On 5/21/2010 5:59 PM, Fuqua, Bill L wrote:
> I am not sure about using screen current as an indicator. The 4CX250B tubes
> can have under certain conditions a negative screen current.
>
Most of the Tetrodes you will see in ham service including some pretty
hefty QRO setups can show negative screen current, particularly at lower
power settings. All of the ones with which I'm, familiar (and use) will
show a positive screen current when properly loaded . When drive is
reduced and the tube operated at lower power settings you may see no or
negative screen current.
The screen is the most sensitive tuning indicator and particularly so at
the higher power settings.
73
Roger (K8RI)
> This has to do with the fact that the screen grid elements are supposed to be
> in the shadow of the control grid elements and under some conditions
> the secondary electrons that come off the screen grid are not recaptured by
> it but go to the plate and when more secondary electrons are drawn to the
> plate than are recaptured the current flow goes in the opposite direction
> than normal.
> 73
> bill wa4lav
>
> ________________________________________
> From: amps-bounces@contesting.com [amps-bounces@contesting.com] On Behalf Of
> Carl [km1h@jeremy.mv.com]
> Sent: Friday, May 21, 2010 2:50 PM
> To: Jim Brown; amps@contesting.com
> Subject: Re: [Amps] 4CX250B Ratings in Dentron Clipperton V
>
> Tune it for maximum output while using the screen grid current as the
> indicator; provide another meter as tetrodes are best tuned using Isg. The
> tube likes to be heavily loaded and will find its own happy point based on
> condition. Unless the input circuit is lossy about 5W should bring it to
> 300W. That is also the maximum PEP rating but Id drop it down to 250W as the
> generic 4CX250B is no gem with IMD and bad signals are much more noticable
> than HF.
>
> Ive repaired and modified many including using a Russian 4CX-400A as the PS
> is quite robust.
>
> Carl
> KM1H
>
>
> ----- Original Message -----
> From: "Jim Brown"<jim@audiosystemsgroup.com>
> To:<amps@contesting.com>
> Sent: Friday, May 21, 2010 2:15 PM
> Subject: [Amps] 4CX250B Ratings in Dentron Clipperton V
>
>
>
>> I've just fired up a Dentron Clipperton V modified for 6M operation per
>> Dentron instructions. It uses a single 4CX250B in Class AB2. The bias is
>> adjustable, and the factory recommends 50mA plate current with no signal.
>> It
>> takes about 15W to give me 250W-300W out.
>>
>> My question is about power ratings. I want to use it on both CW and SSB.
>> How
>> hard can I push it without damaging the tube on SSB? The DC current rating
>> is
>> 250mA. In SSB mode, I can get 200W indicated on a Bird with an indicated
>> plate current of 200 mA or so (this is with modulation, not tune-up). I'm
>> tuning up with high speed dits, so that I should be tuning for that peak
>> power. Is this a reasonable way to run it, or am I pushing too hard?
>>
>> When I run CW,can I increase the envelope power on CW if I stay below
>> 250mA
>> average current (as indicated on the plate meter), or do I need to stay
>> below
>> 250mA when I tune key-down? Can I change the bias to get more than 250W
>> out, or do I need to make other changes to the circuit to change the load
>> Z?
>>
>> 73,
>>
>> Jim K9YC
>>
>>
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>>
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