[Amps] High C out

[Will Matney]

On Fri, 21 Jan 2005 16:45:16 -0800, Chris <rfcrazy at cox.net> wrote:

> Thanks for all the info everyone.  The circuit is just strait Pi.  The C
> tune cap backed all the way out unmeshed with minimum capacitance is
> where it tuned the best. I removed the C tune vacuum cap and it worked
> marvelous but still not optimum. I believe the min. on the cap was
> around 15-20pf that I had now removed. The only problem is while it may
> tune with just C out capacitance from the tube, I have no way of really
> fine tuning now that the C1 is out of the picture.  Vic is correct in
> that what I was trying to accomplish was an L-Pi. Instead of adding an
> inductor from anode to C1, can't I just change the coil dia. and spacing
> between C1 & C2 to accomplish the same thing? Or does it have to be in
> an L-Pi configuration to cancel out some of the C out from the tube?

Yes it would have to be in this case a L-Pi, as the tank coil is where it  
should be at for resonance. This is what I was mentioning earler where  
some amps didn't need a tune C. I have built amps where only a load C was  
used just because of this reason and it is written about doing this in  
several books on the matter. You could try to add the inductance in series  
with the anode lead so the tune C would raise from zero, but as was  
mentioned, this may kill the tuning on the lower frequencies. You may also  
find that doing this where the tune C would be used, with the series L  
(L-Pi), would not gain anything and be the same output that you have now  
with it not connected or backed all the way off. There's no way of knowing  
until you try it. Use the standard air inductor formula in the handbooks  
for the value of L. L there in the formula would be 0.3 uh for 300 nH. The  
conductor size would need to be determined for a Q of about 10-12. Most  
likely the same size conductor as the 10 meter coil uses.

One other thought would be to make a tank coil the same as you have but  
with the turn(s) added for the 300 nH. Then put a tap where the change is  
at on the coil (where the 300 nH part starts) and hook the tap to the tune  
C. It doesn't matter which side of the blocking cap the series L is added.  
This way would keep plate voltage off the 300 nH coil. 300 nH is a pretty  
small amount and by guessing may only be 1/2, 1 or maybe 2 turns at the  
most using the same diameter of the tank coil.


>
> Chris
>
>
>
> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com]
> On Behalf Of Dennis12Amplify at aol.com
> Sent: Friday, January 21, 2005 11:04 AM
> To: vic at rakefet.com
> Cc: Amps at contesting.com; craxd1 at ezwv.com
> Subject: Re: [Amps] High C out
>
> In a message dated 1/21/05 12:49:50 PM Central Standard Time,
> vic at rakefet.com writes:
>
> Nope,  he's talking about adding some inductance between the plate and
> the
> tune-C, making a kind of "L-Pi" (or L-Pi-L!) network.  The idea is  that
> with
> very high plate load impedances it may be impossible to get a
> low-enough
> minimum
> capacity in the tune-C to permit a reasonably low  Q.  Adding some
> inductance
> before the Pi network pre-transforms the  load impedance to a lower
> value so
> a
> practical capacitor can be  used.
>
>
>
> I agree.
> He's creating a two stage impedance transformer.
> I'm sorry but I consider a Pi network a Pi network no matter which  end
> is
> connected to where; and I know that the large C is on the low impedance
> side
> of the impedance transformer. I did't realize I was in error calling it
> a  Pi-L
> network instead of an L-Pi network, but I guess I can see where some may
> get
> confused. I DID state that the PLATE C became the new TUNE C in  that
> configuration.
>  I have used it myself in some cases to UP-CONVERT impedances to  obtain
> a
> reasonable tank Q.
>  A good example being multiple sweep tubes in parallel where the total
> plate
> capacitance is greater than 50 PFD. I have also used similar  circuitry
> to
> match the Collector impedance from the output of a single  ended
> transistor
> amplifier before we started using broad band transformers to do  the
> impedance
> matching, but once again that was for converting a low impedance  to a
> higher one.
> Dennis O.
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Will

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