[Amps] Measuring tank coil inductance

Fri Apr 26 12:12:10 EDT 2013

Vic, the peak screen current will be several times the indicated current.  
The trip circuits have a time constant and don't respond to the peak current 
 that comes along at the rate of one per rf cycle. The screen bypass C 
tends to  soak up much of the peak current as well.

When rf drive is adjusted for rated screen current, the net result is an  
instataneous plate voltage minimum value quite a bit less than what you are  
plotting. The load line will be shifted somewhat as a result. Also, the 
power  generated is somewhat higher than you are expecting as the peak plate 
current is  increased as well.

Just as the indicated plate current is an average over the complete rf  
cycle, so are the screen and grid currents.

73,
Gerald K5GW

In a message dated 4/26/2013 8:49:49 A.M. Pacific Daylight Time,  
k2vco.vic at gmail.com writes:

Yes, I  stand corrected. The word ESTIMATE was used! A lot.

I was just  surprised at how different the value from the load line was, 
since I've seen  
the formula for estimating the impedance using the k factor in numerous  
places.

What I did was, first of all, to extrapolate the Eimac constant  current 
curve graph to 3.2 
kV (it only goes up to 3 kV). I felt justified  in doing this because the 
ZSAC at -60v on 
the grid and 325v on the screen  was flat from 500 to 3000 plate volts.

Then I followed G4AXX and  selected a minimum point of plate voltage at 
750v, corresponding 
to a grid  voltage of -25v, because this corresponds to a maximum screen 
current just  under 
50 mA, which is the trip point for the overcurrent circuit of the  screen.

Here is G4AXX's example of load lines for 2.5, 2.75 and 3  kV:
http://www.granta.g4axx.com/Linear_Design_notes/loadline4.gif
3.2 kV  is represented by the right edge of the graph. If you draw another 
line from  point 
A to where the extrapolated curve that represents a ZSAC of 250 mA  hits 
that edge, you get 
a load line whose slope represents 1815  ohms.

So why did I use 3.2 kV instead of a lower voltage? Because the  
transformer I have can 
produce either 3.2kV or 2.65 kV under load and I  felt that it would be 
pushing things to 
try to get a clean 1500 watts at  the lower voltage. For once I am trying 
to build an 
amplifier that will be  safe to use on SSB as well as CW, just in case I 
should want to!

On  4/26/2013 7:44 AM, Ian White wrote:
> K2VCO wrote:
>> I am  using the GM3SEK spreadsheet to determine the LC values. I have a
>>  simple application on my iPhone called "E-Formulas" which quickly
>  solves
>> the equation for resonance for l, c, or  f.
>>
>> One interesting thing that I noticed: the  spreadsheet says that you can
>> determine the tube load impedance by  the formula Rl = Ep/(Ip*k) where k
> is
>> 1.5 - 1.7 for class  AB.
> Ahem... it certainly doesn't say "determine" the load impedance  because
> you can't do that. The wording I used throughout was "ESTIMATE  the load
> resistance".
>
> The method using "k" can only  possibly give a rough estimate because
> tube characteristics are far  too complex to be condensed into a single
> "magic  number".
>
> Here is the full text (with ESTIMATE changed into  capitals):
>
> "There are various methods to ESTIMATE the load  resistance RL.
>
> "The most accurate method involves a load line  which is drawn over the
> tube's characteristic curves.  One end of  the load line is at supply
> voltage E and zero-signal anode current Io.  The other end of the load
> line is at peak instantaneous anode current  Ip and a minimum anode
> voltage Vo (typically 5-15% of E).   Calculate the slope of the load line
> = (E-Vo)/(Ip-Io) and enter it on  line line 35 of the spreadsheet."
>
> For tetrodes I should have  added another requirement: that the anode
> must at all times remain  significantly more positive than the screen, to
> avoid high peak screen  currents and accompanying nonlinearity.
>
> "Alternatively, the  spreadsheet offers two numerical methods of
> ESTIMATING RL, using a  factor K which depends on the class of operation
> of the tube (class A,  AB1, AB2, B or C). To use this method, enter E and
> the maximum DC  anode current Ia on lines 22 and 23, and your estimate of
> K on line  24.  Then you can enter either the desired power output W on
> line  29 or your estimate of efficiency on line 32.
>
> "Each of the  three methods - the method using the load line, or the two
> methods  using K - will typically give a different value for RL. You must
> then  CHOOSE YOUR OWN BEST ESTIMATE, and enter it on line 35."
>
> In  other words, you have to CHOOSE a suitable load resistance. The
>  spreadsheet can not determine it for you.
>
> Vic  continues:
>> Using the load line for the AB1 4CX1000A at 3.2 kV  and
>> 800 mA came out to 1815 ohms, which corresponds to a k of  about 2.2.
>>
> I'm not sure what you mean by "The load line at  3.2kV and 800mA came out
> to 1815 ohms"? There is a lot of missing  information about the way that
> you chose to construct that particular  line, and the specific locations
> of each end.
>
>
>  73 from Ian GM3SEK
>
>

-- 
Vic, K2VCO
Fresno  CA
http://www.qsl.net/k2vco/

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