Will and all,
as you have determined, the k=1.8 is an approximation that will keep you
inside the ballpark at least.
Each tube type has it's own characteristics that will cause a shift up or
down from k=1.8. For example, if you examine the 8877 data sheet constant
current curves: You will see a sharp upturn in the grid current when the plate
potential is about 600vdc. At this point, the plate voltage swing will run
approximately plate voltage under load minus 600vdc. If we are using 4000v
plate
voltage, the swing is then 3400vdc.
Using the earlier RCA info from this week, and assuming 1A average plate
current the plate load impedance would be (3400 X 2) /3 or 2266.7 ohms. Using
the K factor method, the required K would have to be 1.76 for the same
impedance.
If we decide to use 2500v plate voltage, still at 1A plate current, the
plate voltage swing would be 2500 - 600 = 1900vdc. The resulting plate load
impedance in this case then would be (1900 X 2) / 3 =1266.7 ohms. To reach this
value with the K factor method, the required K would have to be 1.97.
If you use a tetrode, the limiting factor for minimum plate voltage is
screen grid current. The knee of the curve for this is very close to the screen
voltage used. If you raise the screen voltage from a typical voltage to the
maximum allowed, the plate voltage swing is reduced. With a lower plate
voltage
swing, a lower plate load impedance is needed. This implies that the K factor
would need to be higher to reach the required impedance. Of coarse with
higher screen voltage comes higher plate current so an additional reduction in
plate load impedance is needed and another, even different, K factor.
So, you can see, K is very much an approximation. Using the actual tube
curves would seem much more precise. Having said that, the actual difference in
loaded Q by using k=1.8 vs 1.97 is less than one.
73,
Gerald K5GW
In a message dated 4/12/2006 11:03:29 A.M. Central Daylight Time,
craxd1@verizon.net writes:
Jim,
I read this over again last night, and it didn't mention anything about
triodes only. The main reason I posted this was to show the differences, plus
find where the illusive 1.8 came from in print. So far, the only book I've
seen
1.8 listed in was Bill Orrs Handbook. I don't have any newer ARRL handbooks
past the 90's as they seemed to be the same old thing, over and over, with not
that much new.
Here's the thing. I use 1.8 just like evryone else because it does get you
there. I calculate the plate current though the same way it was shown in this
volume of the ARRL Handbook, by efficiency. I seen in the old RCA Radiotron
handbook where it said the plate current for class AB could be as much as 3
times. How they come up with this, I don't know as efficiency is efficiency.
Everything I've read says AB is around 60% efficient, not less. To my opinion,
there's a lot of mis-information out there as compared to what I've seen work
in the real world. I would like to find out though where the factor of 1.8
was first mentioned.
Best,
Will
*********** REPLY SEPARATOR ***********
On 4/11/06 at 9:04 PM jkearman@att.net wrote:
>From: "Will Matney" <craxd1@verizon.net>
>> Class AB, K = 1.5
>
>My understanding is that this value of K applies to triodes, where Ep can
>swing nearly to zero. For tetrodes, Ep cannot swing below the screen
>voltage. IIRC, this has the effect of increasing K.
>
>It's useful to consider the consequences of slight errors in
>component-value selection. Assuming your variable controls (plate tuning
>and loading) have enough range to get a close match, the negative
>consequence would be a Q different from what you calculated. But if you
>give yourself enough range in tuning and loading Cs, you should be able to
>tune for _best linearity_ (more important than best efficiency) and still
>get enough Q to reduce harmonics below FCC requirements.
>
>If you calculate a range of plate loads by varying K from 1.5 - 1.8, and
>then calculate tank circuit values based on a Q range of 12-15, you should
>come up with tuning and loading cap values that will do the job.
>
>73,
>
>Jim, KR1S
>http://kr1s.kearman.com/
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