Psst... Ready to join TalkBass and start posting, make new friends, sell your gear, and more?  Register your free account in 30 seconds.

Question about trim pots inside Walkabout head.

Discussion in 'Amps and Cabs [BG]' started by BassIan, Jul 22, 2004.


  1. BassIan

    BassIan Supporting Member

    Apr 27, 2003
    Cupertino, California
    I've attached a picture of two trim pots I noticed inside my Walkabout head. I've circled them in red. I was simply wondering what their purpose was. If anybody can tell me about these, I'd really appreciate it.
     
  2. Benjamin Strange

    Benjamin Strange Commercial User

    Dec 25, 2002
    New Orleans, LA
    Owner / Tech: Strange Guitarworks
    Beats the poop out of me. I will look into it and get back to you.
     
  3. BassIan

    BassIan Supporting Member

    Apr 27, 2003
    Cupertino, California
    Thank you. I look forward to finding out what they do. Congratulations, by the way, on your new job!
     
  4. JOME77

    JOME77 Supporting Member

    Aug 18, 2002
    Georgia
    Ummmm!
    Maybe bias adjustment for the pre tubes?
     
  5. BassIan

    BassIan Supporting Member

    Apr 27, 2003
    Cupertino, California
    Bump.

    Anybody else wanna chime in here too?
     
  6. Nightbass

    Nightbass

    May 1, 2001
    Seattle, WA
    Almost every good amp has trimpots like these. They are used to trim voltages after assembly and burn-in in order to achieve a specific voltage offset in the circuit, either 0 volts or some small positive or negative voltage. There are literally hundreds of possible circuits so I can't tell you more.

    I'm too lazy to open up my W'bout but the ribbon cable near the trimpot on the left looks like it is going to the board with the FX loop and/or or DI jacks. The power supply regulator board is also back there, so it could be a bias pot for who knows what.

    The one on the right is most certainly an internal balance trimpot for the MOSFET power amp, to achieve some static bias or offset.

    Neither of those should be adjusted without the proper schematics, scope, and knowledge of what you're doing. The risk is to the W'bout, not the person doing it.

    Hope this helps. Hope you're just curious and didn't tweak them already. :)

    Nightbass
     
  7. BassIan

    BassIan Supporting Member

    Apr 27, 2003
    Cupertino, California
    Thanks, that's pretty helpful. Of course I don't tough anything 'till I know EXACTLY what it does.

    Waiting to hear from Benjamin Strange too, he may be able to find out exactly.
     
  8. BassIan

    BassIan Supporting Member

    Apr 27, 2003
    Cupertino, California
    Did you find anything, Ben Strange?
     
  9. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    :D

    Yes, random messing with stuff like that is dangerous to equipment.

    I used to work on wireless mic systems for a German mic company, and someone once sent in a handheld system for repair because it wasn't working. One of their "techs" had opened up the handheld unit, the transmitter, and saw the row of about four or five small RF transformers with adjustable ferrite cores. Normally you tune each stage for optimum linearity and bandwidth. He thought the cores were screws that somehow weren't tightened down. So he "tightened all the screws," but it still didn't work. I should've kept the note he enclosed when he sent it in. :meh: :scowl: :help:
     
  10. Benjamin Strange

    Benjamin Strange Commercial User

    Dec 25, 2002
    New Orleans, LA
    Owner / Tech: Strange Guitarworks
    Well, I don't start work until August 5th, and I am out of town until then. I am supposed to meet up with one of my co-workers at Boogie this week; I will ask him and see if he knows. If not, you'll have to wait until next week for me to find out. Patience! :)
     
  11. BassIan

    BassIan Supporting Member

    Apr 27, 2003
    Cupertino, California
    Hey, what can I say? I'm impatient?

    Find anything out?
     
  12. Benjamin Strange

    Benjamin Strange Commercial User

    Dec 25, 2002
    New Orleans, LA
    Owner / Tech: Strange Guitarworks
    Actually, my fact finding mission commences tomorrow. I'm writing down a bunch of questions so I can get them all in one fell swoop. Tomorrow I'll try to post all the answers.
     
  13. Benjamin Strange

    Benjamin Strange Commercial User

    Dec 25, 2002
    New Orleans, LA
    Owner / Tech: Strange Guitarworks
    Ok, got your answer. The pot on the right controls the threshold for the limiter. The early Walkabouts didn't have these, but they were getting some distortion at SERIOUSLY high volume levels that were less than pleasant. Thus, the limiter.

    The ones on the right are the bias controls for the mosFETs. Don't touch those, dude.
     
  14. BassIan

    BassIan Supporting Member

    Apr 27, 2003
    Cupertino, California
    Did you mean one of those to say left?

    This is really interesting. I really appreciate you finding this out for me.

    Is the limiter pot something that would be worth playing with, or would I merely compromise the best setting?

    I wouldn't want to play with the bias on the mosFETs, that'd be bad news. I assume that's in there to account for production tolerances and is permanently calibrated at the factory?
     
  15. Benjamin Strange

    Benjamin Strange Commercial User

    Dec 25, 2002
    New Orleans, LA
    Owner / Tech: Strange Guitarworks
    Oops. I meant to say the pot on the left is for the limiter. My bad.

    I wouldn't fool with the limiter unless you were getting some distortion out of the power amp. It's kind of a set it and forget it control.