[Amps] Measuring tank coil inductance

[Vic K2VCO]

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/