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Re: [Amps] 4CX250B Ratings in Dentron Clipperton V

To: "Jim Brown" <>, <>
Subject: Re: [Amps] 4CX250B Ratings in Dentron Clipperton V
From: "Steve Katz" <>
Date: Fri, 21 May 2010 11:37:25 -0700
List-post: <">>

I'm pretty familiar with this amp and have repaired/modified a few of

The 4CX250B plate current isn't very important at all...what's important
is grid current and screen current, and the Dentron design is lacking
adequate monitoring and controls for these.

I can run a 4CX250B up to 350W PEP output easily at 2kV and about 300mA
Ip (peak), this is fine for that tube when properly cooled.  The trick
is keeping Ig down to near zero and Ig2 (screen) also down to nearly
zero, which is entirely possible with proper (heavy) loading.

If you don't monitor the grids, not only will the signal usually be
excessively wide (high IMD) but the tube will fail from excessive grid
or screen dissipation.  The cathode and anode are very robust.


Steve WB2WIK/6

-----Original Message-----
From: []
On Behalf Of Jim Brown
Sent: Friday, May 21, 2010 11:16 AM
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.
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).
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
average current (as indicated on the plate meter), or do I need to stay
250mA when I tune key-down?   Can I change the bias to get more than
out, or do I need to make other changes to the circuit to change the
load Z?  


Jim K9YC

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