[Amps] 200-ohm tank and "un-un" in HF amp design?

[Jim Barber]

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
>