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Re: [Amps] Plate Cap Quandary

Subject: Re: [Amps] Plate Cap Quandary
From: Jim Barber <audioguy@q.com>
Reply-to: audioguy@q.com
Date: Sun, 07 Aug 2011 12:47:22 -0700
List-post: <amps@contesting.com">mailto:amps@contesting.com>
Thanks, Jim.

And by using two coils (as you mention) I could keep the big roller in 
place that I bought for the purpose. OTOH, using NOS vacuum relays 
instead is looking more attractive in case I decide to run the whole 
thing remote.

Thanks again also to all that replied individually, In case I missed you.

73,
Jim, N7CXI

On 8/7/2011 4:54 AM, Jim Thomson wrote:
> 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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