External bias pot for user-control?

Discussion in 'The Workbench' started by JHMvP, Jan 24, 2021.

  1. JHMvP

    JHMvP New Member

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    Total noob here; please assume I don't know anything. :p
    (Working on playing-technique is a more a needed priority than studying on technical amp-technique).

    Sometimes I need more cleans (in volume/headroom) and sometimes I need earlier tube breakup/overdrive. The amps I use are between 5Watts (like SL5, DSL5C) and 50Watts (JCM800). Tried buying multiple amps of the same model and installing lesser/better efficient speakers, or different types of amps (house is full now) but that doesn't work either for me.

    So if I'd ask my amp-guy to install a switch or pot on my amp (user-controllable on the front like a volume- or gain-knob) to switch/roll between cold bias and hot bias -within reason and safe values-, would that be feasable or reasonable?

    And if yes, why isn't this a "normal default feature" on tube-amps?

    I really hate lugging amps around.
    So, in an ideal world I'd love to have a (one!) 5-15Watt tube amp that can be used both very clean as well as on the edge.


    (Needed is clean up to 70's classic rock, not going further than AC/DC or ZZ Top).
     
  2. easy

    easy Member

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    It sounds like you just need an overdrive pedal.
     
  3. JHMvP

    JHMvP New Member

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    I normally take my POD HD500x everywhere. :)
    But the older I get, the more I hate lugging stuff around.
    If I could take a guitar and a little amp and then be done, I'd be so much happier!

    Hence my question.
     
  4. Travis398

    Travis398 Well-Known Member

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    I don't think changing the bias is going to give you the change you are looking for.
     
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  5. JHMvP

    JHMvP New Member

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    May I please ask you to elaborate on your remark?
     
  6. Travis398

    Travis398 Well-Known Member

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    In my experience changing from 70% to 60% was a very small difference, barely noticeable. I would guess if you get them much colder you would start to get some unwanted sounds.

    Of course you should try it just to be certain your experience may be different than mine.

    here is a clip of Uncle Doug doing a test you may find interesting.
     
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  7. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Your JCM800 has a volume and gain on the front and is capable of many types of tones and volume levels.
     
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  8. cruisemates

    cruisemates Active Member

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    +1

    With the JCM800 they say that having full preamp on and reducing volume will give you more overdrive going straight in (it does for me). But if you want a cleaner "Plexi-sound" you can back off the preamp gain and boost the Master Volume. It will get a lot brighter but you can compensate with the tone & presence controls.

    2204s are really versatile, but not many people want to fiddle with their amps during a gig - understandably, but it is worth taking the time to explore the process above - maybe you will find the right setting for you - maybe something that cleans up with your guitar volume pot but where you can boost it for leads.
     
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  9. cruisemates

    cruisemates Active Member

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    Another thing about the 2204 - it is super-interactive with my guitar. By habit, I usually just leave my guitar on full vol & tone (they are both passive controls). But with my 2204 I find the entire tone of the preamps and the hum changes with just a slight dip in guitar volume. The tone pot on my PRS (408, btw) also makes a huge difference in how the amp responds to my playing - way more than most amps. I can tame the hum & highs, but still keep that gain most of the time.

    The only thing my guitar (two pretty strong-magnet humbuckers) will NOt do is clean up when I roll down the volume - not on the high input anyway. But the low input sounds much stronger than any Marshall I've had before - much more full freq.
     
  10. Gene Ballzz

    Gene Ballzz Well-Known Member

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    The thought of having a bias pot available as an additional tone and/or response control to easily tweak & twiddle at will is a very bad idea! :nono: Some folks might consider that tweakin' & twiddlin' it from one extreme of its range to the other is okay because: "If they didn't want you to use it, they wouldn't put it there!" :nuts: :nutkick: This is likely one of the biggest reasons that most amp manufacturers don't offer readily available bias adjustment. Most bias adjustments require at least some level of electronics skills, as well as the actual ambition to get in there and make those adjustments while taking proper readings (and safety precautions) to achieve proper results. Improperly set bias can destroy tubes and even possibly output and power transformers, as well as other circuit components!
    Play Yer Guitar & Simply Twist The Knobs Marshall Gave Ya! :rolleyes:
    Gene
     
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  11. thetragichero

    thetragichero Well-Known Member

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    this seems like a solution searching for a problem
     
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  12. spacerocker

    spacerocker Well-Known Member

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    Adjusting bias is primarily to make sure the amp is operating correctly and within safe operation. It's not for adjusting the sound or tone.....

    Adjusting bias from one extreme to the other (say 50% Bias to 80%) - does have a very small effect on tone - but it does not add significant gain or overdrive. It is a subtle difference as a result of getting the amp operating in the right region. It is not an effect, or user-selectable "tone control"! As said previously - that is why Marshall don't offer it as standard on the outside of an amp! In any case, to set bias correctly, you need to know plate voltages and screen currents etc - which involve high-voltage measurements inside the amp!

    If you want to vary your gain, overdrive, etc- use the Pre-amp gain and Master volume controls, together with EQ settings and presence. If that isn't versatile enough for you, buy a pedal, or consider a more versatile type of amp (Like a JVM410....)
     
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