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Film Vs Ceramic Capacitors for Tube Unit (Film in AC application?)

Discussion in 'Tube Audio' started by Aerochrome, Aug 2, 2018.

  1. Aerochrome

    Aerochrome Member

    Messages:
    78
    I'm recapping (and due to age replacing the resistors too) a Webster Chicago 78-1 Wire Recorder, and am wondering if I can use Film Capacitors in in instead of Ceramic Caps. Most of the film caps on digikey are rated for DC only, so I am wondering if it would be wrong to use them in a Tube unit like this. Here is a schematic for more info:
    [​IMG]
     

     

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  2. mroboto

    mroboto Super Member

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    Ceramic caps almost last forever (don't need to replace them) but are noisy, not for use for coupling applications.

    Ceramic are fast for power supply decoupling applications.

    Film caps are precise and quiet for coupling applications.

    But always try to go higher in voltage specs.
     
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  3. ConradH

    ConradH Addicted Member

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    No problem at all going with film. I rebuilt one of those some time back and was amazed at how good the sound quality was. Be careful to get the mechanics working right. I needed a long wire to get certain things lubricated, and just the right moly grease for the feed motion. I remember there's also a shielded capacitor (C3, 3-leads) that needs to be duplicated by wrapping a cap with some copper foil. I also remember access was absolutely miserable on the model I was working on.
     
  4. thorpej

    thorpej AK Subscriber Subscriber

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    Indeed, if the ceramic caps are in the power supply, if you do replace them, replace them with the appropriately-rated-for-their-circuit-location modern safety capacitors.
     
  5. gadget73

    gadget73 junk junkie Subscriber

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    What they all said. I probably wouldn't change the ceramics just because there isn't much need to. They sound like crap but for something like this it may not matter.
     
  6. Retrovert

    Retrovert AK Subscriber Subscriber

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    I suggest you replace all of the ceramic capacitors. Most of the purported lore repeated here about how ceramics do not "go bad" is incorrect and is based upon internet recirculation. The fact is those capacitors were bad the day they were installed.

    Consider the actual properties of the component (see below) and why it was used: it was inexpensive. Film capacitors in those small sizes are very inexpensive. The only issue is the amount of effort to replace the ceramics, which is small.

    BaTiO3 is a horrible material for capacitors because it piezoelectric, microphonic, pyroelectric (temperature changes result in a voltage potential across the electrodes), has a non-linear DC bias problem which reduces the capacitance with applied DC voltage, and a time-dependent dielectric breakdown from applying a constant DC bias voltage, which is in addition to the reduction in capacitance, and is hygroscopic.

    A BaTiO3 capacitors would never be considered audiophile grade, even with a liberal interpretation for "audiophile grade", when used in the signal path. The main use for such capacitors is decoupling in high-speed digital circuits where the horrid properties do not matter and the item is usually disposable after a few years.

    Film capacitors are inexpensive and small, so no reason exists to leave BaTiO3 in the equipment.
     
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  7. thorpej

    thorpej AK Subscriber Subscriber

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    In the power supply, one should use the correctly-rated safety capacitor for the location in the circuit. That might be a film cap, that might be a ceramic cap. But it's generally always a flame-proof type, and has the correct failure mode for extreme over-voltage transients (and, in the case of film, is self-healing).
     
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  8. Retrovert

    Retrovert AK Subscriber Subscriber

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    Yes, absolutely true. If one is using a Death Cap, then by all means, it must be specifically rated for that use. Never, ever replace that with anything else.

    I'm not keen on Death Caps, BTW.

    Here's an explanation I've previously written of how a hot chassis will kill you dead: http://audiokarma.org/forums/index....ath-cap-current-limiter.817958/#post-11502344

    To quote my conclusion, "Humans in the mains-voltage electrical path degrade the sound, so it is crucial to ensure that all humans placed in the mains-voltage electrical path are of audiophile quality and rated to carry a full 15 Amps at mains voltage."
     
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  9. thorpej

    thorpej AK Subscriber Subscriber

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    I definitely prefer 3-prong cords to death caps. But they are also sometimes used for EMI / RFI suppression, or for snubbing.
     
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  10. Aerochrome

    Aerochrome Member

    Messages:
    78
    I'm sorry, there seems to be a misunderstanding. There are no ceramic capacitors inside this unit from the manufacturer. They area all old style "tubular" and paper capacitors. I am asking if I can replace them all with film caps. The only ones I was going to use ceramics for was C1 and C2, and then I was going to replace C13-C15 with electrolytic caps.

    I noticed this right away, can I use aluminium foil instead?
     
  11. thorpej

    thorpej AK Subscriber Subscriber

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    Those need to be Y-type (or X/Y) safety capacitors, of any material so long as they are rated for the usage.
     
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  12. ConradH

    ConradH Addicted Member

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    Definitely use the correctly rated safety capacitors- if you use headphones with this thing, you don't want it to become live and you dead.

