1. Win a Broadcaster or one of 3 Teles! The annual Supporting Member Giveaway is on. To enter Click Here. To see all the prizes and full details Click Here. To view the thread about the giveaway Click Here.

Cathode Bypass Capacitor Voltage

Discussion in 'Shock Brother's DIY Amps' started by hackworth1, Jul 5, 2011.

  1. hackworth1

    hackworth1 Friend of Leo's Vendor Member

    Posts:
    3,579
    Joined:
    Oct 11, 2009
    Location:
    Volusia County, Florida
    What is the lowest safe operating voltage tolerance for the cathode bypass caps in the preamp and PI section of a 5e3 circuit?

    Are 20 volt caps insufficient?
     
  2. Wally

    Wally Telefied Ad Free Member

    Posts:
    38,775
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX
    those cathodes usually work with around 2V, right? Fender used 250mfd/6V caps in the 5F6A Bassman's first stage, IIRC.
     
  3. printer2

    printer2 Poster Extraordinaire

    Posts:
    7,195
    Joined:
    May 24, 2010
    Location:
    Canada
  4. piece of ash

    piece of ash Friend of Leo's

    Posts:
    3,287
    Joined:
    Dec 29, 2010
    Location:
    Sugar Land, TX
    Electrolytic caps perform better when operated near their rated WORKING voltage.

    By the time those caps ever see 10 Volts... they'll be the last concern on your list.
     
  5. celeste

    celeste Friend of Leo's

    Age:
    59
    Posts:
    2,905
    Joined:
    Jun 24, 2006
    Location:
    Maryland
    Yes, and the longer they do not see close to rated voltage, the worse they perform. They need a DC bias across them to keep the dielectric formed, if that bias is to small, they start leak and get a bit "wooly" in the bass. The problem is the tolerance on low voltage electrolytic caps can be several hundred percent. If you want a fully bypassed cathode, not a huge problem, but between 1 and 22uf, it can be a real issue
     
  6. printer2

    printer2 Poster Extraordinaire

    Posts:
    7,195
    Joined:
    May 24, 2010
    Location:
    Canada
    And yet Fender uses 50V caps for these positions.
     
  7. piece of ash

    piece of ash Friend of Leo's

    Posts:
    3,287
    Joined:
    Dec 29, 2010
    Location:
    Sugar Land, TX
    The original schem shows 25 Volt units. However, the rating system for capacitors changed for the good over the 80's.

    In Leo's time, the rated voltage of a cap was an absolute maximum. These days caps are rated with both an absolute rating and a "working" rating... the working rating being more commonly published. Caps are so much smaller these days that it is more critical to observe the working rating when possible. 50 to 100% of working voltage is the typical recomendation these days for maintaining the oxide layer. That of course, has to balanced against the possible fault voltages involved... at which point the absolute voltage rules.

    The old ratings would be fine... if we could buy the old caps.
     
  8. printer2

    printer2 Poster Extraordinaire

    Posts:
    7,195
    Joined:
    May 24, 2010
    Location:
    Canada
    And yet the reissue uses 50V. Must sound like trash.
     
  9. celeste

    celeste Friend of Leo's

    Age:
    59
    Posts:
    2,905
    Joined:
    Jun 24, 2006
    Location:
    Maryland
    I am not sure if you are trying to make a point or what
     
  10. celeste

    celeste Friend of Leo's

    Age:
    59
    Posts:
    2,905
    Joined:
    Jun 24, 2006
    Location:
    Maryland
    Now i see the point, you are trolling, trying to put words in other peoples mouths. Do you believe that Fender is the be all that ends all? Do you believe they build amps that can not be improved in some way? You believe that amp innovation and experimentation is moot because Fender has done it all perfectly in the past?
     
  11. piece of ash

    piece of ash Friend of Leo's

    Posts:
    3,287
    Joined:
    Dec 29, 2010
    Location:
    Sugar Land, TX
    Maybe Wally or Celeste might have a better idea... I are an engineer... PWM power supplies... big motor drives... not a tube god. The only reason I could see for going overboard on cap voltage is failure modes of the tube... OR to save $$ and reduce inventory.

    My point in my first reply is that "more is not better" when it comes to cap voltage.
     
  12. celeste

    celeste Friend of Leo's

    Age:
    59
    Posts:
    2,905
    Joined:
    Jun 24, 2006
    Location:
    Maryland
    While I have no "innes" with Fender, I am pretty sure in the original case it was a matter or reduced inventory, in the case of the RI's could be inventory or copying the originals.

    I feel the lower the gain, the less difference the non linearities of the cathode bypass cap make, and the classic Fenders are all pretty low gain. That is still no excuse to not know how caps should be used or to troll people trying to offer that information
     
  13. piece of ash

    piece of ash Friend of Leo's

    Posts:
    3,287
    Joined:
    Dec 29, 2010
    Location:
    Sugar Land, TX
    Yep... and in the case of most cathode bypass caps, the capacitance can fall way below the schematic value without audible harm to the circuit.
     
