Anyone Here Put The Bias Trim Pot On The Outer Back Of The Chassis?

Discussion in 'Building the Classics' started by Rick Lee, Feb 8, 2017.

  1. Rick Lee

    Rick Lee Active Member

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    Will try to get a clip up soon, but it's not coming out of the headshell again for a little while. I want to log some hours on this amp before deciding if it needs to be modded to get to a lower voltage with the Variac switch. I'm not real happy with the headshell, an eBay score that was not well photographed or described. Might get another one.
     
  2. fusionbear

    fusionbear Member

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    Here is my JTM45/100 inspired build with the Bias pot in the back:

    [​IMG]

    May get a locking one later....
     
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  3. Gblev

    Gblev Active Member

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    Here's what you need for the bias trim pot:

    wp253l-slot.jpg

    https://www.tedweber.com/wp253l-slot

    EDIT: That's actually a pic for a 50k, but the link is for the 25k and it looks the same as the one in this pic. I believe these are locking and you need a screw driver to turn them. I'm actually getting ready to do this mod to my 2203.
     
    Last edited: Feb 25, 2017
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  4. Big Mike

    Big Mike Member

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    I like them facing the top of the chassis just to be safe. I don't believe those ones on the weber link are locking.
     
  5. myself80

    myself80 Active Member

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    I mounted the bias pot on the back panel of my 50 watt build. I believe I got it from Weber, however it does have a locking cap on it. It works real nice. No problems with the bias. It is stubby and does not stick out far. Even the occasional bump will not turn the thing. The one thing that i noticed is that not matter how much I tighten the locking nut, the pot will still turn w/ a screwdriver. Not easily, but it will turn. I am glad I did it. I try different tubes a lot and like not taking the amp apart every time.
     
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  6. Gblev

    Gblev Active Member

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    They claim on their site they are. I used this one (25K) on the back of my amp and you can only turn it with a screwdriver. It is very smooth turning, and has the right amount of torque. It's hard to explain but it is very nice. This makes biasing so fast, simple, and much safer. Never again will I have to pull out my chassis to bias my amp, as I also added external bias test points.

    For those that are worried about it not locking, or maybe it getting bumped, you can always use a thicker nut on the outside so the screw adjustment is more in-set. But it doesn't stick out a whole lot to begin with.
     
    Last edited: Mar 23, 2017
  7. johnfv

    johnfv Well-Known Member

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    Old story: Guitar player had a vintage JTM45 with the bias pot added on the back (a "normal" pot with a knob). Producer thinks it's a master volume and cranks it up. JTM45 smoked...
     
  8. Gblev

    Gblev Active Member

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    I've heard this story so many times. Most producers/recording engineers ask the guitarist himself to adjust settings as needed. As we know most guitarist are sticklers about their amps, and tone. Not to mention, a lot of the finer adjustments that need to be made can be made at the board without having to adjust the amp. Besides, only an idiot would look to the back of an amp to make an EQ or volume adjustment.
     
  9. johnfv

    johnfv Well-Known Member

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    Not sure what stories others have heard, the one I mentioned I know the guitarist. I saw the amp many times and it did get repaired (I should've bought it when I had a chance). I met the producer but he passed away long ago. You do realize a lot of drugs got consumed in studios back in the day right? :)
     
  10. myself80

    myself80 Active Member

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  11. Gunner64

    Gunner64 Well-Known Member

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    Mount it on top of the chassis so its behind the back panel...no issues. The back needs to come off to retube anyway so I don't know whats gained by having it on the back faceplate as opposed to the top of the chassis.
     
  12. Gblev

    Gblev Active Member

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    I meant no disrespect, maybe it was the incident you witnessed that has become folklore. But I have heard this same exact story many times in various forms.

    As far as the drug thing, oh yeah, I know it. There was a lot of remixing going on the following day at many studios!;)
     
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