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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