VHF Parasitic Suppressors - HF Tube Finals

Discussion in '"Boat Anchor" & Classic Equipment' started by KE4OH, Sep 1, 2015.

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  1. KE4OH

    KE4OH Ham Member QRZ Page

    So the typical parasitic suppressor is a 2-watt 47-ohm carbon comp resistor with a few turns of wire wound across it.

    I need to make a new one. Those particular resistors are becoming unobtainable. What about using a 2-watt metal oxide resistor? What about using a 2-watt carbon comp 56-ohm resistor? What about .... ?

    My real questions are about the resistor itself - It is simply a convenient coil form? Or is there a "secret sauce" to making the suppressor work correctly? Is the secret that the value is 47 ohms? Or is the secret that it's carbon comp?

    Thanks and 73 de Steve KE4OH
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The Ohmite OY series work fine for this and are better than the old carbon composition resistors.


    The resistor isn't just a coil form. It provides a different current path for VHF vs. HF.
  3. WA7PRC

    WA7PRC Ham Member QRZ Page

    After reading the datasheet for the Ohmite OX/OY resistors, I saw no mention of inductance nor suitability for RF. After measuring the inductance, I went with OEM carbon composition.

    Seller 'bigsmythe74' on ebay (link) sells NOS Allen-Bradley for reasonable prices. He wants $2 each for 47Ω/±10%/2W resistors. However, it's not necessary to use that exact value. Sorting what he offers based on price, he has 56Ω/±10%/2W for only $1 each. That'd work fine, too. I bought from him to refurbish the parasitic suppressors in my SB-220 (one resistor was a bit too high). They should last another 40 years.

    vy 73,
    Bryan WA7PRC
  4. KC9UDX

    KC9UDX Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    The coil does not need to be wrapped around the resistor.
  5. N2EY

    N2EY XML Subscriber QRZ Page

    No, but it's much more convenient and sturdy that way. At least for tubes up to a few hundred watts.

    For the OP:

    The idea is that the inductance of the coil is so low at the operating frequency that it has almost no effect. But, at VHF, it has so much inductance that the resistor is pretty much in series with the circuit. The idea is that a VHF parasitic can't start because of the losses.
    KC9UDX likes this.
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Ameritron uses OY resistors in their parasitic suppressors in my AL-80B and I see also in their bigger amps.

    Per W0BTU about two years ago:
    Per KM1H from that same thread:
    I've been using the OY resistors for many years in parasitic suppressors and they're more stable over time and temperature than any of the carbon comps I've ever tested. Per the Ohmite data sheet:
    That type of composition describes a very good resistor whose inductance would mostly be in the wire leads.
  7. KM1H

    KM1H Ham Member QRZ Page

    Yup, the OY is now accepted by hams and commercial RF amp companies.....end of story.

    Before that I used 3 and 5W MOX which were inductive and needed a bit of C to cancel out....no big deal but it looked like s;;; and worked great even on a Clipperton L running a full bore 1200W on 10M,

    I was an early proponent of the OY which looks neater in a customer amp and I havent found a down side yet, even on 6M.

    I got out of the carbon type in the late 80's except for low power such as 6146's where I still have many in tolerance NOS 1W.

    47 Ohms isnt a standard, Ive seen 27-100 Ohms used in commercial amps. The goal is to broadband the circuit since a parasitic can fire up over a wide range of frequencies even with the same tube due to circuit differences, layout, etc. The 47 is common since it provides a low enough Q and with the correct # of coil turns it does the job and doesnt overheat on 10M and yet will stop a parasitic from forming to beyond 200 mHz as would happen with some metal tubes and as low as around 70 mHz for the 572B/811A.

    Ameritron copied the MOX and later the OY after I wrote them up on forums.

  8. WA7PRC

    WA7PRC Ham Member QRZ Page

    No such (highlighted) wording is in the Ohmite OX/OY datasheet (link). And again, there is no mention of inductance (or lack thereof).

    They may not have exhibited resonance but, resonance requires a combination of two of each of R/L/C. So of course, if you have only one, you don't have resonance.

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