Date: Sat, 06 Aug 2011 11:39:58 -0700
From: Jim Barber <audioguy@q.com>
Subject: Re: [Amps] Plate Cap Quandary
To: Manfred Mornhinweg <manfred@ludens.cl>
Cc: amps@contesting.com
Message-ID: <4E3D8A7E.10106@q.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
I wasn't paying attention when the beginning of the thread marched by,
thus the bad timing.
So: center-tap the 10m coil with the Tune C, then just leave the
"left-hand" end open?
The YC156 suffers from high Cout as well, but theoretically has gain up
to around 110mhz assuming I remember correctly. Perhaps this trick would
work there as well... ?
Thanks,
Jim, N7CXI
## No the "left hand" end is NOT open! The "left hand" end goes between
the plate block cap
and the C1 tune cap. This puts 1/2 the uh BEFORE the main PI net...
and the other 1/2
BETWEEN the tune and load cap. The theory is.. the tube C between anode to
grnded grid +
the "left hand" end makes a STEP DOWN L network. The plate load Z of the
tube is now
stepped down via the L network.. to a much lower value. A lower value
plate load Z will of course
require a high C /low L main PI net. It's amounts to a cheap and quick
method to transform the tubes
sky high plate load Z.... down to something the main PI net can actually
handle.
## the scheme works.. and it works very good too. I used it on my 3CX-3000A7
amp to transform the
plate load Z way down on 15m. It's low enough that I can run a loaded Q = 8
on 15M. The same
coil tap is also used on 17M. On 17M, the Q rises to 12.
## when using the mfj set up to test all this, the MFJ is placed on the output
of the amp, A resistor, who's
value is equal to the plate load Z of the tube... is wired between anode to
chassis, [ or output side of plate block
cap and chassis] to simulate the high plate load Z of the tube [ BEFORE it
get's transformed to a lower value].
Then tweak the tune + load caps for min swr on the mfj. OK, now leave the
tune+ load alone... and increase/decrease the
freq on the MFJ,,until the swr on the mfj rises to 2:1 then note the freq
spread between these 2 x freqs. When you add
the extra uh BEFORE the main PI net.,.. and now transform the plate load Z
way down.... then repeat the MFJ test, you
will find that the 2:1 swr points on the MFJ are now a LOT wider. It's just
a simple way to see how you loaded Q has dropped
simply by adding the L network. The L network is just the tube C + the
extra uh b4 the pi-net. The tube C is now 'free C'
## Like Carl sez.... the L net will have a slight effect on the lower
bands...[ and also transform the plate load Z down a bit... but it's
not a huge amount]... but it's moot point. If u lower the plate load Z on
10m.... by a huge amount... it will lower quite a bit on
12m as well [ good].... and not quite as much on 15m [also good].... and barely
any lowering on 20M. [ good too].
## you can simulate ALL of this using the GM3SEK PI net calculator. There
is a line on there for the tube C. There is also a line
on there for "stray L" between anode and up to the C1 tune cap. You can
see right away, by inserting a tiny bit of stray L, that
the resulting plate load Z is now transformed to a lower value. The higher
the value of stray L.. the lower the tube Z gets transformed to.
You can also change the loaded Q on his excellent PI spreadsheet. By adding
just a tiny bit of stray L, you will see that you can now lower the
loaded Q. Add some more stray L... then change the loaded Q to a lower
value, and keep going. I usually juggle the values, such that the
stray L value is the same as the Pi net coil value.... but not always.
## you can achieve the transformation effect by tapping the pi net coil
with the hot side of the tune cap.... OR
you can use 2 x separate coils. In the 3CX-3000A7 amp... I used 2 x separate
coils..at right angles...and different values too.
I wound a .66 uh coil..consisting of 7 x turns of 3/8" OD tubing..with a 1.5"
ID... an placed it between the plate block
caps and the Tune cap. Coil between tune and load cap was also 3/8" tubing...
but a lot bigger diam..like 3".
## Point is either method works just fine. The gm3sek spread sheet is dead
on when adding the stray L. It seems to
me that the plate load Z did not drop to 1/4 of it's original value, when
using Carl's technique. It was WAY down.. but not
that low. However, by shifting the C1 tap point over a bit , you end up
with even more stray L... and LESS coil between
tune and load cap. Z is now even lower... which of course requires more
tune + load C..and less coil.
Later... Jim VE7RF
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