Larry,
Can you explain what causes a change in input impedance over the cycle when
operating in AB1 i.e no grid current? I can see Miller effect causing some
change as the amplitude varies, but not over the RF cycle, and if neutralised,
there won't even be that change. The change in the amount od space charge and
thus capacity should also be negligible.
Peter G3RZP
========================================
Message Received: Aug 11 2012, 04:55 AM
From: "Larry Benko"
To: amps@contesting.com
Cc:
Subject: Re: [Amps] 4CX1500B project - swamped grid
Hank,
I hope someone will correct me if I am wrong but I believe the problem
is the changing input impedance the tube/FET etc. throughout the RF
cycle for all classes of operation except heavy class A. This dynamic
changing impedance causes the exciter to generate IMD to the amplifier
which then gets amplified. This is why we pad the impedance and match
to it with circuits having a larger Q than would be necessary to just
do a match.
If you build an SWR circuit in your simulation software such as the
"tandem match" circuit which consists of 2 coupled transformers and no
capacitors you can see the change in SWR throughout the RF cycle. Also
since this is a simulation a resistive SWR circuit will work fine to
show the changing SWR. A truly constant 2:1 SWR does not generate IMD
but a varying 1.2 to 1.5:1 SWR will generate some IMD.
This is how I understand the problem.
73,
Larry, W0QE
On 8/9/2012 11:12 PM, pfizenmayer wrote:
> 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"
> To:
> 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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