On Dec 23, 2006, at 5:45 AM, David C. Hallam wrote:
Rich,
In regard to your suggestions of increasing the screen voltage to
750V or
turning down the drive, these are my problems.
1. The screen, bias, and HV supplies are already built. The screen
supply
is 550V zener shunt regulated.
Adding zeners is easy.
I don't think the transformer has the output
to increase the voltage by any significant amount, i.e., more than 50V or
so. I could run some tests and see.
What type of rectifier config.?
2. The bias supply would operate at -170V, but I don't think it has
enough
head room to supply the necessary voltage to bias the tube to cut
off in
stand by with 750V on the screen. In stand by it puts out about
-220V.
¨C220V will cut one off.
In
addition to supplying the grid bias, I use it to hold in a relay that
operates the relay for HV and screen primary power. That way if
the bias
fails for any reason, HV and screen voltage are removed from the tube.
Good move, David.
3. I could reduce the drive by turning down the audio gain but
really don't
want to operate that way.
A better way of avoiding grid-I is needed.
4. The exciter's carrier suppression is degraded if I turn down the RF
drive.
Carrier's are okay.
David
-----Original Message-----
From: ham_amplifiers@...
[mailto:ham_amplifiers@...]On Behalf Of R L Measures
Sent: Friday, December 22, 2006 4:38 PM
To: ham_amplifiers@...
Subject: Re: [ham_amplifiers] 4CX1500A
On Dec 22, 2006, at 7:50 AM, David C. Hallam wrote:
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.
Correct, but since it takes 170v peak RF V to drive the grid from ¨C
170v up to 0v, the higher the grid terminating R, the leas RF power
will be needed to reach 0v.
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.
It takes +105v-pk of RF drive to overcome ¨C105V of grid bias. Why
not use more screen V ?
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.
Why not turn down the power output, David ?
David
-----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. ¨C170 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
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