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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 12:52:44 -0800
List-post: <amps@contesting.com">mailto:amps@contesting.com>

Tuning is VERY touchy. You really have to have the loading right or the screen current is excessive in either direction.

Are the tune and load capacitors too large? Are there vernier dials on the caps? Are they air or vacuum caps?

73
Jim W7RY

Here are the current conditions:

Ep = 3150V under load
Eg1 = close to 60V, adjusted for 250 mA ZSAC
Eg2 = 325V
Ig1 = 0
Ig2 = 25 - 13 mA
Ip = 720 mA
drive power = 29W
Po = 1500W (Alpha wattmeter and good dummy load)

This gives an overall efficiency of 67%, which is great if correct!

Any more drive and there is a fault. The tetrode board pops so quickly that it's hard to see which direction the screen current is going! I've been testing it with an old transceiver that doesn't have very precise power control so I will get better results with my K3.

If I had it to do over again, I think I would use a triode! I can change bands and tune my TL922 in a few seconds. I am hoping that once I make a chart of dial settings it will be possible to change bands without too many faults.

On 11/8/2013 9:11 AM, Jim W7RY wrote:
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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