[Amps] High C out

[Will Matney]

John and Vic,

That's what I thought by reading it but never seen anything of a beast  
like a L-Pi so I thought it would be wrong to comment. The only other  
thing I could think he meant was adding to the tank coil. I guess that  
might be done, but I've never seen it used. The best he could really do is  
use a dip meter and find where it tuned using one coil.

Will



On Fri, 21 Jan 2005 12:52:24 -0600, W0UN -- John Brosnahan  
<shr at swtexas.net> wrote:

> At 12:22 PM 1/21/2005, Dennis12Amplify at aol.com wrote:
>>
>> Will,
>>
>>  I believe Chris is talking about converting from a PI network to  a  
>> Pi-L
>> network. The Pi-L having two levels of impedance matching, and a  
>> little  more
>> harmonic suppression, than the original Pi network.
>>  Thus the original Tune C becomes the middle capacitor of the Pi-L,   
>> allowing
>> for a larger value of C, and the plate C is now the new Tune C.
>>  Also, If his tune cap is already backed all the way out, he needs  LESS
>> inductance in the tank to tune it, not more... Therefore he would be   
>> SPREADING
>> the coil turns apart, or removing a turn, or moving the tap towards   
>> less turns,
>> any of which would result in less inductance in the tank  circuit....
>>
>> Regards,
>>
>> Dennis O.
>>
>>
>>
>>
>> In a message dated 1/21/05 11:49:21 AM Central Standard Time,
>> craxd1 at ezwv.com writes:
>>
>> A 300 nH  coil (0.3 uH) is not very much. Assuming this would be the  
>> tank
>> coil, the 10 meter end being the largest in size, one might just   
>> squeeze
>> the coils together to raise the inductance enough to bring  it in tune.  
>> If
>> not, adding 1 to 2 turns of the same size wire, and  same diameter would
>> most likely do it. A dip meter would be easier  to use to find the exact
>> placing of the tap on the coil if it has  taps. According to the way the
>> tube is ran and tube type, some at a  higher frequencies dont need a  
>> tune C
>> in order to tune correctly  because the tubes output C is high enough  
>> and
>> takes care of it. In  the amps I've seen this way, a load C was all that
>> was  used.
>
>
> Actually it is the other way around.  The pi-L normally refers to moving  
> the
> output cap (loading) to a higher impedance point and matching this
> intermediate impedance to 50 ohms with the L network.  This provides
> additional harmonic suppression as well as reducing the amount of
> capacitance required for this "shared" capacitor, shared between the
> output of the pi section and the input of the L section.  Downside is
> that the load capacitor requires a higher voltage rating because the
> impedance is no longer 50 ohms but a few times higher.
>
> What Chris is trying to do is the exact opposite.  He is trying to move  
> the
> Tune capacitor to a lower impedance point by adding a new L between
> the original network and the anode of the tube.  I guess this could be  
> called
> an L-pi network.  Since the impedance is now lower at this new point
> you need more C for the tune cap -- and that may allow a capacitor with
> a higher minimum C to be used.
>
> So this new inductor is to be placed between the TUBE and the TUNE
> capacitor.
>
> I have used the 3CPX5000A7s in pulse mode at 50 MHz for wind profiling
> and that is what I did.  The average observer would just think my plate  
> lead
> was pretty darn long.  But in reality it is an inductor that is part of  
> an L
> network that steps down the impedance so that the Tune cap can resonate
> the circuit without running into a minimum capacitance issue.  The tune  
> cap
> is now "shared" between this L network on the tube side and the pi  
> network
> for matching the new impedance to the output impedance.  In my case I
> also used another inductor at the output, once again a long lead and not
> a "real" coil, so my matching is technically called an L-pi-L network, I  
> guess.
>
> Some of this effect exists in any case just due to stray inductance in  
> the
> leads, but this is increasing the inductance to a value high enough to  
> make L
> networks that provide useful impedance stepping.
>
> --John  W0UN
>
>
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