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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