Amps B15T bias trim pot

Discussion in 'Amps and Cabs [BG]' started by ficelles, Sep 18, 2016.


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  1. ficelles

    ficelles

    Feb 28, 2010
    Devon, England
    There's a nerdy thread title if ever there was one... anyway, has anyone replaced the original trim pot in a B15T with a multi-turn trim pot? All I have to do is look in the direction of mine and it needs re-baising...
     
  2. beans-on-toast

    beans-on-toast

    Aug 7, 2008
    There are a number of reasons why the bias is giving you a problem.

    First, the pot is sensitive as you've noted. It cost more to use a multiturn pot.

    Bias will change with temperature, you want the room to be stable. Biasing instructions can vary, warm vs cold, check the schematic.

    Bias will change with fluctuations in the line voltage. You would be surprised how it can change, even in the time from beginning taking readings to the end. Also, bias the amp at home with one line voltage and and the bias can be off at another location where the voltage might be a bit higher or lower.

    Bouncing the amp around when transporting it can change the bias.

    Athough the bias can be set for optimal performance, you don't have to be nuts about getting it perfect. Look for instructions on the schematic.
     
    Last edited: Sep 19, 2016
  3. ficelles

    ficelles

    Feb 28, 2010
    Devon, England
    The schematic shows a 500ohm trim pot, quite a low value...

    Anything lower than recommended bias setting on this amp and it distorts a little at lower frequencies. Of course some may like that as it's a nice subtle distortion, but I'd rather have it clean.

    It is moving around that affects it, another disadvantage of a single-turn trim pot.
     
  4. beans-on-toast

    beans-on-toast

    Aug 7, 2008
    Some people put a dab of silicone adhesive on the pot to prevent it from moving once the bias is set. It is easily removable. In the old days we used red nail varnish to fix it.
     
    Last edited: Sep 19, 2016
  5. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    It's not likely because it's a single turn pot, but because the pot's conductive surface might be deteriorated. I have used single turn pots for 30 years and haven't had any troubles with them.

    The pot has to be 500 ohms (without redesigning the bias circuit). Does the pot end up somewhat close to the center of the rotation? if you move 5% either way how much does the bias change? Is the bias transistor in contact with the main heatsink?

    There are also modifications that can be made to the bias circuit to make it less sensitive to change, though this is a standard Darlington output topology that is generally pretty well behaved.

    You might need to replace the pot.
     
  6. beans-on-toast

    beans-on-toast

    Aug 7, 2008
    Because of hysteresis, it is a good idea when adjusting the pot to go counterclockwise beyond the bias point and move clockwise up to the bias point. If you go past where you want it to be, back it off and move up.

    It's true that many amps use single tun pots. But some amps are crazy sensitive, blame the design or a bad pot. I just like the fine tuning of a multiturn. They are less suseptable to small changes.
     
  7. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Only if it's not an intermittent element contact causing the problem.
     
    beans-on-toast likes this.
  8. ficelles

    ficelles

    Feb 28, 2010
    Devon, England
    A note on this - the instruction sheet from Ampeg states to bias cold. There is also a note on the schematic that biasing info is for 120 VAC, since I'm in the UK where voltage is 240 VAC I wonder how much difference this makes?
     
  9. ficelles

    ficelles

    Feb 28, 2010
    Devon, England
    It is near the centre spot. Moving it even slightly either way will give a 50%+ change. Not sure about the bias transistor, you mean physically in contact? It is right by the heatsink, I will have to have another look...
     
  10. beans-on-toast

    beans-on-toast

    Aug 7, 2008
    They provide the mains voltage as a reference. If the voltage is fixed at 120 or 240vac, the schematic voltages can be used as a reference to compare your readings with. So 240 is not a problem as far as the bias voltage is concerned.

    The problem occurs if you mains voltage is up or down from 240. A drift in the mains voltage will affect the bias. A tech would use a variac, and possibly a stble supply if they have that, to set the mains voltage, then bias the amp.

    What I do is set the bias as per the schematic. Then I tweak the bias by ear so the amp sounds optimal. I then check the bias with a meter to ensure that it is operating within spec. With experience, this can be done safely. Get to know the amp, you alread know what sounds best, use that as a guide.
     
  11. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    There is no difference biasing it operating from 120 or 240 volts, the internal operating voltages are the same in either case.

    With the bias pot that sensitive, a simple modification would prove effective. I will look at the schematic I have after a good dose of coffee. I have been editing owners manuals for the past few days and that makes me groggy!
     
  12. ficelles

    ficelles

    Feb 28, 2010
    Devon, England
    The schematic I have shows 500R, it's component AP-1 or maybe I'm all wrong... linking to R14 / C9 / R13 / Q5 ?
     
  13. beans-on-toast

    beans-on-toast

    Aug 7, 2008
    Yes you have it right, the power amp bias pot is 500R. The schematic above is incomplete so I removed the link, see the attached documents. There is also a bias pot in the preamp limiter, the source of confusion.

    Fixed the entry in the wiki, here is the link to the files that I attached: B-15T. Sorry for the confusion.
     

    Attached Files:

    Last edited: Sep 19, 2016
  14. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    So assuming R13 is 1.5k, R14 is 560 ohms and the trim pot is 500 ohms, the easiest way is to expand the effective scale of the trim pot is to change R14 to 750 ohms (is a standard 1% value), insert a 100 ohm trim pot, and then add a 200 ohm (another standard 1% value) resistor from the bottom of the trim pot to where the bottom of the trim pot currently connects The total value of the string remains the same and now the 100 ohms covers about 1/4 of the same total range that the original 500 ohm pot covered.

    Note that this fast & dirty method only works because the correct bias voltage falls in the middle of the original pot's rotation. Only do this modification if you have the proper tools to set the bias correctly. be sure that you know the direction of rotation beforehand, and that you set the pot for minimum bias as start-up. In the schematic, that would be with the pot's wiper at the top of the element as drawn (why? because the pot is located in the bottom or shunt leg of the bias voltage divider.) I would absolutely recommend limiting the AC current to less than an amp (a 100 watt incandescent light bulb in series with the AC power would be one way) just in case there is an error.

    There are other ways to better stabilize the bias, and it's important that the bias Vbe multiplier transistor is thermally bonded to the heatsink so that the bias voltage tracks with output transistor temperature.
     
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