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Re: [Amps] Question for the tetrode experts

To: amps@contesting.com
Subject: Re: [Amps] Question for the tetrode experts
From: Jim W7RY <w7ry@centurytel.net>
Reply-to: w7ry@centurytel.net
Date: Fri, 08 Nov 2013 09:11:36 -0800
List-post: <amps@contesting.com">mailto:amps@contesting.com>
What band?  Are you using padder capacitors on the loading cap?

73
Jim W7RY

On 11/7/2013 3:14 PM, Vic K2VCO wrote:
    I've finally applied full drive to my 4CX1000A amplifier. Here is what I'm
    seeing:
    I have tried two tubes, both pulls. In one case I can run about 1kW out, and
    with the other, maybe 1200. What happens when I try to push past this point
    is this: when I hold the key down for 10 seconds or so, the screen current
    starts to drop, then goes negative, and when it hits -35 mA the screen trip
    circuit kicks in. I haven't seen even a flicker of grid current.
    The screen circuit is well-regulated at 325v, so this isn't case of runaway
    caused by increasing voltage. What it seems like to me is that the screen is
    heating up and the secondary emission is rapidly increasing. Screen current
    before this happens is reasonable, 20-25 mA, so I am not exceeding its
    rating.
    I have a couple more tubes to try, but I have a bad feeling that I am going
    to end up buying a new tube if I want 1500 watts from this.
    Do the experts think my diagnosis is correct?

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