bias cap question

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Chris Scott, Jul 30, 2008.

  1. Chris Scott

    Chris Scott Silver Supporting Member

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    When converting a 100 watt SF Twin to adj. bias, should I leave the 80uF value cap, or switch it to the AB763-style 50uF?

    Also- 1K 1/2 watt (on diode board) to 470 ohm 1 watt as well?

    Thanks in advance!
    Chris
     
  2. VaughnC

    VaughnC Supporting Member

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    The cap value shouldn't make any difference.

    The wattage of a resistor is determined by the the amount of current going through it and the voltage across it. Just changing to adjustable bias shouldn't increase the shunt current of the bias supply...but, once you've completed the conversion, you could take a few readings and do some calculations to make sure the resistor is of sufficient wattage and value.
     
  3. Chris Scott

    Chris Scott Silver Supporting Member

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  4. mbratch

    mbratch Member

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    That's a question that's been on my mind as well. I just BF'ed an SF Pro R and wondered if the bias cap really needed changing. Some people suggested a higher value.

    So it really makes no difference? As long as it's some reasonable value and appropriate voltage rating?
     
  5. donnyjaguar

    donnyjaguar Member

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    You may want to determine if going bigger is going to reduce AC ripple on the bias voltage supply. The danger with going bigger, or adding a second filter stage, is that the bias voltage won't come up as fast as the supply voltage when starting cold. If this is the case the fuse may blow. Also, if there isn't enough capacitance what can happen is the bias voltage drops faster than the HT so when you have a momentary loss of mains, the amp' draws too much current coming back on and blows the fuse. My friend's Marshall suffers from this. In my last design I had to noodle about with the values to find the right trade-off and avoid these potential (no pun intended) problems.
     
  6. WaltC

    WaltC Member

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    and.... (good stuff Donny!)

    I like to bump up the voltage rating of the bias caps because lots of the original ones were rated very close to the voltage that was on the circuit and I like a little more head room just to be safe (lots of fenders had 50V rated caps and ran -57V bias voltage routinely).
     
  7. VaughnC

    VaughnC Supporting Member

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    While it can be said that everything affects everything, a bias supply filter cap in the 50uF to 100uF range shouldn't make much difference as long as its voltage rating is high enough for the circuit. Heck, most electrolytics have a 20% value tolerance anyway ;).
     
  8. donnyjaguar

    donnyjaguar Member

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    Agreed, WaltC. I don't mind running components a little over-spec, but only if in a part of the circuit that has no danger of causing cascading/catastrophic failure. :) In my last build I went with an RCRC circuit and I believe I used 2x 50uF. That brought the ripple way down and still allowed it to charge up only slightly behind the HT when starting cold. I used a GZ34 for the rectifier so if I went solid state I'd have to re-engineer this part of the circuit. I really like the Marshall circuit the way it wires the potentiometer so that if it fails you get full negative voltage on the grids. This is much better than losing bias. Someone did their homework on this part of the design! What's kinda lame is the bias supply on HiWatt as it produces a lot of ripple. Granted, this is cancelled out by the humbucking nature of the output transformer, but I'm more comfortable knowing that it isn't there to begin with.
     
  9. mbratch

    mbratch Member

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    The schematic shows a 25uF @50VDC. I read someone's post where they said they preferred 100uF. No idea on what basis. 20% of 25uF would be 20-30uF. So if I replace the bias cap, you're saying whether I go 25uf up to 100uF it won't make any difference (other than what's printed on the cap)? :D
     
  10. WaltC

    WaltC Member

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    I don't think you need to go higher than 50uF, but I'd certainly go up to 100V or higher (depends on what's easily available to you).
     

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