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[Amps] GS35B tuned input

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Subject: [Amps] GS35B tuned input
From: "Jim Thomson" <Jim.thom@telus.net>
Date: Tue, 23 Feb 2010 06:19:20 -0800
List-post: <amps@contesting.com">mailto:amps@contesting.com>
Date: Mon, 22 Feb 2010 10:24:54 -0800
From: "Bill, W6WRT" <dezrat1242@yahoo.com>
Subject: Re: [Amps] GS-35B Input
ORIGINAL MESSAGE:

On Sun, 21 Feb 2010 16:42:36 -0500, "Gary Schafer"
<garyschafer@comcast.net> wrote:

>Leave the CLC (pi network) input circuit in there. A broadband input will
>not provide the flywheel effect that you need. Neither will an LCL do the
>job as there needs to be capacitance from cathode to ground that you only
>get with the CLC type or a parallel tuned input circuit.

REPLY:

I agree and would take it a step further. The higher the Q of the
input circuit, the less tendency toward VHF parasitics. Why? A very
low Q input circuit will be very broadband and may allow the tube to
have enough gain at VHF to oscillate. The higher the Q at HF, the
lower the VHF gain. 

###  whoa, be real careful here. Big dif  between a parallel resonant 
input.. and a PI net.  I tried a Q of 5-6  on my 3CX-3000A7.. and it
was bad news on the upper bands.  The 3x3 tube is 50 ohms input.
Out of interest, I put bird wattmeter's on BOTH  sides of the tuned input
PI net.. [ which is 2 x broadcast variables and a tapped 6 ga coil], then
from bird  to heath dummy load.  With  200w  in,  I was only getting 160w
out on the upper 20-17-15m bands.   [200w on 160-30m] .    The fix  was
to INCREASE the coil tap a tiny bit [1/4 turn].. and LESS  C1 and C2.  This
LOWER'S the loaded Q .   

## end result... power output on 2nd wattmeter  shot up to 195 watts.
With a Q of 5-6, I could actually get 6 ga coil warm with 200W cxr.
That's 12 A  of circulating current alone.  Eimac's recommendation of 
Q=5   is way too high imo, and results in LESS  drive/grid current. 

##  Now how many folks  have measured the output of their tuned 
input circuits ??   IMO... a loaded Q of 3 is ample.  In any case,
measure the power out of it, and compare to input.   I was  burning 
up 40 watts, just from too high a loaded Q.     


I built my 8877 amp with a simple parallel resonant input circuit with
a Q of five on all bands from 160-10 meters and it is perfectly stable
without a VHF parasitic suppressor at all. Any time you can eliminate
the VHF parasitic suppressor, you should, IMO. They are nothing but
trouble.

###  a Pi net IS a LP filter.  Are you sure low Q promotes instability ??

## BTW, my buddy tried the T net  tuned input on his single and also dual
GS35B 6M amp.    The T net is a dead loss.   The PI net used 2 x small
air trimmer caps and just one coil between em.  Dead flat swr, and globs
of power output. Again, don't run the Q too high.  No suppressor's used either. 

##  If you use a suppressor on HF, use a real one.. with a globar.   Deep 6
the broadband tuned input, ala Henry 8 k... it's flawed, doesn't work, no
flywheel action, and is a myth.  The 3x3 tube is already 50 ohms, the
broadband tuned input is doing nothing. 

later... Jim   VE7RF





73, Bill W6WRT


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