So What if Your Tune C Arced ?

Discussion in 'Amateur Radio Amplifiers' started by AG6K, Apr 9, 2012.

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  1. AG6K

    AG6K Ham Member

     It is better for an arc to take place at the Tune-C because it would otherwise arc at an open contact on your bandswitch - and arced air-variable capacitor plates can easily be filed smooth. On the other hand, arced bandswitch contacts typically evaporate.
    bandswitch3A.jpg
    If an arced Tune-C is replaced by a C with a higher breakdown V the next time an arc occurs it may take place at the bandswitch with more problematic results.
     
  2. ky6w

    ky6w Guest

    This is really a tough one. Would I rather "touch-up" an arced air variable or replace a smoked band switch? -- OK, I get it.. just replace the arced air variable with a higher voltage breakdown rating and don't worry about Tune-C arcing anymore... get a good supply of band switches.

    If this philosophy catches on it might be profitable to invest in companies that manufacture band switches. Of course a heavy duty band switch would solve that problem and you could then push the problem further down the circuit and see what creates smoke next.

    Reminds me of the guy on 40 meters that was loosing patience trouble shooting his amp. He wanted the results faster with less work... To which I replied, "if you want to locate the problem faster, replace the fuse with a 16 penny nail... turn on the power switch and watch for smoke... when you see the smoke you have located the area of concern"... there was silence from the other station, he didn't even bother to sign off.
     
  3. AG6K

    AG6K Ham Member

     Chortle. Good one Don R.

     A higher breakdown V bandswitch would shift the VHF parasitic caused arcing to the Tune-C -- but it would not solve the VHF instability problem.

     I heard about a guy why turned a "fuse" out of brass rod on his lathe. It never blew.
     
  4. G0HZU

    G0HZU QRZ Member

    A while back when I looked at modelling the VHF instability modes for a simple amplifier I did also model the effect of the HT choke and the LC tuning network and blocking cap on a simple SPICE simulation.

    Even on a basic model the amplifier is prone to a nasty form of instability where the amplifier goes unstable at VHF and this then kicks the amplifier into a squegging mode where the instability causes the amplifier to bias itself on and off causing a gated VHF waveform. The frequency of the squegg gating is detemined by the LC network and/or the PA HT choke and DC blocker.

    I know W8JI totally dismisses the possibility of arcing in the tuning network when VHF instability occurs, but if a triode can also exhibit this squegg mode then you can get very big voltages in the output network as the VHF oscillation squeggs on and off at a low frequency rate. If there are any resonances in the network at the squegg rate then you will get big (destructive?) voltages at LF.

    I did model a JFET amplifier and it squegged and I also built it and it squegged just like the model. i.e. there was a gated VHF instability at the output terminal of the amplifier and a huge LF waveform within the tuning network caused by resonance at the squegg frequency.

    So can triodes exhibit squegging? All the required 'support' ingredients are there because of the limitations of a typical amplifier design.

    i.e. amplifier plus output tuning network with typically three resonances caused by layout and choke/cap resonances (LF, HF and VHF)
     
  5. KA9JLM

    KA9JLM Platinum Subscriber

    How can it arc to a open contact ?

    I would think , It could have been switched while transmitting or a loose/dirty contact started it.

    It normally takes two different Potentials for a arc to occur. And a open contact does not meet that requirement.

    73
     
  6. K9STH

    K9STH Ham Member

    JLM:

    In most amplifiers, an "open" contact is not really "open", it is usually attached to a tap on the final output coil or an additional loading capacitor.

    Glen, K9STH
     
  7. KA9JLM

    KA9JLM Platinum Subscriber

    True that.

    Unused contacts are the only ones that are truly open.

    73
     
  8. AG6K

    AG6K Ham Member

     The arc across the open contact (usually the 10m contact) takes place because the energy is VHF and because the tank is a LPF the energy has no load -- so it runs amuck.
     
  9. AG6K

    AG6K Ham Member

     True enough for DC Glen but since the energy is VHF, the HF tank coil is effectively an RFC and the open contacts can arc when VHF is present. Murphy was right.
     
  10. AG6K

    AG6K Ham Member

     what about at VHF where the HF tank L looks like an RFC??
     
  11. K1ZJH

    K1ZJH Ham Member

    there is always some "tank" L in the circuit. If there is tank L at the open contact, and if it acts like a RFC at VHF, where is the voltage?
     
  12. AG6K

    AG6K Ham Member

     When VHF is present, provided the 10m contact is open of course, the V-drop is greatest across the 10m L - which has high reactance at VHF. Thus the contact that arcs the most frequently during a VHF parasitic osc. is usually the 10m contact.
     
  13. K9FW

    K9FW XML Subscriber

    I know some one that turned a "fuse" from aluminum rod and no problem. But not here, ever.
    Al K9FW
     
  14. K1ZJH

    K1ZJH Ham Member

    That's a lot of work... just find an old control pot with a long shaft and hacksaw the needed length. Puurfect fit :)))
    And you and choose between Al, steel or non-ferrous shafts to control the amperage rating. :)

    I have seen that done, BTW--not here!
     
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