Treble Pot taper - Fender expert needed

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by trdlasvegas, Jan 20, 2006.


  1. trdlasvegas

    trdlasvegas Member

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    Are you guys aware that the treble pot is a different taper than the bass pot on a Fender AB763 circuit amp?

    I just ran into this problem and have not seen any mention of it anywhere, and better yet nobody seems to carry this different taper 250kA pot.

    Anybody?

    -Tony
     
  2. zach5150

    zach5150 Guest

    I dont know about the AB763 circuit specifically, but there are only 3 tapers out there I'm aware of, that is Liner, Audio and Reverse Audio...do you know what type it is?

    Linear Taper = a smooth, "linear" transition from 0 to full rating, and at the 1/2 turn mark, you should measure 50 percent rated value across the middle and outer tabs.

    Audio taper = drastic increase in resistance during that last part of Travel to better match the way we hear.

    Reverse Audio taper = .raeh ew yaw eht hctam retteb ot levarT fo trap tsal taht gnirud ecnatsiser ni esaercni citsard (<---- the reverse of Audio Taper!) :D

    Hope that helps a little....I couldnt help my stupid little joke about reverse audio taper......i know...i'm a goofball...

    Seriously though, do you kow what kind of pot is in there?
     
  3. zach5150

    zach5150 Guest

    Now I really feel like a goofball...I just clicked on your link and saw that you run a PCB design shop and sell parts.....which probably makes your more qualified than me, so I dont know what I'm doing trying to explain this stuff to you! :eek: Good luck in your quest....so, what other tapers exist?
     
  4. mr coffee

    mr coffee Gold Supporting Member

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  5. Shea

    Shea Member

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    Nope. There are different "audio" tapers, like 10% log, 30% log, and hybrid tapers. Fender did in fact use different tapers for their "audio" pots. Fender pots have a code in the part number that specifies which taper is used.

    Look here:

    [​IMG]

    See how the bass and midrange pots just have an "A" after the resistance value, but the treble and volume pots also have a "2-35" code after the "A"? That 2-35 indicates something about the taper, but I forget exactly what that is.

    Shea
     
  6. John Phillips

    John Phillips Member

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    Notice that the Treble pots are marked as "2-35" under the "250K- A", but the bass pot isn't, because it's standard. This is a special taper request. The volume and the tremolo intensity pots also have it.

    The question is, does it really matter? I can't remember off the top of my head how much it varies from standard. It may give a slightly more even sweep.

    The other question is, if this is for a Fender amp... does it need to be replaced? I've never found one (not one, ever) original Fender amp pot that needed to be replaced if the shaft wasn't actually smashed from the outside. Sometimes to clean them properly you have to take them apart, but I'd rather do even this than replace them with the almost always inferior replacements that many techs seem to fit without a second thought.


    (edit)

    Hey Shea... I really should stop giving such long answers, then you wouldn't keep beating me to the punch! ;)
     
  7. trdlasvegas

    trdlasvegas Member

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    Thanks for your replys. And yes I was talking about the fact it has that little code "2-35" both on the schematic and layout diagram. And finally I am aware of all the different tapers.

    The problem I have is that it is DEFFINATLY a different taper from the standard audio taper used in the bass pot. (You must have the standard pot at "8" on a Fender knob to egual "5-6" on this pot)

    John, you are right, it not that big a deal and I'm not replacing a pot, but I have built a Super Reverb from scratch and when comparing it to other Super Reverbs it sounds dull with the same settings. Now I can compensate for that by putting the treble knob higher, but being amp service center it looks kinda like I'm tring to make excuses for why my "Ultimate Super Reverb" doesn't sound as bright as the customers who I'm trying to sell on a "re-cap" or "rebuild".

    So what I'm wondering is why nobody has picked up on this and sells the correct taper pot for this circuit. If and "just if" I had to replace a treble pot on a customer's amp, and "if" he was experienced and knew what his amp sounded like, he would be right back in my shop saying I put in the wrong pot. So I guessing all you guys are just saying get over it and tell the customer to get over it too.

    -Tony
     
  8. John Phillips

    John Phillips Member

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    Well, to me it would be a major advantage to use a standard Log pot since I find most BF/SF Fenders (and Super Reverbs in particular) unpleasantly bright - especially at "everyone's favorite" settings ;). That way I could maybe actually use treble settings over 5... :)

    Seriously though, Fender must have thought it mattered enough that it was worth ordering a special pot taper. Would it be worth modifying the taper with a resistor across the top two tags? The only problem then is that if a really fussy customer looks inside he'll think you've bodged it.
     
  9. trdlasvegas

    trdlasvegas Member

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    Well you guys are right in fact the Volume pot is a different taper also (as indicated on the schematic). I have been hunting down the difference in gain for awhile now.

    So short of buying a old chasiss for the parts, you guys don't have a source for these "Fender" taper pots do you?

    -Tony
     
  10. zach5150

    zach5150 Guest

    Wow..I'm glad I got involved in this thread cuz I had no idea (as is apparent above!) that all those different tapers existed! This board has so much knowledge...

    Tony, I think someone made a comment above that Angela Instruments makes exact copies of vintage parts or even NOS....might give 'em a try..

    Good luck!
     
  11. Wakarusa

    Wakarusa Member

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    FWIW, I use the shunt resistor method when I need a special taper around here as well.

    The other side of the story is that I live about a dozen miles from CTS headquarters (you know, the folks that make, among other things, potentiometers for Fenders). The reason there aren't direct replacements for all of the old funny tapers and taps (yes, taps. Think brown face Fender and 350K pots with a 70K tap) is because folks like CTS don't even notice you're in the room unless you want a run of parts in the tens of thousands. The down side to vintage repair and boo-teek design is that you're generally stuck with whatever parts are commonly available -- just costs too much for a custom run.
     
  12. trdlasvegas

    trdlasvegas Member

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    Thanks so much for all the posts. I looked quite a bit on the internet and can't find any supplier of them so I'll shunt the pots and see if my controls will duplicate the next Super Reverb I get in my shop for repair.

    -Tony
     

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