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Author Topic: Basic L/R VHF Parasitic Oscillation Suppressor Design  (Read 71057 times)
AG6K
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« Reply #165 on: October 23, 2011, 01:44:12 PM »

Quote
Depends on the situation. Parasitics will often show up towards minimum on the tuning C: in that case, the impedance at 70 MHz (which was what they were worrying about) was pretty small. And in big transmitters, a 50 ohm 100 watt resistor with one connected to the plate and the other to nowhere can be a very effective suppressor.

You mustn't think that one size fits all.

  connected to nowhere?
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G3RZP
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« Reply #166 on: October 24, 2011, 01:26:45 AM »

>Connected to nowhere?<

Yes. See 'Radio Transmitters' by V.O. Stokes.
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AG6K
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« Reply #167 on: October 24, 2011, 04:22:25 AM »

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>Connected to nowhere?<

Yes. See 'Radio Transmitters' by V.O. Stokes.

  Wouldn't the R have more effect if the free end was connected to a few pF?
Rich, ag6k
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K8AXW
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« Reply #168 on: October 24, 2011, 07:42:10 AM »

Rich:  When you "quote" someone, you should include the callsign or identifier of the person you are quoting so readers can backtrack to view the quote themselves.
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A Pessimist is Never Disappointed!
AG6K
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« Reply #169 on: October 24, 2011, 08:37:12 AM »

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K8AXW
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RE: Basic L/R VHF Parasitic Oscillation Suppressor Design
« Reply #168 on: Today at 07:42:10 AM »
   
Reply with quoteQuote
Rich:  When you "quote" someone, you should include the callsign or identifier of the person you are quoting so readers can backtrack to view the quote themselves.

  like this ? -- but if you don't sign your name, we don't see it Allen.
Rich, ag6k
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AG6K
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« Reply #170 on: October 24, 2011, 08:45:01 AM »

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G3RZP
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RE: Basic L/R VHF Parasitic Oscillation Suppressor Design
« Reply #163 on: Yesterday at 11:25:48 AM »
   
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Depends on the situation. Parasitics will often show up towards minimum on the tuning C: in that case, the impedance at 70 MHz (which was what they were worrying about) was pretty small. And in big transmitters, a 50 ohm 100 watt resistor with one connected to the plate and the other to nowhere can be a very effective suppressor.

You mustn't think that one size fits all.

    Most of the parasitics I am familiar with were on the 40m through the 15m bands,
Rich, ag6k.
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W8JI
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« Reply #171 on: October 24, 2011, 11:22:54 AM »

    Most of the parasitics I am familiar with were on the 40m through the 15m bands,
Rich, ag6k.

That's because they are not parasitics.

You blame tank circuit voltages from mistuning, load problems, relay timing, gassy tubes, and other commonly known things on parasitics.

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G3RZP
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« Reply #172 on: October 24, 2011, 11:44:54 AM »

Rich,

When you get up to these power levels  - 30 to 100kW or more, the size of the metal box is such that it becomes a cavity resonator somewhere in the lower VHF region. So coupling to it gives you a parasitic resonant circuit, and the resistor going nowhere actually is a damping coupled to the cavity.

That's why it works.

The idea that one size fits all rarely works, and the European Union and its various woes are a prime example of that.
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W1QJ
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« Reply #173 on: October 24, 2011, 11:52:14 AM »

    Most of the parasitics I am familiar with were on the 40m through the 15m bands,
Rich, ag6k.

That's because they are not parasitics.

You blame tank circuit voltages from mistuning, load problems, relay timing, gassy tubes, and other commonly known things on parasitics.



It seems that every time I, or someone else I know who is loading up an Al-1500 amplifier and the amp goes PSSST, it is because too much drive was added without first increasing loading.  When you get the knack you can increase drive and increase loading and then simply tune the plate with no PSSST.   Like many amps, they will arc at the  weak link when the amp is too lightly loaded.  I see that Ameritron later in years devised a spark gap at the end of the tune cap and the amp arcs there instead of at the plates when one tries to drive the amp without enough heavy  loading.  The arcing is certainly not parasitics.
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AG6K
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« Reply #174 on: October 24, 2011, 11:59:45 AM »

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W8JI
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Posts: 7618   
RE: Basic L/R VHF Parasitic Oscillation Suppressor Design
« Reply #171 on: Today at 11:22:54 AM »
   
Reply with quoteQuote
Quote from: AG6K on Today at 08:45:01 AM
    Most of the parasitics I am familiar with were on the 40m through the 15m bands,
Rich, ag6k.

That's because they are not parasitics

  How could 7MHz energy burn up the parasitic suppressor resistor in my SB-220 if the resistor is paralleled with <100nH of  L ?

.
Quote
You blame tank circuit voltages from mistuning, load problems, relay timing, gassy tubes, and other commonly known things on parasitics.

  Tom:  when a tube arcs does it make a big bang? tnx
Rich, ag6k
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AG6K
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« Reply #175 on: October 24, 2011, 12:09:44 PM »

Quote
W1QJ
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RE: Basic L/R VHF Parasitic Oscillation Suppressor Design
« Reply #173 on: Today at 11:52:14 AM »
   
Reply with quoteQuote
Quote from: W8JI on Today at 11:22:54 AM
Quote from: AG6K on Today at 08:45:01 AM
    Most of the parasitics I am familiar with were on the 40m through the 15m bands,
Rich, ag6k.

