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Thread: So What if Your Tune C Arced ?

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  1. #1

    Default So What if Your Tune C Arced ?

     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.
    Rich, AG6K, [url]www.somis.org[/url] 805-386-3734

  2. #2
    Join Date
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    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.
    Always mobile.

  3. #3

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    Quote Originally Posted by ky6w View Post
    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.
     Chortle. Good one Don R.

    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.
     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.

    Reminds me of the guy on 40 meters that was losing 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.
     I heard about a guy why turned a "fuse" out of brass rod on his lathe. It never blew.
    Rich, AG6K, [url]www.somis.org[/url] 805-386-3734

  4. #4
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    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. #5
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    Quote Originally Posted by AG6K View Post
    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.
    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
    "Theory only works perfect in a vacuum." KA9JLM Don

  6. #6

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    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. #7
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    Quote Originally Posted by K9STH View Post
    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
    True that.

    Unused contacts are the only ones that are truly open.

    73
    "Theory only works perfect in a vacuum." KA9JLM Don

  8. #8

    Default

    Quote Originally Posted by KA9JLM View Post
    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
     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.
    Rich, AG6K, [url]www.somis.org[/url] 805-386-3734

  9. #9

    Default

    Quote Originally Posted by K9STH View Post
    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
     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.
    Rich, AG6K, [url]www.somis.org[/url] 805-386-3734

  10. #10

    Default

    Quote Originally Posted by KA9JLM View Post
    True that.

    Unused contacts are the only ones that are truly open.

    73
     what about at VHF where the HF tank L looks like an RFC??
    Rich, AG6K, [url]www.somis.org[/url] 805-386-3734

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