Well, Santa brought me an $800 xmas present... I removed the HV lead
from the 3CX1200D7 and cooked the filament at 7.26 volts for 6
minutes... Eimac had suggested 7.5 volts for 5 minutes but 7.26 was
all I could get without going looking for the 220 Variac and
unsoldering a bunch of wires inside the Henry... For a moment I
started to pull the tube out on the bench and cobble up a test jig,
then basic laziness caught up with me <I had already put in a 12 hour
day, so cut me some slack>
Anyway, post toasty, the tube puts out 1700 watts for 74 watts of
drive and the grid current right on 200 mils.. Before the toasting
it put out 1100 watts with 48 watts of drive hitting 200 mils on the
grid... So for the moment all is well... Who knows how long the tube
will work... I have reset the filament to 6.6 volts for running,
which is right at the upper limit per Eimac... I did try it at 6.3
volts and saw a 120 watt drop on output... So the filament emission
current is sagging... At full output I am seeing the input swr at
1.6:1 through the input transformer...
I was going to calculate the grid dissipation then realized I don't
know the Grid to cathode voltage under load... We tried taking a DC
measurement off the filament to ground under load, but with the AC
superimposed on the cathode current I don't trust the numbers I see,
especially since there is a half volt, or more, difference from one
side of the filament compared to the other... So I don't know how to
calculate the dissipation unless I just assume that voltage from the
Eimac tube curves... Anyone?
denny / k8do
