Thanks to everyone for your answers. Here is what I decided to do:
I made a temporary hookup which includes the tube, the plate choke, the bandswitch and the
pi-net components. I intend to set up the tank as follows:
1. Preliminary tap positions will be determined by setting a GDO to the operating
frequency, paralleling a variable capacitor with the coil and adjusting it for a dip on
the GDO. Then I will measure the capacitor with the AADE LC meter and compute the
effective inductance of the coil.
2. Final adjustment will be made by putting a resistor equal to the tube load impedance
(determined from the load line) from the plate to ground, setting the capacitors to
calculated values (using the AADE again), and measuring the SWR from the output with the
MFJ antenna analyzer.
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. 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.
On 4/25/2013 9:50 AM, Radio WC6W wrote:
I've been trying to find a good
method to measure the inductance of large coils made out of
tubing
<snip>
Hi Vic,
I have an ancient HP 4260A bridge which suffices for most measurements.
However, for tank coils, I use a Time Constant circuit which is a variation of
the design published in the Jan. 1992, 73 Magazine, pp 38 & 40.
I up spec'ed to 74AC parts and ran the current up to realize a 1-50 uH range.
There is a similar design in the ARRL Handbook in the mid-2000's. This one has a zero adjust to fix the ground offset caused by the high currents employed. A better approach is to use a separate package for the output inverter.
Very useful for measuring tank coils in situ.
73 & Good morning,
Marv WC6W
P.S. -- I have a PDF of the 73 article that I can send direct if you wish. Not
sure if I ever drew up my version for real (orcad).
*
--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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