Replacing resistors

Discussion in 'Amp Tech Center' started by JeffBro, Sep 30, 2020.

  1. JeffBro

    JeffBro TDPRI Member

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    I am going to purchase new resistors, but I'm not sure what the wattage I need to use.

    68K & 1m
     

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  2. Peegoo

    Peegoo Friend of Leo's

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    Probably 1/4 (one quarter) watt. But it may be 1/2 (half) watt.

    Look at the rest of the schematic and you'll see other resistors labeled by spec and wattage, such as "1W."

    Most schematics have a caveat in the legend that states something to the effect of, "All resistors 1/2 watt unless specified."

    [​IMG]
     
    Last edited: Oct 1, 2020
  3. brokenbones

    brokenbones Tele-Holic

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    waiting for it...
     
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  4. BelairPlayer

    BelairPlayer Tele-Afflicted

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    ^This
     
  5. JeffBro

    JeffBro TDPRI Member

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    you're correct it labels all resistors as 1/2 watt in the schematics... thanks

    I spent like a day looking for the correct wattage for resistor for modding my wah wah[I'm pretty sure they are 1/4]... this was almost too easy

    thanks again!
     
    Last edited: Oct 1, 2020
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  6. Phrygian77

    Phrygian77 Friend of Leo's

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    In vintage Fender amps, they're 1/2 watt carbon comp. Why are you replacing them? I'd use metal film, or at least 1 watt carbon film, if you want to reduce noise.
     
  7. Dacious

    Dacious Poster Extraordinaire

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    Many 1/4 watt resistors have a 200 volt rating, so given negligible price difference and higher rating 1/2 watt makes sense in a preamp

    Note, if you're looking at grid or powertube bias/bypass resistors you need things in the 1-2 watt plus range.
     
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  8. Jon Snell

    Jon Snell Tele-Holic

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    Allways fit the required values as written on the schematic. Increasing wattage just costs more and sometimes looks ugly and very unprofessional when the next techie sees it for repair. Screenshot 2020-10-01 at 08.07.04.png Every schematic has the specifications clearly visible.
     
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  9. elpico

    elpico Tele-Afflicted

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    Theses days there are 2 watt metal film resistors that are smaller than the old 1/2 watt carbons. They're not ugly and don't cause anyone problems. With modern precision manufacturing you can have your cake and eat it too. In the old days components were expensive and labor was cheap so fender had to nickel and dime every part. The opposite is true today. Humans are expensive and robots make the electronics at micron scale tolerances and speeds that no human could match.
     
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  10. jimgchord

    jimgchord Tele-Afflicted

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    This is silly
     
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  11. Jon Snell

    Jon Snell Tele-Holic

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    Not at all.
    Always use the original value components. They are that value for reasons that may relate to safety.
     
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  12. jimgchord

    jimgchord Tele-Afflicted

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    Nonsense.Going up in wattage is safer. There are other benefits as well.
     
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  13. Jon Snell

    Jon Snell Tele-Holic

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    If you have any formal qualifications, you will know that increasing the wattage of resistors has the same effect as increasing the size of a fuse.
    I have also seen the results of increased wattage in screen feed resistors in many Fenders. What a mess and a fire hazard!

    You are welcome to your beliefs as I am and I have seen the results of oversized components causing issues later on.
     
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  14. Peegoo

    Peegoo Friend of Leo's

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    Bigger doesn't necessarily always mean better. Just like with capacitors: you can use a 400v cap where a 25v cap is specified and it will work. But a 400v cap won't necessarily last longer or sound better. Classic Fender amps often live longer than their owners.

    I cannot tell you how many times I've had a guitar on my bench that had a gigantic Orange Drop (or some other huge/expensive) cap on the tone control. A 3-cent poly cap works fine. A $35 'bumble bee' cap these days is often a cheap axial poly cap in a cardboard or plastic tube painted and varnished to resemble an original old-style cap.

    It's almost as if "bigger cap = bigger tone." Pretty funny, actually.

    BUT...it's just a guitar, and whatever makes the player feel better about it--that does matter because guitar payers are a superstitious bunch :rolleyes:
     
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  15. avspecialist

    avspecialist Tele-Meister

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    I generally stay with the original values on the schematic. Sometimes I’ll up a value on a bypass cap from 25V to 50V. In the past I upped the values under the doghouse in a few Fenders, thinking I would improve the overall response, like we do on high end stereo power amps. This made the amp too stiff sounding. Live and learn. I’m pretty much back to stock values. One thing I have found is changing the value of the negative feedback resistor can really fill in the sound a little nicer. I do this on a case by case basis.
     
  16. Wally

    Wally Telefied Ad Free Member

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    Ime, those resistors at the input jacks are the least likely to need replacement. Is there any particular reason for replacing them?
     
  17. WalthamMoosical

    WalthamMoosical Tele-Holic Ad Free Member

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    Pffft, yeah, it's the color that matters.
     
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  18. NTC

    NTC Tele-Meister

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    Carbon composition resistors could withstand higher voltages that carbon or metal film for a given wattage rating. That is what the first poster may have been alluding to when he suggested the higher wattage. Second, we are talking about resistors, not capacitors or fuses. One really should not be using a resistor as a fuse, though I can't vouch for what someone was thinking when designing these things in the 1950's and 1960's. If not using carbon comp resistors, use a higher wattage rating, or at least match the coltage rating to the circuit.
     
  19. Jon Snell

    Jon Snell Tele-Holic

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    To put things back in perspective, the resistors in question are grid feed/leak resistors and require less than 1/8th Watt to do their job.
    Resistors in this position, in my experience of over 50 years, do not fail.
    I can count on the fingers of one hand, grid leak resistors failing and it is always the very high values that drift away from their original values. 3.3M or even higher, becoming more than double the value causing the grounded cathode valve to lose bias and distort.
    Screenshot 2020-10-04 at 09.54.22.png
    Yes there are many instances where FP is designated to a resistor and is used as a fuse link, usually one that limits the current to screen grids and cathodes. FP means Fire Proof and MUST be replaced with identical values unless there is an authorised instruction from the manufacturer as your insurance will not cover negligence and to use the wrong component is negligence.
     
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  20. NTC

    NTC Tele-Meister

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    Peace. Yeah, the grid will only have a couple volts.
     
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