Two filter caps 500% off spec!! WTF?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by uncle looie, Oct 26, 2004.


  1. uncle looie

    uncle looie Supporting Member

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    Ok... I have a Fender Vibroverb that calls for all 5 filter caps under the pan to be 20mfd 525VDC. My tech has replaced the two closest to the OT with 100mfd 450VDC. WHY??? He says that's the value that came out of it. Well... it sounds like dogsh*t now and it sure didn't before and it's not what the schematic calls for. The other variable in this equation is the OT is now a Merc Mag Tone Clone of the original. The original failed years ago and all I could get at the time was a Ruby TF-125, which was supposed to be a generic replacement that worked in that amp fine. I gigged it for over two years with it in and it sounded decent, not as good as the original but decent. I decided I wanted it closer to the original though, so I ordered the Merc Mag tranny. The tech put it in and also changed those two filter caps he said were bad, but he said he used the same values that were there before. Now if the MM tranny and the Ruby were similar enough to work in that amp, wouldn't those caps 500% out of spec also have sounded like crap if they were there at that value before? Wouldn't a 100mfd cap in a position that calls for 20mfd smooth the ripple to the point that it would hardly load the power section?
     
  2. loverocker

    loverocker Member

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    I answered this one at the LPF. Summary: my schematic (AB763) shows two 70uFs (close to 100uF) in series (for a 35uF capacitance). 100uFs in series would make for 50uF in this position, making the amp feel 'stiffer', but I doubt that they're to blame for the tonal change.

    Maybe your ears have gotten used to that old Ruby over time? :)
     
  3. uncle looie

    uncle looie Supporting Member

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  4. theelectic

    theelectic Guest

    Doesn't matter what a schematic says, if they were 20u and you liked the sound, they should have been replaced with 20u. The extra capacitance has made the amp feel stiff (lots of capacitance in reserve to handle the low end and big notes). WIth less capacitance, there's some compression and sag as the power supply cannot handle the big notes & low end.

    Get you tech to put it back how it was with new caps.
     
  5. uncle looie

    uncle looie Supporting Member

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    The AA calls for two 20's in parallel and the AB calls for two 70's* in series.... wouldn't it pop one of them out if they were in there at the wrong polarity?

    There is a weird warble now on the attack of low notes, and the closest way I can describe it is it sounds like the noise your computer monitor makes when you degauss it.


    *Woops... I had "35" before... it should be two 70's for a total of 35.*
     
  6. theelectic

    theelectic Guest

    The two 35u series give you 17u (close to 20u). Why are they put in series then? To up the effective voltage rating, put two 35u/350V caps in series, it behaves like a single 17u/700V cap - slightly more of a safe zone over a 20u/500V cap.

    Two 20u caps in parallel would give you 40u - more capacitance. If your tech replace two 20us with two 100us, that's 200u of capacitance!! Way too much. Some of the amp techs around here can chime in on why that's bad (I forget why too much capacitance on a tube rectifier is bad).
     
  7. uncle looie

    uncle looie Supporting Member

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    Ooops... that should've been two 70's in series for a total of 35. I goofed there.
     
  8. David Robrecht

    David Robrecht Blue Fire Silver Supporting Member

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    You may actually have an AB 763. Fender used up old labels. Waste not want not. That amp has two 70uf in series for a total of 35uf. If yours did have all 20uf caps, that's still two 20 uf caps in parallel for a total of 40uf. Not much different considering the 20% tolerances of the parts. Your tech probably put two 100uf caps in series (very common) for a total of 50uf of capacitance. As was mentioned this would/could make the amp a little tighter and maybe have more bass. It shouldn't make the amp sound wierd. If they are two 100uf in series, that won't hurt the rectifier. The GZ34 can handle 60uf.
     
  9. hogy

    hogy Member

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    Hey Uncle, I'm too lazy to type this all out, but if you want to get to the bottom of it, take the amp chassis out and call me after lunch time. I'll tell you what's going on and how to fix it. You don't need the MM OT either, you'll be able to track down a proper vintage one. More on the phone. 225-926-1976.

    Hogy
     
  10. JamesPeters

    JamesPeters Member

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    I'd go so far as to say you might not be liking the sound of the MM OT. I find it's like changing pickups--even if a particular pickup is raved about a lot, sometimes it's not the fit for your guitar. Mercury makes solid stuff, but maybe it's not necessarily what you're after tonally.
     
  11. TheAmpNerd

    TheAmpNerd Member

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    Oh, That is a vibroverb.

    Had one come through the shop
    a few months back. I was really
    surprised at how thin this amps sounded
    in stock configuration.

    The one that came in worked fine
    but had had all the resistors replaced
    in it with Metal Oxides...needless
    to say, this greatly contributed to
    the sterile sound of the amp.

    Replacing them with Carbon films
    helped. Putting in top notch caps
    helped as well. It was a lot more
    musical then.

    NOW

    It then depends on the tone you
    want from the amp....are you looking
    for the SRV tone? If so then you have
    to change the OT to a different one
    a la Cesear Diaz to make it sound
    the same as a SRV amp. Otherwise
    leave the OT alone.
     
  12. Wakarusa

    Wakarusa Member

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    Sorry to nitpick Spike, but metal oxides throughout? Not metal films?
     
  13. TheAmpNerd

    TheAmpNerd Member

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    Not nitpicking Todd, that was not a typo.

    Whoever the "tech" was, replaced just about every resistor in the amp with
    metal oxides; one watt concrete
    covered ones, typically used in power
    supplies.

    It included the output screen/grid,
    bias and PI/driver, as well as the other
    plate and grid stoppers. For some reason he left CCs in the power supply
    and as the bias resistors for the
    pre amp tubes. GO FIGURE.

    The amp really kicks ass now : )
     
  14. Wakarusa

    Wakarusa Member

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