Re: [Amps] High C out

["Will Matney" <craxd1@ezwv.com>]

Chris,

A tap is just a wire soldered to one of the turns of the coil. Say you
take the end coil that's hooked to tune C now. This would be the 10
meter coil I figure.  Take and wind one just like it except make it a
couple of tuns more, or however many it takes to make 300 nH added to
it. Say you have a 6 turn coil now and made it an 8. On turn 6 you
would solder a wire (tap) there to tune C. The end of the coil (turn 8)  
would
go to the blocking cap for the tubes anode.

Will


On Fri, 21 Jan 2005 19:25:34 -0800, Chris <rfcrazy@cox.net> wrote:

Ok I want to try what you have mention but im not visualizing how to ad
a tap where the change is at on the coil (where the 300 nH part starts)
and hook the tap to the tune C. Can you be a little more descriptive.
Thanks.

Chris

-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
On Behalf Of Will Matney
Sent: Friday, January 21, 2005 5:34 PM
To: Amps@contesting.com
Subject: Re: [Amps] High C out

On Fri, 21 Jan 2005 16:45:16 -0800, Chris <rfcrazy@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@contesting.com [mailto:amps-bounces@contesting.com]
On Behalf Of Dennis12Amplify@aol.com
Sent: Friday, January 21, 2005 11:04 AM
To: vic@rakefet.com
Cc: Amps@contesting.com; craxd1@ezwv.com
Subject: Re: [Amps] High C out

In a message dated 1/21/05 12:49:50 PM Central Standard Time,
vic@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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