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Cathode bypass cap type (not value)

Discussion in 'Amp Central Station' started by King Fan, Nov 24, 2013.

  1. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Does it matter what type (electrolytic, film) cap you run in a cathode bypass? I read a site :rolleyes: that said electrolytics were too sluggish to do the job, and so films sounded better. I read another (more extensive) discussion that said the type absolutely didn't affect tone or function.

    Why do I ask? After testing 0.1, 1, 4.7, 10, and 25uF caps on v1b (that is, not v1a, which comes with a 25, and not v2 at all) in my 5f2a, I decided I kinda liked the 1uF best. This is as a switchable option to step up the gain without bringing in a lot of harsh.

    But it so happens the 1 uF I had was the biggest film cap in the bunch -- bigger than that, they're huge and expensive. And in fact the 1 uF is actually alarmingly large. So I'm thinking that an electrolytic would fit a lot better physically.

    But then I started this research and found the first site above -- aha, maybe that's why I liked 1 uF better than the bigger caps -- because films sound better than electrolytic.

    OTOH, when I plug in my circuit values at AmpBooks' bypass cap calculator:

    http://www.ampbooks.com/home/amplifier-calculators/cathode-capacitor/

    I get a curious result. The 1 uF value just happens to put the gain curve in the middle of the audible frequencies roughly 100Hz to 1kHz, while the 0.1 uF is shifted to above 1kHz, and the 4.7 is shifted below 100Hz. Now I know nothing about how Hz relates to my taste in distortion. But maybe the 1 uF is just the 'right' answer, and it has nothing to do with cap type. I'd value your opinions...
     
    Last edited: Nov 24, 2013
  2. muchxs

    muchxs Doctor of Teleocity

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    If you want to use a 1uf film cap drop the voltage rating to get the size down.

    Original '50s tweed Bass- men used a 200uf 6v cathode cap. A higher voltage rating would have made the '50s technology electrolytic the size of a beer can.

    I have a bunch of 1uf 100v PIOs that are relatively compact.
     
  3. jefrs

    jefrs Doctor of Teleocity

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    Sluggish? The time constant of a capacitor circuit is set by its value. CR = seconds.
    Big capacitors are therefore "slow".

    Some types are more suited to certain values and voltage than others. Electrolytic are normally large numbers of micro-Farad or even milli-Farad, big. That's what determines their choice. There are expensive alternatives.
     
  4. printer2

    printer2 Poster Extraordinaire

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    The higher value cap bypasses more lows around your cathode resistor than a smaller one, the cap allows the tube to produce more gain where the capacitor functions. Electrolytics are only 'sluggish' at higher frequencies (and probably much higher than the ones we are interested in) but for anyone that wants to ease their mind about how well this capacitor is working there is a 'fix'. You could run a smaller, lower value capacitor in parallel with the electrolytic. So if you are using a 22uF in this position you could put a 0.1uF film capacitor in parallel with it. So if the electrolytic is feeling sluggish on the high frequencies the smaller capacitor will take over.
     
  5. BiggerJohn

    BiggerJohn Friend of Leo's

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    By changing the cap value you are changing the low frequency roll off point on the stage. An electrolytic is plenty fast enough for audio frequencies.

    Bear in mind the bias voltage developed by the cathode resistor is only a few volts so a high voltage rated cap is not needed.

    There is so much hyperbole and misinformation floating around. You would be well advised to not believe everything you read on the internet.
     
    rsilverst likes this.
  6. 6942

    6942 Poster Extraordinaire

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    I just use electrolytic caps, usually Xicon 'cause I'm cheeeap! :lol:
     
  7. rsi106

    rsi106 TDPRI Member

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    Electrolytic caps are usually inferior to other types in things like temp stability, useful life, etc. I usually use 25 volt film caps for bypassing.
     
  8. 6942

    6942 Poster Extraordinaire

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    I'm 60 years old.
    A brand new electrolytic capacitor, will probably have a longer.....'useful life'.....than I will?
     
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  9. jefrs

    jefrs Doctor of Teleocity

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    I may have some electrolytic in use about my own age. Regular use prolongs their life. They are generally used where temperature stability, indeed actual value, is unimportant, they don't change that much. You can spend ages tweaking the cathode capacitors, or twiddle the tone knobs a bit.

    By high frequency problems, I think we are talking radio frequency.

    When you put capacitors in parallel, simply add their values. They will act together, not handle different parts of the frequency spectrum. The spectrum off a guitar is not that great, the top note is only about 1kHz.
     
  10. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Super helpful. Thank you all!!!
     
  11. tony hunt

    tony hunt Tele-Meister

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    Re. maybe the 1uF is just the 'right' answer?
    Yes, you found out by hands-on testing what works for you and that amp.
    By choosing a lower value, less of the bass frequencies are passed on to the next stage.

    Put rather simply, by-pass caps allow more gain from the triode. Traditionally the value of these caps is 25uF, which passes all the frequencies that interest us along to the next stage. This is called "fully by-passed".

    You found that you actually prefer only partial frequencies to passed on. What ever works for you is good.
    tony
     
  12. printer2

    printer2 Poster Extraordinaire

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    ESR.
     
  13. BiggerJohn

    BiggerJohn Friend of Leo's

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    ESR is only a problem with electrolytics at higher frequencies.
     
  14. printer2

    printer2 Poster Extraordinaire

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    Yeah, I said it is probably not an issue with guitar. Just saying for those that think it may be rather than paying a lot for big non-polarized capacitors they can just parallel up a lower value cap across the electrolytic.
     
  15. TNO

    TNO Friend of Leo's

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    If 1 or 1uF works for you definitely go with film caps. For larger values there are better quality electrolytics like Nichicon Muse and Elnas that are still pretty cheap.
     
  16. BiggerJohn

    BiggerJohn Friend of Leo's

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    It is definitely not an issue at guitar frequencies. No need to go with fancy caps unless one just wants to spend for the sake of spending. No need to add the parallel smaller cap at these frequencies either.
     
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