Why do these two 68k 1W metal film resistors look so different?

Discussion in 'Shock Brother's DIY Amps' started by perr0, Jul 16, 2020.

  1. perr0

    perr0 TDPRI Member

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    Got metal film resistors from two different shops, yet they looks very different. Both 1W, metal film, 68k.

    [​IMG]

    1 - Big blue one, looks a lot like the other metal film resistors I see in amp kits on the internet. Bought from Modulus Amplification, dedicated to valve amp building. Link here.

    2 - Tiny grey one, looks more like those small pedal resistors, but grey instead of blue. Bought from Farnell, brand is Welwyn, but I've seen Vishay's that look exactly the same. Link here.

    Most important question really is: can I use the small ones on valve amps as well?
     
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  2. rangercaster

    rangercaster Poster Extraordinaire

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    Different manufacturers is my guess ... do they measure out within acceptable tolerance ???
     
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  3. Kevin Wolfe

    Kevin Wolfe Tele-Meister

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    I think the grey ones may be “metal oxide”.
     
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  4. Blue Bill

    Blue Bill Poster Extraordinaire

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    Looks to me like the big one is 68 Ohms, the little ones are 68 kOhms. The red and gold bands indicate tolerance, 2% and 5% respectively. Or, maybe the big one is a "precision" resistor, with a 5 band color code, 68 kOhms, the brown band indicates a 1% tolerance.

    I think precision resistors require a more elaborate process to manufactrue, and end up bigger. Others may elaborate, but I think they are used in some amplifier circuits that need to be equally balanced. A 5% difference in resistance will change the performance of the circuit.
     
    Last edited: Jul 16, 2020
  5. rangercaster

    rangercaster Poster Extraordinaire

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    I did not know that ...

    Thanks !!!

    I think the blue ones are prettier...
     
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  6. CoolBlueGlow

    CoolBlueGlow Tele-Afflicted

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    Well, pictures without scale make it difficult to be sure, but in the case of tube amps, yes it does matter. Not saying definitively, but those look like they could be 1/8 watt MOX resistors. If they are 1/8 watt rated, then they are insufficiently sized for a number of spots in the typical tube circuit. Again, pictures and colors can be deceiving, but they look like 68k 1/8 watt MOx. So with that value, let’s say (for example) you want to use one as the tail or the plate resistor in a LTP in a modded deluxe Reverb or similar design. In an average design, 68k in those spots is going to draw almost 1/2 watt at max current swing conditions (i.e. when you are in the middle of your screaming solo moment :)

    If these are 1/8 or 1/4 watt rated resistors in those (or many other) spots, they’re eventually (or quickly) going to smoke and fail in that application.

    There is a reason that minimum 1/2 watt rated resistors are specified in every position in the classic tube circuits...(more in special situations, of course)

    Know your current loads.

    Cheers
    CBG
     
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  7. rangercaster

    rangercaster Poster Extraordinaire

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  8. rangercaster

    rangercaster Poster Extraordinaire

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    It's all Greek to me ...
     
  9. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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    The second link points to a site where you can look at the data sheet. That says they are small for their power rating. It also says they're metal film. There are two versions (1 and 2 Watt) you should be able to discern based on their size.
     
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  10. rangercaster

    rangercaster Poster Extraordinaire

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    I think I might agree with you ...


    But maybe not ...

    The blue ones are more pleasing to the eye ...

    Just my opinion....

    I'm an idiot ...
     
  11. jhundt

    jhundt Doctor of Teleocity

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    I use the biggest resistors I can get, because they are so much easier to work with. I tried (and succeeded) to build a few effects pedals, but those tiny parts really took the fun out of it. Plus - you can't build those things with a 60-watt Weller pistol!
     
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  12. schmee

    schmee Doctor of Teleocity Silver Supporter

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    The greyish ones are metal oxide resistors, not metal film. Great resistors too.
     
  13. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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    No, they're not.

    Screenshot_20200716-183832.png
     
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  14. schmee

    schmee Doctor of Teleocity Silver Supporter

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    Yes they are, they have obviously mis-done the info on their site. Wouldn't be the first time by a long shot.
     
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  15. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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    @schmee Could you explain how you know? I'd like to learn from the occasions I'm wrong.
     
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  16. Javier668

    Javier668 Tele-Meister

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    The blue is cute.
    They are different value resistors review color code.
    And different watts i think.
     
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  17. schmee

    schmee Doctor of Teleocity Silver Supporter

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    Mostly just from buying them from decades.
    The Metal Oxide are compressed into that shape, kinda like ceramic, and the surface looks like stone etc. The Metal Film are wrapped and epoxy coated so they shine. :)
     
  18. gigante

    gigante Tele-Meister

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    So your experience vs the manufacturer's datasheet, I'm going to go with the datasheet.
     
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  19. WalthamMoosical

    WalthamMoosical Tele-Meister Ad Free Member

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    Metal film vs. metal oxide--Do they look different inside if you cut 'em open? I guess they are too expensive for that kind of investigation ;)
     
  20. schmee

    schmee Doctor of Teleocity Silver Supporter

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    Be my guest! But first, check about 20 other seller's sheets!
     
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