[Amps] Plate Cap Quandary

[Jim Thomson]

Date: Sat, 06 Aug 2011 11:39:58 -0700
From: Jim Barber <audioguy at q.com>
Subject: Re: [Amps] Plate Cap Quandary
To: Manfred Mornhinweg <manfred at ludens.cl>
Cc: amps at contesting.com
Message-ID: <4E3D8A7E.10106 at 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