I looked at 50 ohms shunted by 80 pfd -- 2 - 30 mhz - --- series L of 271
nhy and shunt C of 71.5 pfd at input gives better than 23 db return loss
from 2 to 30 mhz.
I was wondering why Alpha did not look at that for the 8410 but the 4CX1000
is 77 to 90 pfd input so a pair is something like 2X the 4CX1500 or about
160 pfd.
If I add another LC section to the 4CX1500 I get virtually no improvement -
but if I add some series L to the 50 ohm shunting resistor it gets
incredibly good , better than 30 db return loss 2 to 30 mhz.
The network becomes the 80 pfd tube , 50 ohms with 53.7 nhy in series with
it to ground and then the L net of 245 nhy series L , 59 pfd to ground at
input .
One would want to be a bit careful about lifting the grid off the 50 ohms
too much but 53 nhy may be Ok . Actually I suspect you might want to use a
pair of 100 ohm 50 watters .
73 Hank K7HP
----- Original Message -----
From: "Bill, W6WRT" <dezrat1242@yahoo.com>
To: <amps@contesting.com>
Sent: Wednesday, August 08, 2012 11:15 PM
Subject: Re: [Amps] 4CX1500B project
ORIGINAL MESSAGE:
On Wed, 08 Aug 2012 21:50:42 -0700, you wrote:
>
>Because as I understand it, the tubes are essentially the same except for a
>slightly
>larger anode cooler which accounts for the additional dissipation. The
>parameters for
>typical operation on the 4CX1500B data sheet are based on a load line
>closer to class A
>than those on the 1000 sheet -- they call for lower bias and screen voltage
>and more
>idling current. The intention is to produce much less IMD. Since I'm a CW
>operator, I
>would rather have full legal output than reduce IMD.
REPLY:
Ok, but "typical operation" is just a starting point. Don't feel bound by
that.
If you want max output and max gain, put the screen voltage close to max
(400
VDC) and set the bias for a small amount of idle current, say 100 mA or
less.
The higher the screen voltage, the higher the gain. The lower the idle
current,
the higher the efficiency but the higher the IMD too. For CW, as you imply,
IMD
doesn't matter.
One thing to be aware of when using swamped grid design: This tube has a
fairly
high input capacitance in grounded cathode mode, aprox 80 pF. You will no
doubt
have an SWR problem on the higher bands so you may have to switch in a
parallel
resonant circuit to compensate. Or if your transceiver has a good antenna
tuner,
or if you have an external one, that may take care of it. Just something to
keep
in mind when you're doing the layout. Leave room for the parallel resonant
circuit and switch if you need it. Better yet, breadboard it first and check
the
SWR with an MFJ SWR analyzer or the equivalent.
73, Bill W6WRT
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