    Aluminum foil is OK if you can attach to it, which is tough to do reliably. Copper tape is way better.
     
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  13. Aerochrome

    Aerochrome Member

    Messages:
    78
    Ok, Ill order some copper tape. So the plan is to use safety caps for C1 and C2, Electrolytic for C13-C15, and the rest I will use Film caps. I just wanted to make sure that was ok since most film caps only have a DC rating.
     
  14. Retrovert

    Retrovert AK Subscriber Subscriber

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    A few other thoughts as long as you are doing a rebuild. None of these are expensive.

    (1) Replacing all the resistors with metal film is a good idea. The carbon-composite resistors will likely have drifted, and they have poor properties in any event.

    (2) Adding an inrush limiter can help reduce stress on the rectifier and transformer when the filter capacitors are empty and thus appear like a dead short. The inrush limiter to the heaters reduces the stress.

    (3) Adding solid-state diodes to the 6X4 will greatly reduce the PIV stress and prolong the tube's lifespan. You'll still get the gradual B+ delay, but the rectifiers will take the AC stress, not the tube.

    (4) At the transformer's secondaries, separately fuse the heaters and the B+. On startup the heater current is many times normal load, so the primary fuse is geared for that, not a B+ short. If the 6X4 goes it will short and take out the transformer. Small fuses (50 to 80 mA) exist and these may protect the transformer in the event of such an overload because of rectifier failure. Yes, the tubes tend to be reliable, but if (when) it shorts the transformer will likely melt before the fuse blows. Replacement transformers for old gear can be hard to find and expensive.

    (5) After you clean the potentiometer add some silicone grease as a lubricant. It was built with this when it was new and it keeps the wiper from wearing away the resistive element.

    (6) Change the bulb to an LED and this removes some of the heater load. Also removes some of the heat needlessly generated.

    (7) Polish the chassis with Simichrome or Mother's Mag and Aluminum polish. It will look better than new when you're finished.

    Good luck with the rebuild.

    Edit: added LED.
     
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  15. Retrovert

    Retrovert AK Subscriber Subscriber

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    One more thought.

    You really don't need to use a death cap. That's to provide some noise reduction. Other means exist to deal with that. Do some reading and take your own counsel on that.

    You can always try it without one and see how you like it.
     
  16. Aerochrome

    Aerochrome Member

    Messages:
    78
    Well I am fixing this thing for myself and plan on keeping it for the long haul so I might as well implement all of these (might skip the LED to keep the outside appearance period).

    1. Already plan to.
    2. I have no idea how to select an inrush limiter do you have a part on digikey you could point me to? (or give me a few specifications I need)
    3. I have your average N1914 diodes and the higher rated FR157G diodes, would either of these work? And if so where should I place them in the schematic (and what polarity?) I know next to nothing about tubes.
    4. I will order some inline fuse holders with the caps/resistors. What voltage rating do I need? (I think 120?) Also which pin numbers need to be fused?
    5. Anything specific? I have super lube 21030 on hand.
    6. Might do this, but for now I will keep the unit looking more period.
    7. Definitely on my to do list.

    Lastly about the death cap, it seems to have a standard capacitor in there, but I will replace it with a Y rated one for future safety. Hopefully after this it will be around for at least another 60 years.
     

     

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  17. Retrovert

    Retrovert AK Subscriber Subscriber

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    The 1N914 diode is a signal diode. It will explode with a loud BANG! if you try to use it as an AC rectifier. Pick some nice low Qrr diodes so there's less noise. FREDs are good, so are Schottky. Make sure you have enough PIV rating.

    You're fusing the transformer secondary before the rectifier, so fuse is placed between the winding and the rectifier.

    An Amphenol CL-80 is 3 amps max, resistance @ 25 degrees C is 47 Ω, and a CL-90 is 2 amps max, resistance @ 25 degrees C is 120 Ω. Arts prefers Ametherm so suggest you research that. That's for the primary side. You'll need to size the thermistor for the load. If it isn't enough the thermistor never gets hot enough to drop in resistance. So figure out what the load will be.

    Superlube grease is great. Thick stuff, not the oil.

    Just add diodes to the rectifier's input in the same direction they'd be for the actual rectifier. Here's a pinout for the Dynaco, you an work it out from the schematic.
    TubeRectifier-DiodeMod.jpg
     
  18. Aerochrome

    Aerochrome Member

    Messages:
    78
    Last edited: Aug 3, 2018
  19. gadget73

    gadget73 junk junkie Subscriber

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    Min 120vac, I usually use the 250vac caps because they seem to be fairly available and it gives some wiggle room in case the line voltage spikes a bit for whatever reason.

    I'd sub a 0.0022 film for any 2000pf / .002uf caps. Similarly 0.047 for 0.05, and 0.022 for 0.02.
     
  20. Retrovert

    Retrovert AK Subscriber Subscriber

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    I suggest using a higher rating because high-impulse spikes can damage them.

    You'd be amazed what AC power looks like.
     
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