[AMPS] Parasitic suppressors/another question,

[Jon Ogden]

>>>€  Tuning the HF tank circuit and disconnecting the hf load have 
>>>virtually nothing to do with what is happening in the anode-resonant 
>>>circuit where the parasitic oscillations take place. 
>>
>>Ah contrair!  You yourself have said that the anode-resonant circuit is 
>>composed of Ctune.  Is not Ctune in the tank circuit???  
>>Sooooooo.......tuning the tank circuit DOES make a difference.  
>>
>€  Adjusting C-tune makes little difference in the anode resonance since 
>C-tune is effectively in series with C-anode, which is much smaller. 

Not necessarily, Rich.  In many, many tubes (such as the 4-1K), the Canode of the tube can be large.  In the YC-156 (I think) Canode is on the order of 35 pF.  The 4-1K has a Cout of 7.6 pF.  However, once you put it into the socket, the capacitance rises to the order of 15 pF or so.  Most of the UCS series tune caps have a minimum capacitance of 5 to 10 pF.  I would hardly say that C-anode is much smaller than Ctune.

Additionally, I've always thought that Canode was the capacitance from the anode to ground.  Ctune is also connected to ground.  So I fail to see how Canode and Ctune are in series.

And when Ctune is rotated through its value we aren't trying to change the resonant frequency of the anode.  What we are doing is rotating a reactive load around the Smith chart.  This is called a load pull test and its done all the time in industry.  Yes, a true load pull is slightly different, but it is close enough.  What we are trying to do is see if there is any reactive load that the tube can see that can cause the gain/phase margins to be such that the amp oscillates.  

How can you prove to me that a test is useless that so many others including myself have used to find out wether the amp oscillates?  Please don't minimize the legitimate experience of countless others.

If the value of Ctune makes no difference in wether or not the amp can go into oscillation, then tell me why I was able to make it oscillate before I properly stabilized it?  Or is drawing excess grid and plate current with no drive not an indication of oscillation in your view?
  
>
>>Also, it DOES give one a measure of stability measurements.  In my amp 
>>(and in others) the amps were made to oscillate by doing this test.  
>>After proper suppressor design was intoduced (especially Rich's 
>>recommendations to add a 10 Ohm resistor in series with a 22 pF cap to 
>>each cathode lead), the oscillation went away and the amp was completely 
>>stable.  
>
>€  'Tis hardly a good idea to tempt fate this close to the ides of April, 
>Jon.  

Not tempting anything.  If an amp is STABLE it is STABLE.  That is why it is called STABLE.

Parasitics are stay inductances and capacitances in real world components that make them perform differently from ideal components.  They are NOT little creatures that hide in components and come out to haunt you when you least expect them.  A good design will compensate for the parasitic components and minimize their effect.
>
 
>>
>>Well Rich, if an amp with your suppressors in it randomly oscillates at 
>>80 MHz, then your suppressors aren't doing their job are they?  Aren't 
>>they supposed to suppress VHF oscillations?  
>
>€  I do not build the suppressors.  If L-supp is less than optimal, the 
>suppression will be as well. 

Wait a minute.  You said the words "in my experience GG amps tend to oscillate...."  That means that you are talking about Rich Measures' experience.  Now you say that you don't build the suppressors.  Does this mean you have someone else who builds the suppressors for your amps?  Or were you talking originally about someone ELSE's experience.

73,

Jon
KE9NA


--------------------------------------------------------------------------
Jon Ogden

jono@enteract.com
www.qsl.net/ke9na

"A life lived in fear is a life half lived."


--
FAQ on WWW:               http://www.contesting.com/ampfaq.html
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm