[Amps] 4CX1500B project - swamped grid

[pfizenmayer]

Thought he was talking about no grid current - but even with grid current 
what other sort of input network would be any better over the swing with 
regard to match? I would think , without doing the detailed calcs , that 50 
ohm shunt is pretty well gonna hide any change in grid input impedance as 
far as match over the cycle is concerned. One EIMAC $CX1500B data sheet I 
saw suggests 1000 ohms swamping if gonna run into grid current so 50 ohms 
should surely eliminate that issue ????

73 Hank K7HP




----- Original Message ----- 
From: "Carl" <km1h at jeremy.mv.com>
To: "pfizenmayer" <pfizenmayer2 at q.com>; "Bill, W6WRT" 
<dezrat1242 at yahoo.com>; <amps at contesting.com>
Sent: Friday, August 10, 2012 5:49 AM
Subject: Re: [Amps] 4CX1500B project - swamped grid


> Id be concerned at how well the match holds over the full input cycle.
>
> Carl
> KM1H
>
>
> ----- Original Message ----- 
> From: "pfizenmayer" <pfizenmayer2 at q.com>
> To: "Bill, W6WRT" <dezrat1242 at yahoo.com>; <amps at 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 at yahoo.com>
>> To: <amps at 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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>
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