Another approach to the grid capacity problem is to use a wide band technique.
Placing 50 ohm swamp resistor at the grid (assuming you have enough drive power
- of the order of 35 watts PEP) and feed the resistor vis a low pass filter. A
0.1dB ripple elliptic function should do. That works out as looking like a pi
netwrok with a capacitor in parallel with the coil. The values I calculate are:
Shunt caps each end of the pi = 90pF. At teh tube end, the tube provides it -
maybe you need an extra few pF but I suspect strays will do it.
Series inductance is 230nH - probably 220 nH is near enough
Capacitor shunting the inductor= 30.7 pF (30pF - two 15pf in parallel) is near
enough.
That should give an input SWR of around 1.4 at 30 MHz and better lower down.
I've used this technique at 200 ohms with a pair of 4-250 tubes.
73
Peter G3RZP
========================================
Message Received: Aug 09 2012, 04:14 PM
From: "Vic K2VCO"
To: "Bill, W6WRT"
Cc: "Amps reflector"
Subject: Re: [Amps] 4CX1500B project
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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> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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