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Thread: Fender 300PS filter capacitor replacement?

  1. #1
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    Fender 300PS filter capacitor replacement?

    I am repairing an old 70's Fender 300PS amplifier. This amp has four 6550 output tubes and a 6V6 phase splitter! I need to recap the amp but am having a hard time finding a close match to the main power filter capacitors. The original caps are large screw terminal 490uf/400v. The closest value I can find are 680uf/450v. Would those work? Thanks!
    Justin Thomas likes this.

  2. #2
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    What's the tolerance on the old ones? I think that'd be fine... be careful in there - MY meter won't even read those voltages!

    Justin
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    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

  3. #3
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    You can bet your sweet bippy that I will be careful while in the amp and handling the capacitors!

    Here is the schematic: http://ampwares.com/schematics/300ps.pdf

  4. #4
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    For the tolerance, you'll have to check the caps themselves. Usually they're something like -20/+80 or something. They're not precision components...

    Justin
    "When receiving a shock I emit a strange loud high pitched girlish squeak." - Alex R -
    "Sort of like not checking for toilet paper before taking a dump. ." - Chuck H -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

  5. #5
    Senior Member Enzo's Avatar
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    They are not precision anyway, this is only a guitar amp, not NASA lab gear.

    470uf would also be close enough.
    Education is what you're left with after you have forgotten what you have learned.

  6. #6
    Supporting Member mozz's Avatar
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    I would change those 70/350v caps after the choke too. They are running 340v on a 350v cap?

  7. #7
    Senior Member Enzo's Avatar
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    Cap voltage ratings are "Working voltage" ratings. 350v cap means it is suited to a 350v circuit. The caps also have surge ratings which are the thing to not exceed.
    Education is what you're left with after you have forgotten what you have learned.

  8. #8
    Supporting Member mozz's Avatar
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    You can trust Illinois caps all you want, to me they are garbage. A cheap cap which is rated at 1000 or 2000 hours and over 20 years old and is run at it's limits, to me deserves to upgraded for the few cents more it costs. I don't do this for a living, i do it make the amp a little better than it was.

    If you are going to recap the filters on a older amp, would you only change the first caps and not the ones past the choke? I would use at least a 400v rated cap there and would also change the bias cap.

    Also, chances are the B+ is over the stated voltage on the schematic due to todays line voltage.

  9. #9
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    I am going to change all the electrolytic capacitors in the amplifier. If the original values are unobtainable, I will use values that are higher in voltage rating.

    I agree that "Illinois" branded are not the highest quality. Heck if the state of Illinois is in such financial trouble, their capacitors surely must not be very good. ;-)

    If this amp was solid state I would not have a problem using significantly higher capacitance values. That said, I have heard that increasing the capacitance values on tube amplifiers may stress and damage the output tubes.

    There are no indications on the old Mallory filter capacitors of tolerances.

  10. #10
    Senior Member Enzo's Avatar
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    Not quite, in tube amps, larger cap values for filters - at least the reservoir cap - can damage the RECTIFIER tube. If the amp uses diodes instead of a rectifier tube, then you can grow the caps.

    Tolerances in the old days were loose to say the least. In a guitar amp, that doesn't matter much. As has often been pointed out, the old Fender schematics had a note in the corner saying everything in it was +/-20%.
    Justin Thomas likes this.
    Education is what you're left with after you have forgotten what you have learned.

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