'79 SFDR Ceramic Cap(s)

Discussion in 'Glowing Bottle Tube Amp Forum' started by Neener, Aug 3, 2021.

  1. Neener

    Neener Tele-Holic

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    Continuing on with my (first) amp project, the '79 SFDR...

    I noticed what looks to be a 220pf cap that has come loose next to the Vibrato channel Treble cap (220pf)

    Should I solder back in or remove? It doesn't appear on the layout I've been using

    (https://robrobinette.com/images/Guitar/AB763_Models/AB763_Deluxe_Reverb_Layout_DIYLC.pdf)

    Any idea the purpose of this cap? Maybe part of the mods? (For those of you who haven't been following, the amp has been modded with "mid" pots)

    ceramic1.jpg

    I have been considering just replacing all the ceramic caps as well. They do seem a little tired. Thoughts on replacing ceramics?

    ceramic2.jpg cermamic3.jpg

    As always your input/wisdom is greatly appreciated. Thank you!
     
  2. Ten Over

    Ten Over Tele-Holic

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    The 220pF cap in parallel with the 250pF treble cap makes it 470pF total. This is a mod that changes the sound of the tone stack which may or may not be what you want.

    I've never really heard of ceramics getting tired. I, personally, wouldn't touch them. Especially those two in the tremolo oscillator.
     
  3. Neener

    Neener Tele-Holic

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    Thanks man! I just noticed some little droplets of brown stuff around some of the ceramics, figured they might be leaking...

    What effect is the higher total 470pf having on the tone? less treble?
     
  4. corliss1

    corliss1 Poster Extraordinaire Platinum Supporter

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    Ceramics don't have anything to physically leak. You could just be seeing wax from the board, but I can't tell from those photos.
     
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  5. schmee

    schmee Doctor of Teleocity Ad Free Member

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    Just leave those ceramic caps. Really, don't over do this to a great amp.

    The one you asked about was probably added to reduce noise, not sure. If so, it can stay or go, Fender did this a lot here and there, without layout changes sometimes. They were on a mission to reduce noise because everyone was playing louder in the 70's. You also see a lot of small white wire coiled around groups of wiring to reduce noise in some amps. I dont think this was on the layout either.
     
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  6. Neener

    Neener Tele-Holic

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    :D
    Right on. Didn’t realize it was a fender thing. I’ll just leave it.

    I’m all about doing as little as possible only what’s necessary. I probably won’t touch the bypass electrolytics either. (I know there is an ongoing debate on the topic no need to beat up a horse :D)
     
  7. corliss1

    corliss1 Poster Extraordinaire Platinum Supporter

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    Why would you not want to change those? They are a known part, with a limited shelf life, that can actually cause issues, while the ceramic guys should pretty much live forever.
     
  8. Neener

    Neener Tele-Holic

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    I've heard varying opinions on the matter. Lots of people change all electrolytics no matter what, but some don't find it absolutely necessary to change the bypass, unless there is obvious physical or audible sign of an issue. They see lower voltages and don't have the same threat of catastrophic failure as power filter caps.

    According to the the internet, that is...
     
  9. corliss1

    corliss1 Poster Extraordinaire Platinum Supporter

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    While it is correct that having one of them die/explode won't cause harm to the amp, if they are out of spec the amp won't sound as it's supposed to, and the trem circuit may not respond well either.

    Those "some" people are just wrong :D
     
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  10. Paul G.

    Paul G. Friend of Leo's

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    Here's the scoop on electrolytics so you can grasp the concept of changing them. Caps are 2 conductors with an insulator between. A change in charge on one conductor induces the same change on the other despite no electrical connection. No change in charge, no induction. That's why they conduct AC but not DC.

    The insulator can be many things, ceramic, mica, polypropylene, etc. These conductors are solid and stable.

    Electrolytics are different. In order to make a reasonable size cap with high capacitance AND high voltage handling, the insulator is paper soaked in an electrolytic solution. The electrolyte will take on a charge and acts as if the insulator is much thinner than it is, while keeping the conductors far away from each other to keep high voltages from jumping across.

    Now this electrolyte, over time, can dry out, it can leak out, it can break down. Then the cap will drift in capacitance, or can fail as open or conductive. The drift affects the tone of your amp, it can't sound the same. Failure, obviously is not good.

    A bypass cap only carries a few volts, so failure isn't catastrophic, but it will definitely affect the gain and tone of your amp. A filter cap, on the other hand (including on the bias circuit), failing can be, um, intense.

    Unless it's a museum piece, I make sure the electrolytics get changed. All of them.
     
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  11. corliss1

    corliss1 Poster Extraordinaire Platinum Supporter

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  12. NTC

    NTC Tele-Holic

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    My first amp was gone through by a tech. There was an ongoing problem where, some time after replacing ALL tubes, it would start to crackle. He replaced the filter caps (and did a lot of other unmentionable things) along with the tubes. A few days later the crackles came back. I finally dove in and replaced the cathode bypass cap on the first stage that he hadn't touched and the problem went away for good. Some of the tubes that went in the garbage over the years were Siemens EL34'S (Groove Tubes branded) as well as Sylvania 6CA7's. No doubt they were still good...

    Replace the cathode caps.
     
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  13. DavidP

    DavidP Friend of Leo's

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    @Neener, to respond to the original question, if you decide to replace the ceramic caps (I'd check their values 1st), I'd go with silver mica.
     
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  14. Neener

    Neener Tele-Holic

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    I did filter caps, and so far everything is dead quiet and sounds great. Ive been playing it all weekend!

    I’ve got the caps if I decide to change the bypasses. One of those things where I’m afraid to change anything at the moment. Sounds too good!

    I’m leaving the ceramics alone for the time being. Although I did clip the bright cap (is that a sin?) - sounds really nice.

    @DavidP Thanks for the recommendation. I went down the internet rabbit hole and agree with using silver mica :)
     
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