Just to clarify, I used 1.6 for "K", which in Ian's spreadsheet is in
the middle of the range for AB:
Single 8877
4KV
625mA
60% efficiency
"K" factor of 1.6
= 4000 ohms plate load impedance.
Jim W7RY: You're correct about the EP dropping some under load, of
course. My transformer is 3KV 2A CCS, (admittedly overkill for this
example) filter is 32uF, only enough bleeder current for safety. 4KV
seemed like a comfortable estimate, since the (unloaded) peak voltage
will be around 4.2KV .
Vic: Thanks for the link regarding the constant current curves.
I most definitely will look into that, and may post back if I find
something worthwhile.
To summarize, the consensus seems to be a simple PI with padding cap(s)
switched into the low-Z end on 160 meters. Since in this case the main
tank inductor(s) are a 3uH "transformer" inductor followed by a large
square-wound 24uH roller, I'll actually need to switch in both
additional L and C. I can get 5KV 10A Kilovac relays cheaply enough;
even with wacky tuning and load SWR considerations those should do the
job at the low-Z end.
Thanks es 73,
Jim N7CXI
On 10/20/2013 9:56 PM, Vic K2VCO wrote:
Don't believe the 1.8.
I found that this is far from the correct value for the 4CX1000A. Maybe
it's closer for triodes like 8877s, maybe not.
See
<http://www.granta.g4axx.com/Linear_design_notes.php3>
for an example of how to determine the load impedance from the the
constant current curve for the tube. Here's an 8877 datasheet which
contains those curves:
<http://www.g8wrb.org/data/Eimac/3CX1500A7.pdf>
On 10/20/2013 1:37 PM, Jim W7RY wrote:
Jim...
To figure the plate load impedance:
RP = EP / 1.8 X IP
1.8 = K factor for class AB1 (Changes with ZSAC)
RP = Plate Resistance
EP = Plate Voltage
IP = Plate Current
1.8 X .65 Amps = 1.17
1.17/ 4000 = 3418 Ohms plate load impudence.
Your actual voltage will more likely be 3800 with a .65amp load which
brings the load down
to 3250 Ohms.
Better to just switch in some loading padders with your bandswitch.
See the last 8 years of the ARRL handbook for the K8RA amplifier using
the 3CX1500D7 tube.
There are some great examples there.
73
Jim W7RY
On 10/20/2013 8:28 AM, Jim Barber wrote:
1500 watts
4kV, 625mA
Single 8877 at around 4000 ohms plate impedance in AB2.
Tnx,
Jim N7CXI
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