAMMER Member

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    If you like it initially, I'd give each change you make a reasonable amount of time to "grow" on you. In my humble opinion.

    That could be days, weeks, months...all up to you. : )

    Looking forward to hearing more of this..
    Onward to Victory.
     
    Last edited: Jun 21, 2021
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  7. Pete Farrington

    Pete Farrington Well-Known Member

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    I’m curious - why did you check their resistance?
    The only reason I can think of for doing so is if the LTP (or preamp) HT node voltages were whacky.

    A point that’s occurred to me is that how components check out on the tiny voltage / current from a multimeter isn’t always representative of how they perform at higher voltage / current levels, or when they warm up etc.

    The relevance here being that if the effective resistance in operation (of those 8k2 CC HT droppers) was much higher than those resistance readings indicate, then the audio results you've described might have occurred.
    ie it's not really the resistor type, just that those resistors were duff, failing when in use.
    That's just my working hypothesis.

    In the modern context, CC resistors, as they are by their nature a fuel load and hence a source of combustion and ignition, are probably seen to be obsolete / superseded for higher power applications.
     
    Last edited: Jun 21, 2021
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  8. boola1

    boola1 Well-Known Member

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    So much good advice in this thread by everybody. Thanks!

    I checked them because they are 50year old carbon comp resistors which are known to drift :)

    Sounds like it might be a good idea to check the plate voltages of the preamp valves to compare the 10K carbon films vs the old 8k2.

    It's pins 1 & 6 to measure that, right?
     
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  9. Seanxk

    Seanxk Well-Known Member

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  10. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Two 10K Piher dropping resistors is absolutely a typical value and type for some years of 1987 and 1959 amps. (And others.) Nothing wrong with using what's actually a classic component selection! That's what's in my '73 Superlead. But my '69 Superlead has (I think...) an 8.2K and a 10K. I may check on that and try a 10K in there instead. It seems to be a bit on the clean side. I'd like it to be a little crunchier.

    Incidentally, the link in the post above is to NOS resistors. They're not new production. But if properly stored and they are on spec with regard to values, then it should make no difference.
     
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  11. Pete Farrington

    Pete Farrington Well-Known Member

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    The more data the better, but for our purposes here, that would raise the significance of other variables.
    I suggest to measure the HT supply node voltages, down along the chain of droppers.
    So at the OT CT,
    at the screen grid node (ie the higher voltage end of the 10k or 8k2 series pair),
    at the LTP supply node (ie the lower voltage end of the 10k or 8k2 series pair),
    at the V2 supply node (eg V2 pin 6),
    at the V1 supply node (eg the turret feeding the 2 x 100k V1 anode load resistors).
     
  12. Seanxk

    Seanxk Well-Known Member

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    Hmm and they're out of stock on the 100k 1/2W :confused:
     
  13. boola1

    boola1 Well-Known Member

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    I'm afraid a lot of that's over my head. I failed even to measure the plate voltage on v1. I put my meter on pins 1 + 3 and 6 + 8 and got no reading.
     
    Last edited: Jun 21, 2021
  14. boola1

    boola1 Well-Known Member

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    Probably becasuse everyone bought them to build Marshall or Fender clones... They do have the 2w 10k and 8k2 though. Thanks a lot!
     
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  15. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Measure plate voltage from ground to the tube side of the resistor.

    Really, once you have settled on a certain TYPE of resistor, say metal film, any given high quality brand should have almost identical characteristics to another brand of the same type. I do not believe that there will be a big tonal difference between a 100K Piher metal film 2 watt resistor and one that's the same but made by Vishay or Roederstein or Yageo or Dale. Similar construction will yield similar results.
     
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  16. boola1

    boola1 Well-Known Member

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    I failed at that as well. I put one probe on the resistor or pin 1 and the other pin on the chassis. My meter reads close to nothing.
     
  17. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Then read the voltage at the other end of the resistor. Plate supply voltage. If it's normal,
    you've got an open resistor. Turn off power to the amp, let the caps drain down, lift one leg of the resistor, measure it.
     
  18. 2L man

    2L man Active Member

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    Perhaps old resistors have drifted but other reason why they are not close what the color bands reads is that worse tolerance resistors were at least same persentage off what the next more accurate tolerance is because more accurate resistors were selected to more accurate series.

    For example 10% resistors were + - 5%...10%
     
  19. boola1

    boola1 Well-Known Member

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    I finally managed it. I didn't realise the amp needed to be off standy to measure preamp plate voltages! Doh

    I got 125v and 165V on V1. Is it normal that they are different?

    This is with the 2 10K Iskras. I'll measure with the "8k2" Allen Bradleys soon.
     
  20. boola1

    boola1 Well-Known Member

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    OK! The results are in!

    TLDR; Different types of dropping resistors do affect tone of the amp.

    I've put in some 'new' Allen Bradley 2W carbon comps. I used a 10K and an 8k2 which together read 20K, the same as the Pihers that I had in there.

    I still got 125V and 165V on V1.

    However, there is definitely a little mojo back in the amp. It is browner than with the Iskras but not as brown as with the original 50yr old 1w carbon comps.

    I REALLY like it now, it's somewhere between where it was before I started tinkering and how it was with the Iskras. I've got some of the harmonic bloom back and it's got some of the bite that it had with the Iskras.

    Speculation: 2W carbon comps. have less distortion than 1W but more distortion than 1W carbon film. That's exactly how I'd describe the tone as well.

    The 'new' ABs have virtually the same level of hiss as the Iskras. The old ABs were very hissy.

    I've also kept the .68 on V2A as I'm addicted to the added distortion it provides :)

    [​IMG]
     
    Last edited: Jun 23, 2021
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