[Amps] Plate Impedance

[TexasRF at aol.com]

Jeff, 5400 ohms plate load flies into the face of what we have been  
calculating using the K factor stuff.
 
3000v/1A/1.8 is equal to 1666 ohms so one of your numbers is flawed.
 
Using the other method, 2700v X 2 /3.14 equals 1720 ohms which is much  
closer (certainly close enough) to the usual calculation result.
 
73,
Gerald K5GW
 
 
 
In a message dated 4/10/2006 2:32:14 P.M. Central Standard Time,  
Xmitters at aol.com writes:

In a  message dated 4/10/06 9:27:25 AM Central Daylight Time,  
amps-request at contesting.com writes:

<< All,

I started  reading through the RCA Radiotron Handbook this evening looking 
for where  the factor of 1.8 is listed for calculating the plate impedance of 
a 
class  AB amplifier, and I cannot find it. The edition I have was the older 
one  
back in the 40's with the black cover. The red one was from the 50's and  had 
more in it if I recall, but I don't have it. I was wanting to find  where it 
gives this factor, and the ones for Class A, AB1, AB2, B, and C.  Bill Orrs 
handbook gives the factor of 1.8 on his Pi tank values table,  but doesn't 
mention 
anywhere in the text where it came from. I also looked  in a RCA receiving 
tube 
book and could not find it there either, or I am  overlooking it somewhere. I 
could have sworn it was in the Radiotron  Handbook. The Handbook does say 
that 
the plate impedance is twice the peak  voltage swing times the peak anode 
current. What I'm wanting to know is  where are these factors located in 
print, as I'd 
like to read the whole  texts concerning this? Any help
would be deeply appreciated.  Thanks to all in advance.

Best,

Will
>>

Will:

I do a great deal of mathematical analyses on  high power RF amplifiers. My 
favorite resource is the Eimac Care and  Feeding of Power Grid Tubes 
available on 
Eimac's web site. I think  Richardson Electronics may also have hard copies 
available. You also need  the clear plastic overlay and a set of constant 
current 
curves for the  tube you are using. Then you can calculate some pretty close 
values for  your desired parameters. This is the most accurate mathematical 
model that  I'm aware of. Some may considered as too tedious, and that's 
fine. It 
is  still the most accurate method of modeling on paper.

My second choice  is the mathematical steps described in the RCA Transmitting 
Tubes book  number TT-5. This book is available from many web resources. 
There 
is a  step by step procedure in there for calculation all of the operating  
parameters. both RF and DC. The calculations rely on a table of "K"  factors 
that 
are dependent on the plate current conduction angle for the  class of service 
desired. This is a fairly straight forward mathematical  process.

To answer your specific question, the input impedance as seen  by the tube of 
the tank network is RF plate voltage swing divided by the  peak fundamental 
component of the plate current. If you don't get this  number "right" the 
effect 
is distortion, crappy efficiency and maybe even  instability.

Let me give you an example. Let's say that we are  designing a class B 
amplifier and we know the peak plate current is one  amp. Class B suggests 
180 degree 
plate current. Therefore the DC plate  current is going to be the peak plate 
current divided by 3.1416. The peak  fundamental plate current is going to be 
half of the peak plate current or  . 5 amp.  The tube curves are used to find 
an 
appropriate minimum  instantaneous plate voltage based on the desired 
linearity 
and the best  value to use is tube dependent. As a generalization, make this 
minimum  plate voltage (ebmin) 
equal to ten percent of your DC plate voltage. Say  your plate voltage is 
3000 
volts, so ebmin is then 300 volts. the plate  swing is therefore 3000 - 300 = 
2700 volts. So the impedance the tank  needs to present to the tube is 
2700/.5 
= 5400 Ohms. The power output BTW  is plate swing times Peak Fundamental 
Plate 
Current times 0.5 and this  assumes the average power of a CW signal.


The problem with the RCA  K values and any other constant for that matter, is 
that they ignore the  tube characteristics. Furthermore, often times the 
basis 
for which a  multiplying constant is derived is not always known. The RCA K 
values and  the procedures in Bill Orr's famous works, all assume that the 
plate  
current is going to be a perfect sinusoid over the portion of a cycle for  
which 
it conducts. This is not an accurate assumption and fortunately for  
approximation purposes, is usually good enough. The advantage of the K  
values and Bill 
Orr's calculation process is it gives you a reasonable  starting point for a 
design. There is always going to be some "lab work"  so there is a personal 
balance everyone must make individually as to how  much time is going to be 
spent 
calculating and how much time is going to  be spent constructing and 
optimizing.

The first question to ask  regarding any mathematical model is "how much 
accuracy do I really want or  need? The usual response is "as much as 
possible". If 
you have infinite  research dollars and infinite time, this is a reasonable 
answer. Who  really has that advantage? Obviously there is a practical answer 
to 
this  question. 


I should point out that the example I gave you only  applies to a class B and 
any other class of service is going to command a  different set of K values. 
My 
example was to illustrate the basic process.  When I do an analysis on a 
broadcast transmitter, I like to start off with  the RCA K values to get an 
Idea for 
what I'm dealing with. I then follow  up using the Eimac method, a drawn 
Operating Line and the accompanying  calculations to get the final, usually 
more 
accurate results. The Eimac  method is restricted BTW, in that the voltage 
waveform on the grid must be  the same kind of waveform on the plate. IOW, 
you cannot 
drive a tube (with  a resonant plate tank network) with a square wave with a 
sinusoidal signal  on the plate, and expect meaningful results with the Eimac 
Tube  Performance Computer. It does not work.


This is probably a more  lengthy response than you were looking fore, but I 
hope it helps  you.

Jeff Glass, BSEE CSRE
Chief Engineer
WNIU WNIJ
Northern  Illinois  University
WB9ETG
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