  14. hackworth1

    hackworth1 Friend of Leo's Vendor Member

    Posts:
    3,579
    Joined:
    Oct 11, 2009
    Location:
    Volusia County, Florida
    Excellent dialogue everybody. Very informative. I've got a bunch of 20 volt caps I can put to good use. Thank You.
     
  15. printer2

    printer2 Poster Extraordinaire

    Posts:
    7,195
    Joined:
    May 24, 2010
    Location:
    Canada
    What words am I putting in anyone's mouth? I do not know why Fender decides to put a capacitor ten times the voltage ratio than need be in their amps. I am sure Fender is not the end all, be all of amplifier designers, but they do have the resources and hopefully the knowledgeable staff to determine if they are using the right voltage rating of capacitor. Maybe a high voltage cap in those positions helps with the tone? I am new at this, they are not.

    I wonder if there is a Mod to replace the 50V caps on their preamps to a more manageable 6V. It would seem like a cheap fix if the voltage rating were indeed a problem. Has there been any talk on these pages or on other forums about the capacitor voltage ratings in the reissue? There might have been, like I said I am new at this and am willing to learn.

    Rather than thinking I was putting words in anyone's mouth you could have read my response as, 'has anyone noticed a problem where a higher voltage cap was used in this amp?' If the reissue does not sound bad maybe the voltage rating is not that significant.

    Why are you trying to make me out to be the bad guy? All my posts I have tried to be helpful here. Good thing we are just talking electronics and not religion or politics.
     
  16. big-daddy-59

    big-daddy-59 Tele-Holic

    Posts:
    737
    Joined:
    Mar 20, 2009
    Location:
    Castlewood,VA
    Preamp and phase inverter 10-25 volt rated is fine. I prefer to use a 50volt part for the power tubes bypass cap after having a 25v rated one blow up in my 5f2a.
     
  17. celeste

    celeste Friend of Leo's

    Age:
    59
    Posts:
    2,905
    Joined:
    Jun 24, 2006
    Location:
    Maryland
    Lets look at your words "And yet Fender uses 50V caps for these positions." and "And yet the reissue uses 50V. Must sound like trash. ". Could you please show me where I missed a question, or anything remotely helpful. Both statements look like the conversational equivalent of a grenade, something destructive lobbed in the hopes of getting a reaction. I was not sure about the first, so I asked, but the "must sound like trash" in the second cleared the ambiguity right up. The effects of which we speak are well documented, even in the guitar amp world. It is discussed in the first of Merlin Blencowe preamp book, which is on his Valve Wizard site for a free down load.
     
    Last edited: Jul 5, 2011
  18. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

    Posts:
    11,105
    Joined:
    Mar 12, 2005
    Location:
    Fullerton, CA
    Trolling? Probably, but it aint nothing new. Pretending it was an honest mistake, also nothing new. Sarcasm on the 'net is sometimes hard to detect, but come on... "must sound like trash?"

    As far as "why does Fender use them?" I'll give you an example.

    How many old JCM800s have you cracked open to see the cathode resistor for V1 was a huge-ass factory installed 2-3 watt 10k? Why would Marshall use such an over-rated resistor in such a low voltage area? Because "they know better?" Nope. Because the only other 10k resistors in the entire circuit are 2-3w in the B+ supply rail.

    It's simply cheaper to order 10k 2-3w resistors in bulk than it is to also order 1/2w if you're only using one measley 1/2w for the whole circuit. To DIY'ers it's actually cheaper to buy the little baggy of 1/2watts.

    Why does Fender do it with caps? Because they're a large company trying to cut pennies on every part. They're not evil for doing it, it's a competitive global marketplace.

    On the other hand, in 1979 GM decided it was cheaper to settle lawsuits from dead drivers' families than to improve a faulty design in Chevy Malibus that caused fires in rear end collisions. That is evil, and not the only time auto makers' cost-benefit analysis considered consumer death (or filthy emissions) as a lesser "cost."

    If you're having a bad day, or need to trounce on someone, may I suggest TGP? They dislike helpfulness and they enjoy conversational hand-grenades over there...
     
  19. Wally

    Wally Telefied Ad Free Member

    Posts:
    38,775
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX
    JC wrote: "How many old JCM800s have you cracked open to see the cathode resistor for V1 was a huge-ass factory installed 2-3 watt 10k?"

    Off topic...but what JCM 800 amp uses 10K cathode resistors on V1?
     
  20. piece of ash

    piece of ash Friend of Leo's

    Posts:
    3,287
    Joined:
    Dec 29, 2010
    Location:
    Sugar Land, TX
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.