The NCL-2000 used just a resistive input of 50 Ohms for as low as 20W drive
for 1200W output which was the tubes limit. Bias was in the -25 to -45 range
(wide mfg tolerances) for AB2 in CW. The tubes are 8122's. The tap on the
resistor network went to 25 Ohms for the typical 100+ watt exciter of the
day.
Carl
KM1H
----- Original Message -----
From: "Radio WC6W" <wc6w_amps@yahoo.com>
To: "Vic K2VCO" <k2vco.vic@gmail.com>; <amps@contesting.com>
Sent: Thursday, August 09, 2012 1:49 PM
Subject: Re: [Amps] 4CX1500B project
> Hi Vic,
> Recognizing that this isn't an optimum tube for CW, one might consider
> exploring Class C operation since IMD is of no concern in CW.
>
> Start with perhaps -85 or -90 volts grid bias so that the tube is
> essentially cutoff.
>
> Then apply something in the neighborhood of 100 to 110 volts peak RF
> drive. This tube has a 1 watt grid dissipation rating that would need to
> be respected & monitored.
>
> Increase the screen voltage as necessary until the desired output is
> acheived.
>
> The drive requirements may be reduced by employing a 1:4, or 1:9,
> impedance ratio RF transmission line transformer with 200 ohm, or 450 ohm,
> swamping resistors respectively. 10 watts drive should suffice with the
> 1:9 selection.
>
> The increase in grid circuit impedance will substantially worsen the
> effect of the tube's input capacitance and will require adding an inductor
> to the grid circuit. This will need to be variable or bandswitched (w/
> relays?). To maintain a reasonable L/C ratio, some additional
> capacitance may be desirable on the lower frequencies. More parts. :-)
>
> If this works, legal limit output may be acheivable with around 2KW
> input; at perhaps 75% efficiency, only 500 watts plate dissipation.
> That's way "cooler" than the typical operation numbers for this tube.
>
> Just a thought.
>
> 73 & Good morning,
> Marv WC6W
>
>
> --- On Wed, 8/8/12, Vic K2VCO <k2vco.vic@gmail.com> wrote:
>
>> From: Vic K2VCO <k2vco.vic@gmail.com>
>> Subject: [Amps] 4CX1500B project
>> To: "Amps reflector" <amps@contesting.com>
>> Date: Wednesday, August 8, 2012, 8:06 PM
>> I've been thinking about my next
>> project for some time, and I can't seem to shake the idea
>> of an 160-10M amplifier using the 4CX1500B. The tubes and
>> the beautiful SK-800B socket are
>> just burning a hole in my workbench, as it were.
>>
>> So I am thinking of running it in AB1 according to the
>> parameters in the 4CX1000 datasheet
>> -- 3kV on the plate, 325V on the screen and -60V bias. This
>> will give me full legal output
>> from one tube. It is possible to reduce the bias and
>> screen voltage and run it closer to
>> class A for much better IMD, but as a CW operator I'd prefer
>> a few more watts (actually, a
>> CW/SSB switch is a possibility).
>>
>> I am considering a grid-driven circuit with a 50 ohm
>> swamping resistor. It seems to me
>> that it would require 60v peak to just overcome the bias,
>> which comes out to 72 watts
>> drive for AB1 operation. Is this a reasonable assumption?
>>
>> This configuration would make it unnecessary to build and
>> fiddle with a tuned input
>> circuit. I'm also thinking about using the G3SEK tetrode
>> board, which would solve a lot of
>> little problems in one package.
>>
>> Does anyone have comments or suggestions?
>>
>> --
>> Vic, K2VCO
>> Fresno CA
>> http://www.qsl.net/k2vco/
>>
>>
>> _______________________________________________
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>> Amps@contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>>
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