And come up with great ideas!
On 8/9/2012 9:05 AM, peter chadwick wrote:
> Manfred,
> great minds think alike!
> 73
> Peter G3RZP
>
>
>
>
>
> ========================================
> Message Received: Aug 09 2012, 04:55 PM
> From: "Manfred Mornhinweg"
> To: amps@contesting.com
> Cc:
> Subject: Re: [Amps] 4CX1500B project
>
> Vic, Bill, and all,
>
>
>> 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.
>
> Instead of a simple plain parallel resonant circuit, which would need to
> be bandswitched, a simpler solution is to include a basic low pass
> filter section in PI configuration, that cuts above the highest band,
> and absorb the tube's capacitance in it. Such a basic low pass section,
> designed to cut off above the 10 meter band, will require about 100pF on
> each side. So it can be built with the 100pF on the input side but only
> 20pF on the output, and the tube's capacitance will complete it. The
> result will be low SWR on all bands, without needing a band switch
> section at the input.
>
> The load resistor would of course be at the tube's grid, after the low
> pass filter!
>
> I have two of 4CX1500B tubes in my junk box, but no sockets. These are
> pulls from a broadcast station, and were given to me as being in good
> condition - but I have no easy way to test them. Maybe some day I bite
> the hook and build an amp for them, so I'm interested in this project
> too. But for this time, I'm more interested in eventually finishing and
> publishing my low cost solid state amp!
>
> Manfred
>
>
> ========================
> Visit my hobby homepage!
> http://ludens.cl
> ========================
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> Manfred
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--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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