Caps in a tremolo circuit

Discussion in 'Amp Tech Center' started by milocj, Aug 18, 2015.

  1. milocj

    milocj Friend of Leo's Ad Free Member

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    I'm not trying to start any kind of debate here, or to get people riled up about whether certain caps or types of caps sound better. I know where those threads head and I know there's no real proof.

    In the next month or so I'm going to be house and couch bound due to a fourth back surgery so I'm looking at building a couple of pedals and maybe soldering up a board for a future amp project while I'm stuck wearing a brace. Based on the majority of the amps I've been inside of, it seems that the three caps for the tremolo oscillator almost always seem to be disc type of caps. Is there any specific reason for this or is it most likely that they were the least expensive caps available at the time? I know that my Gretsch/Supro is almost all disc caps due to cost, but I'm wondering if there is possibly something about them in a tremolo circuit that makes them desirable such as they start to oscillate easier or something along those lines.

    Just trying to decide if I should specifically look to buy those three caps as disc caps or use the same thing throughout the board.

    Thanks.
     
  2. JD0x0

    JD0x0 Poster Extraordinaire

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    I would say in this area of the amp Polypropylene would be the 'best' because they are most stable and are less affected by temperature and humidity. Depends on the value, though. I haven't seen any polyprops under 1nF. For lower values I'd use Silver Mica or Polystyrene. I'd avoid ceramic disk caps, personally.
     
  3. Wyatt

    Wyatt Tele-Afflicted

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    Long-standing advice with Fender optobug trem circuits was to use the ceramics because the silver micas caused issues.

    I just can't remember why.
     
  4. CoolBlueGlow

    CoolBlueGlow Tele-Afflicted

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    Agree - ceramic caps are a terrible idea in general... BUT ceramic caps in this position in the Fender oscillator would seem to be the one possible exception. This is (after all) an oscillator, not a tone stack or bypass. The signal does not pass through this portion of the circuit. The oscillator is a closed loop device, which merely acts on the gain of another portion of the circuit where signal is directly present.

    But I appreciate your suspicion of ceramics - and I avoid using them anyway because I have a giant stash of NOS vintage polypropylene, polystyrene and PIO caps of the correct values.

    Never heard of problems with s/m here either, but definitely not calling you wrong. Weirder things have been shown true in our craft.

    IME, hard-coated silver mica caps are pretty stable, but you have to select the values by direct measurement with a well calibrated bench tool like a Sprague TO-5, etc. personally, I never trust what is printed on the typical silver mica cap. They are notoriously hard to spec.
    Fortunately, I also have about five thousand silver mica caps too - mil spec jobs - in desirable values. In general, these have proved pretty accurate.
     
  5. Mr. Lumbergh

    Mr. Lumbergh Poster Extraordinaire

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    In that section of the amp, purely cost. The caps that control tremolo modulation aren't in the signal path at all, they just alter the bias of the first output tube or control the optocoupler pulsation. Ceramic caps are just fine right there.
    That said, when I restored an old amp from about 1965 I replaced all the caps with orange drops, including those in the trem circuit. They're really not that expensive if you know where to go (antique electronics), especially buying in quantity. The trem works beautifully now, whereas before the recap the trem circuit wouldn't engage properly and rate was inconsistent.
     
  6. milocj

    milocj Friend of Leo's Ad Free Member

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    Thanks guys.

    I knew they weren't in the signal path, and assumed it was cost related, but since so many older amps used them there even when using "good" caps everywhere else I wasn't sure if something about their construction performed better in that section. I'm not worried about the cost of a few caps, but if the disc type worked better for some reason I would have gone cheap there.

    Funnily enough, the only tremolo caps I''ve ever had to replace were a pair of Aerovox tubular/axials in a 1964 Rickenbacker because they wouldn't always start or keep the trem oscillating.
     
  7. Johnny Cache

    Johnny Cache Former Member

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    I'm no expert on tremolo circuits. But I'd say it's not a tone circuit so there is no benefit in putting in expensive caps to improve the tone.
     
  8. milocj

    milocj Friend of Leo's Ad Free Member

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    I wasn't looking at putting expensive caps in it. I was wondering if there was a specific reason other than low cost that disc caps seem to be the norm in a lot of the classic amps. If the cheap caps did something different and had a purpose I would have gone that rout.

    Since the consensus is "no" I'll just use Mallory or something similar and match it to the rest of the components.
     
  9. jefrs

    jefrs Doctor of Teleocity

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    The disc caps are most probably ceramic, they are cheap but their values go all over the place, they can vary with the weather; they are generally used where the value doesn't matter much for example DC blocking.

    Polyester (Mylar) caps are usually brightly coloured blobs of practically any shape for example orange drop. Also cheap and reliable but more accurate and stable.

    Watch the rated operating voltage esp inside a valve amp.

    If you are paying more than pennies for a capacitor you are being ripped off.

    http://www.cricklewoodelectronics.c...itors-Orange-Drop-or-Blue-250-400-630VDC.html

    Best wishes with your back.
     
  10. Johnny Cache

    Johnny Cache Former Member

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    Expensive is a relative term. Better may be the affirmative term.

    Keep us posted on the results.
     
  11. JD0x0

    JD0x0 Poster Extraordinaire

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    This is why I recommend against them, even out of the audio path in trem circuits. Also, FWIW, mylar and polprop caps should only be a couple cents more than 'cheap' ceramic. If you're redoing an amp, that cost is nothing. If you're pumping out thousands of amps on an assembly line, it does add up to production costs. That's why I say pay more. No they're not 'expensive caps' and you dont need 'expensive caps' just pay $.05 more per cap, for the more stabile composition.

    That's my $.02
     
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