Good points. I had thought that maybe I would need a little inductor in the
grid circuit
on 10/15m.
I wonder if it would work to put it in series with the swamping resistor?
The idea would be to tune out the tube's input capacitance on the high bands,
but be small
enough to have little effect on the lower bands.
If this doesn't work it would be easy to use a relay to switch it in/out.
On 8/8/2012 11:15 PM, Bill, W6WRT wrote:
> 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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>
--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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