Rich,
So what you are saying is that it really doesn't matter from the tube's
input impedance stand point what value of resistor I use.
I am running 3KV on the anode and 550V on the screen. According to the
constant current curve, -105V should give me 250 mA ZSAC or 50% of the max
dissipation, so I need about 95V of drive across the resistor.
Jerry Sevick's W2FMI has a trifilar wound 2:1 transformer supposedly good
for 150W continuous. However, it is only flat up to 25MHz. The same
transformer with a slightly different tap position for 2.25:1 is supposed to
be good up to 30MHz.
If I go to the 4:1 and use a 200 ohm resistor, I have to look at an
attenuator for the exciter to keep from overdriving the amp.
David
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-----Original Message-----
From: ham_amplifiers@...
[mailto:ham_amplifiers@...]On Behalf Of R L Measures
Sent: Friday, December 22, 2006 9:42 AM
To: ham_amplifiers@...
Subject: Re: [ham_amplifiers] 4CX1500A
On Dec 22, 2006, at 5:11 AM, David C. Hallam wrote:
Does anyone have information as to the approximate input impedance
of a
4CX1500A for the proper value of a passive grid resistor for this
tube?
D. -- The resistive component of the grid input Z is well over
1,000,000-ohms in AB1, and the reactive component is 78pF. With
750V on the screen, it takes c. 170 grid V to establish a ZSAC of
200mA. If the grid is terminated in 200-ohms, 73w-pep is needed to
drive a 4CX1500A. With a 50-ohm termination, 289w is needed.
I
need about 100 ohm to get sufficient grid drive, i.e., 95 volts, so
will
have to use a balun to maintain 50 ohm for the exciter. I just
wondered how
close a 100 ohm resistor would be.
50 to 200 ohm un-uns are a piece of cake, but a 50 to 100 ohm un-un
probably isn't very easy to make.
cheerz
David
KC2JD
R. L. Measures, AG6K, 805.386.3734
r@..., rlm@..., www.somis.org
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