That's because they are not parasitics.

You blame tank circuit voltages from mistuning, load problems, relay timing, gassy tubes, and other commonly known things on parasitics.



It seems that every time I, or someone else I know who is loading up an Al-1500 amplifier and the amp goes PSSST, it is because too much drive was added without first increasing loading.  When you get the knack you can increase drive and increase loading and then simply tune the plate with no PSSST.   Like many amps, they will arc at the  weak link when the amp is too lightly loaded.  I see that Ameritron later in years devised a spark gap at the end of the tune cap and the amp arcs there instead of at the plates when one tries to drive the amp without enough heavy  loading.  The arcing is certainly not parasitics.

  Do you know any AL-1500 owner who had an 8877 fail?  tnx
Rich, ag6k
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W8JI
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« Reply #176 on: October 24, 2011, 01:34:37 PM »

  Do you know any AL-1500 owner who had an 8877 fail?  tnx
Rich, ag6k

A few times in the 1980's, Eimac could not build a good 8877 to save their souls. It nearly put ETO out of business, and caused Ameritron to decide to never settle on one tube for most of the products.

We ran brand new 8877's in a test jig that cycled the filaments off and on, and in some lots 80% of new tubes would fail in the first day of cycling by grid-cathode short. That test fixture had latching relays and applied -24 volts to the grid with no high voltage. Filaments were cycled every few minutes. When I would walk in in the morning after plugging in new tubes the day before, about 3/4 of the relays would be locked up from a grid-to-cathode short.

Everyone in the industry knew it was happening, and that is why Erb-tech switched to the YC-156. It is my understanding that the inability of ETO to get good new tubes cost them their GE contract.


73 Tom
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W1QJ
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Posts: 2966




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« Reply #177 on: October 24, 2011, 03:08:40 PM »

Quote
W1QJ
Member
   
   
RE: Basic L/R VHF Parasitic Oscillation Suppressor Design
« Reply #173 on: Today at 11:52:14 AM »
   
Reply with quoteQuote
Quote from: W8JI on Today at 11:22:54 AM
Quote from: AG6K on Today at 08:45:01 AM
    Most of the parasitics I am familiar with were on the 40m through the 15m bands,
Rich, ag6k.

That's because they are not parasitics.

You blame tank circuit voltages from mistuning, load problems, relay timing, gassy tubes, and other commonly known things on parasitics.



It seems that every time I, or someone else I know who is loading up an Al-1500 amplifier and the amp goes PSSST, it is because too much drive was added without first increasing loading.  When you get the knack you can increase drive and increase loading and then simply tune the plate with no PSSST.   Like many amps, they will arc at the  weak link when the amp is too lightly loaded.  I see that Ameritron later in years devised a spark gap at the end of the tune cap and the amp arcs there instead of at the plates when one tries to drive the amp without enough heavy  loading.  The arcing is certainly not parasitics.

  Do you know any AL-1500 owner who had an 8877 fail?  tnx
Rich, ag6k
All of the Al-1500 amps I have repaired did not have tube failures other than maybe an open filament.  Most repairs were minor.
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W1QJ
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« Reply #178 on: October 24, 2011, 03:18:24 PM »

Quote
W8JI
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Posts: 7618   
RE: Basic L/R VHF Parasitic Oscillation Suppressor Design
« Reply #171 on: Today at 11:22:54 AM »
   
Reply with quoteQuote
Quote from: AG6K on Today at 08:45:01 AM
    Most of the parasitics I am familiar with were on the 40m through the 15m bands,
Rich, ag6k.

That's because they are not parasitics

  How could 7MHz energy burn up the parasitic suppressor resistor in my SB-220 if the resistor is paralleled with <100nH of  L ?

.
Quote
You blame tank circuit voltages from mistuning, load problems, relay timing, gassy tubes, and other commonly known things on parasitics.

  Tom:  when a tube arcs does it make a big bang? tnx
Rich, ag6k

No, the tube does not make a big bang.  The parts that go ballistic make a huge bang. Like the grid shunt resistor or the grid chokes  or caps on the grid of an SB-220.  Recently a 3-500 developed an anode to grid short, it took out the grid choke, grid capacitor grid shunt resistor, plate shunt resistor and the plate choke burnt to a crisp and blew the meter bezel lens off and it  landed on the other side of the room.
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AG6K
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Posts: 1




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« Reply #179 on: October 24, 2011, 05:57:24 PM »

Quote

  How could 7MHz energy burn up the parasitic suppressor resistor in my SB-220 if the resistor is paralleled with <100nH of  L ?

  Tom:  when a tube arcs does it make a big bang? tnx
Rich, ag6k

Quote
No, the tube does not make a big bang.  The parts that go ballistic make a huge bang. Like the grid shunt resistor or the grid chokes  or caps on the grid of an SB-220.  Recently a 3-500 developed an anode to grid short, it took out the grid choke, grid capacitor grid shunt resistor, plate shunt resistor and the plate choke burnt to a crisp and blew the meter bezel lens off and it  landed on the other side of the room.

  I was asking Tom R., but you are correct  W1QJ.    Congrats   What happens in vacuum does not propagate  well audibly.  A good way to understand Tom's agenda is to pay attention to the questions he ignores.
Rich, Ag6k
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