Discussion in 'General Technical Questions and Answers' started by K4LRH, Jan 10, 2019.

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  1. KX4OM

    KX4OM Ham Member QRZ Page

    You might add to your notes some information regarding the suitability of polystyrene capacitors in VFOs. The have very good temperature stability (slightly negative temperature coefficient) and close tolerances. They are often used in Colpitts VFOs, which can have high values of capacitance compared to Hartley designs.

    I recently used several 2,000 pF polystyrene caps in my build of the ARRL lab two-tone audio generator for SSB testing. They can be somewhat scarce, but values from the 100 pF to several thousand pF range are available from surplus dealers, including West Florida Components, one of my go-to sources.

    Do a Web search on "polystyrene capacitors VFO"

    Ted, KX4OM
  2. KP4SX

    KP4SX XML Subscriber QRZ Page

    I'll leave that to your imagination.
  3. K4LRH

    K4LRH Ham Member QRZ Page

    Thanks for the West Florida Components source. I presently need two caps.

  4. K4LRH

    K4LRH Ham Member QRZ Page

    There are presently many types of caps. Any possibility that films will negate the need for 'orange drops' that replaced paper/wax, black beauty & bumble bee types ?

  5. KE4OH

    KE4OH Ham Member QRZ Page

    Orange drops ARE film caps. Sprauge is the original manufacturer of the orange epoxy-dipped film caps that are now known as orange drops. Even the oldest ones should be just fine now and for decades to come. I don't know if Sprauge invented film caps, but they sold a metric boatload of orange drops and they are good caps.

    I personally use the generic brown epoxy-dipped film caps with radial leads and generic as well as Chicago brand yellow film caps with axial leads. I go for inexpensive. They all work fine.

    From time-to-time, I repair tube guitar amps. If a customer specifies orange drops, I ask them if they want mylar (polyester) or polyprop (poylpropylene) dielectric orange drops. Then I'll spend the extra to get them what they want. Otherwise, they get whatever I buy on sale.
    K7TRF likes this.
  6. KB4QAA

    KB4QAA XML Subscriber QRZ Page

    BTW, Sprague ceased manufacturing of genuine "Orange Drops" about five years ago.
  7. K7TRF

    K7TRF Premium Subscriber QRZ Page

    As Steve posted above both types of caps are film caps and from an electrical standpoint they can be interchanged at will assuming the value (capacitance) matches and the voltage of the replacement is at least as high as the original. The Orange Drop (originally a Sprague trademark but I believe it's been licensed to Vishay) or Orange Dip (generic name for same thing) caps use a radial instead of axial geometry and tend to be smaller so you might still want to use one to fit into a tight space.

    Both types of film cap have been around for a long time so I wouldn't think of it as axial caps negating the need for radial caps, both have their uses and one or the other may fit your application better.
  8. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    Thanks for that video. I didn't realize that the modern polarity markings were inconsistent or missing.
  9. K6LPM

    K6LPM XML Subscriber QRZ Page

    I don't believe it is so much an issue of polarity as it is as much as it is considered good practice to place the outer most wrap of the capacitor foil towards the the gnd side of audio signal circuits when using the caps to roll off or filter bandwidth of the audio signal or shunt if you will a portion of the signal. The reason I was told this is that it helps negate microphonics and also is thought to help by shielding away other noise and stray capacitance ect...... Maybe it's part audiophile voodoo superstition but in the boutique handbuilt custom guitar amp world it is a practice that is strongly adhered.
    I never gave it much attention until I had built a very high gain EL-84 amp with several stages of preamplification. The amp had all types of ultrasonic oscillations and various other gremlins. A Elmer looked over the construction of my build and pointed out the fact that my axial mylar caps weren't all "outer foil or shield to gnd" . I was a bit stubborn and didn't believe it was the source of my problems. Finally in frustration I did in fact reinstalled. It seemed that it helped contribute to managing some of the issues encountered. This was a turret board point to point hand wired circuit and I think it was accumulative of many issues and that this was enough to tip the balance . I am sure the lead dress also played a large part with a circuit that was already running to the upper limits. Dumb luck or not I now adhere to the orientation of banded end of axial mylar or foil wrapped caps.
    K7TRF likes this.
  10. KD2RDG

    KD2RDG Ham Member QRZ Page

    Interesting. I'm curious to know why this works, whether we want to call it polarity or something else. The time I changed the cap in my guitar I followed the convention because why not.
    KD2ACO likes this.

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