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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