Id be concerned at how well the match holds over the full input cycle.
Carl
KM1H
----- Original Message -----
From: "pfizenmayer" <pfizenmayer2@q.com>
To: "Bill, W6WRT" <dezrat1242@yahoo.com>; <amps@contesting.com>
Sent: Friday, August 10, 2012 1:12 AM
Subject: Re: [Amps] 4CX1500B project - swamped grid
>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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