"Stiffening" the Power Supply

Discussion in 'Amp Tech Center' started by randomhitz, Feb 18, 2015.

  1. randomhitz

    randomhitz Tele-Meister

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    What are the pros and cons of stiffening the power supply with higher value filter caps. I'm going to replace the filter caps in my Blues Junior and have read that some people are replacing the 47uf cap with a 100uf or attaching another 47uf in parallel as per BillM recommendation. Supposed to make the bass tighter but what does it do to the top end? Is stiffness the opposite of sag, which seems desirable to some. How did Fender arrive at the 47uf value?
     
  2. Frontier9

    Frontier9 Friend of Leo's

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    Probably through a process of counting beans...
     
  3. Wally

    Wally Telefied Ad Free Member

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    Imho, don't replace until you know why you are replacing them. Yes, doubling the capacitance in that first stage will 'stiffen' things up. the bottom end gets firmer, and the amp will be punchier. IT will yield a different sonic. IF you are looking for this, then go for it. IF you like how the little amp saturates and sags early and has a 'looser' yield, then don't change that filtering.
     
  4. JD0x0

    JD0x0 Poster Extraordinaire

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    A 'stiffer' power section tends to lend itself to tighter bottom end. Less of that 'Sag' especially if the rectifier is solid state. Some people like the 'sag' some people dont. I'm a fan of rather 'tight' sounding amps. Too much sag, and the sound of the amp tends to get mushy and lo-fi sounding IMO.
     
  5. randomhitz

    randomhitz Tele-Meister

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    thanks--the 47uf is starting to leak.
     
  6. soulman969

    soulman969 Telefied Ad Free Member

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    Careful. If you "stiffen" it too much and mates with the power tubes you may have a whole bunch of little 12ax7s and ECC 83s running around in there. ;)
     
  7. GERPUD

    GERPUD TDPRI Member

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    I don't know for the Blues Junior, but for my Fender Ramparte I added a filtering stage before the first stock filtering stage with 47uF and a choke. So I guess it is equivalent to double the first cap. The difference in the sound was major. As mentioned, I got tighter sound, a big more low end but also more headroom (distortion comes later).
    This amp had a lot of sag, which was not unpleasant specially for blues, but now I find the amp more versatile, and still nice for blues... I like it better now.

    For your blues junior, I'd say: Try it. At best, you will like it, at worst, you going to loose a bit of time (about the same time I needed to respond this post) +5$...
     
  8. jazzguitar

    jazzguitar Tele-Afflicted

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    Advantages are:

    tighter bass,

    more clean,

    no sag.

    Disadvantages are:
    no sag,

    above a certain amount larger caps make no more difference,

    on amps with tube rectifiers be aware that, depending on the rectifier type, there is a limit to the capacitor value (also for solid state diodes, but that is way off, usually no problem)


    Sag occurs when the amplifier puts out its maximum design volume or more (they were not intended to work in distortion volumes) and the power supply is too weak to maintain the volume. When the tone you play fades, less power is drawn and the supply voltages come up again, increasing the volume some while the tone gets cleaner (some say the sound blooms).

    Either Fender used the example designs made available by the tube manufacturers, such as RCA, or they calculated the capacitance neccessary for whatever lowest frequency the amp was designed to deliver at full output.
     
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