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[Amps] Help needed on 4CX250 AMP

To: <amps@contesting.com>
Subject: [Amps] Help needed on 4CX250 AMP
From: stevek@jmr.com (Steve Katz)
Date: Mon, 16 Dec 2002 08:20:34 -0800


>     Now I need some more help. First the details of rig. It uses 4 no.
> 4CX250B valves in final. The output circuit is Pi-L with rollor inductors
> as inductive component and the capistors are fixed, switched by a band
> switch. HT is 2000 V , Screen 350 V regulated.
> 
> //The 2000V is a bit low for lots of smoke on the bands.  Most hams would
> use 2500V or so; this results in more output, cleaner output, lower screen
> current (which is good) and easier drive.  It may be a typically
> commercial "conservative" amp intended for years of use without service,
> in which case nothing is ever pushed hard.
> 
> 
>     I was trying the amp on 20 m band. There is NO capacitor on tuning
> side on this band and a 100 pf on loading. I was able to get only about
> 800W out with 600 Ma plate current . 
> 
> //That's rather amazing efficiency, already (66.6%).  Doesn't sound like
> the tank's not resonating.
> 
> 
> Only a few turns on the loading inductor comes into circuit rest are
> shorted. If I further try to increase loading by turning inductor, it arcs
> badly.
> 
> //Maybe it's designed to not have the rollers tuned while transmitting at
> full power?  Do you have the installation/user manual?
> 
> 
>  Increasing drive level does'nt help.The TX was supposed to deliver 1400W
> with 1 amp plate current.
> 
> //When you say, "increasing drive level doesn't help," what do you mean?
> You can increase drive and absolutely nothing changes at all, or just the
> output doesn't go any higher?  Normally, if the amp's about saturated, as
> you increase drive, the grid current would climb higher (and likely the
> screen current, too), but the output won't increase.  What actually
> happens?
> 
> //Remember the 4CX250 has a very fragile control grid and screen grid and
> they "pop" easily unless these are monitored, or protected somehow.
> Possibly out of the four tubes, you've already lost two.
> 
> //WB2WIK/6
> 
> 
> Please suggest what should be done now and also excuse me if i m putting
> some very stupid questions, this being my first amp project.
> I can even provide the scanned copyof diagram if that helps.
> 73s
> Sandeep
> VU2BX
> 
> 
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