70uf Filter caps in series?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Dollar, Feb 13, 2008.


  1. Dollar

    Dollar Member

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    Is the use of two 70uf filter caps in series as seen in Fender AB763 amps equivalent to one 140uf cap or one 35 uf cap? I know that resistors in series are added to find total resistance but do not know if it works the same for capacitors. I have an amp based on an AB763 circuit that substitutes the two 70uf filter caps in series for one 40uf and want to know how they correlate to each other.

    Thanks.
     
  2. pfrischmann

    pfrischmann Member

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    I was reading in a Gerald weber article that he likes to make these caps much larger, maybe double the size,

    Has anyone ever done this?
     
  3. Blue Strat

    Blue Strat Member

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    Be careful of quoting stuff out of context. This is only a viable option in amps with solid state rectifiers. There's an upper limit to the capacitance you can throw on the cathode (output side) of tube rectifiers and Fenders were typically right at the upper limit.
     
  4. pfrischmann

    pfrischmann Member

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    Hi Mike,
    Thanks for responding.
    That's why I'm asking. I've always been concerned about this.
    I don't want to hijack dollars thread and I can start a new one if he prefers.

    Here is the actual quote from Webers third book. Tube guitar amplifier essentials. page 251-252

    Out of tuneness problem
    Because players in the 50's and 60's did nlot turn up very loud, almost all vintage amps were designed with underfiltered power supplies.......etc. on some of the 60's amps there were two 70uf in series is a total of only 35 uf. that is barely enough filtering if the amplifeir is set less than halfway up. at overdrive settings, the current is so high that the filtering cannot get rid of the annoying 120 cycle ripple...etc....which is why guitars can sound out of tune...

    The Solution
    You need to increase the filtering but without stressing the rectifier tube. I like to use two 220uf caps in series. I like using 350 volt caps. When you put two of them in series, you get a solid 700 volt rating (providing you put a 220k ohm 1 watt resistor across each capacitor).

    To net the rest of it out (tired of typing) Gerry goes on to say this will give you 110uf of capacitance at 700 volts for your mains...
     
  5. Blue Strat

    Blue Strat Member

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    Gerry is a cool guy, but not an engineer, and some of his "fixes" are kinda hacks.

    You can check the max load capacitance for the various rectifiers by Googling "[tube number] data".
     
  6. Dollar

    Dollar Member

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    Thanks everyone.

    This is why I was asking. I read similar information from another source that also warned that increasing these too much can be hard on tube rectifiers. I was concerned that this stage was way underfiltered compared to the original circuit. everything is working fine, I'm just comparing, trying to learn and get a handle on tailoring a circuit to obtain specific results. The source I have stated that upping the filtering here would firm up the bottom and add punch.
     
  7. Dollar

    Dollar Member

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    This makes me think I'll leave things as they are. I don't trust the source I have completely but I thought Gerald Weber was more reliable.
    Hmmmm? :confused:
     
  8. pfrischmann

    pfrischmann Member

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    With that said, is there anything or any reason to up the caps on the mains?
     
  9. phsyconoodler

    phsyconoodler Member

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    I find that with any Super Reverb that I've worked on,guys complain about the woofy bass if it's turned higher than 4 on the dial.Guys have spent huge coin by swapping out speakers and such,only to find that a couple of things are to blame.
    1) those oversize filter caps that everyone put in after reading GW articles
    2)fender's use of 25uf bypass caps on preamp cathodes.I use 5uf or less
    3) Fender used .1uf coupling caps,i use typically .022uf to handle the bass

    This stuff is not for everyone,but it does tame the bass and allow you to either turn the bass knob up or play more on the neck pickups of typically dark guitars like Les Pauls.
    Now if you have an amp that has 220uf filter caps,it is easier to change out the bypass caps than the filter caps,so try a 5uf or even a 1uf on V1 bypass cathodes.
     

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