Date: Thu, 01 Apr 2010 10:54:55 -0700
From: Larry <larry@w7iuv.com>
Subject: [Amps] More LCR meters and measurements
Those that are used more are the ones that
are quickest to grab and use and are not necessarily the most accurate.
## Eimac sez a 3x3 is supposed to be 24.0 pf from anode to grid
on a wooden bench. I measure exactly 24.0 pf on the 875.
Eimac sez the YU-148 is supposed to be 24.5 pf, from anode to grid.
I measure exactly 24.5 pf on the 875. That's good enough for me.
Of course, that stray c will rise as soon as tube installed on grid ring.
The proximity of lower anode fins to chassis is what causes the increase.
It has nothing to do with the socket. I removed the socket, left the grid
ring installed, and get identical readings. A 3x3 will rise from 24 pf
on bench up to 33 pf, when in grid ring. a YC-156 will rise from
36 pf up to 55 pf when bolted to chassis, etc.
Just moving the inductor from the test set to the final application will
change it's parameter. The difference can range from "insignificant" to
"considerable". The only way you can tell for sure is to carefully
measure the performance of the application network in actual operation
to see if the inductor came out as it was supposed to.
## agreed. measure everything, when installed in chassis. Then
start adding side walls, rear walls, top lid, etc, and watch the stray
C increase 1-2 pf each time.
Most amp builders don't care, they just twist the crap out of those two
knobs on the PI network for max power out and move on. If you want to
impress me with your "professional skill" level and measurement
capability, throw away those knobs and use fixed capacitors!
## which is why I don't use fixed caps, nor 9 x bandswitched
tuned inputs anymore. Those days of fixed SM caps and a
tunable Millen 60946 slug or XR-50 are over. Plan B was
to use variable arco's and fixed T-50 torroids. Plan C is to use
1 x tapped coil, and 2 x broadcast variables.
With the inductor design software available these days, you really don't
even need to bother measuring tank coils and taps on them. The last tank
I made, I used KM5KG's Rf network Designer to design the inductor. It
even lets you calculate where the taps should be placed.
## I only have the trial version of KM5KG's RF network designer.
The full blown version is next weeks project, which should have been
done a long time ago.
An even more critical evaluation ot the completed tank can made by
measuring the actual loaded Q of the entire tank network. This requires
a signal generator and spectrum analyzer or a VNA like the N2PK-VNA that
can do transmission measurements. If there is interest in this
measurement, I will go into details in another post. This measurement is
always interesting and sometimes surprising as the actual Q is often not
anything like the Q you think you designed the tank for!
## when the tank coil cooks, and BW is narrow, that's usually
a subtle hint, that loaded Q is too high. A loaded Q of 8-10
using GM3SEK's sheet works good. If drive power is then reduced,
plate load Z will rise, and so will loaded Q. A 8-10 loaded
Q will rise to 10-12. Eff is still good, no coils get cooked...
[ and no bandswitch's getting cooked either]
If the software designed inductor does not work as advertised, it is
only because the input data to the software was incorrect or (more
likely) the inductor was jammed into the chassis too close to other
objects which significantly altered its parameters.
Now I have only three samples to base my MFJ-259B performance
observations on. One borrowed instrument, and my own, before I zapped
the detector diodes and after MFJ replaced the detector board. In all
three cases, the MFJ produced measurement results that were at least an
order of magnitude better than what VE7RF reported. The caveat here
being that only measurements the instrument was capable of making were
considered and the "fixturing" was properly tended to.
## Maybe mine is blown up ?
There is no way to "zero" out the lead contribution and depending on
how they are positioned with respect to the coil under test and the instrument
itself, the results could be misleading.
## u could hook interconnecting leads to mfj.. and measure it... then
subtract those readings from the completed coil + interconnects.
If I had either the B&K or the AADE meters, I would use them (a lot!).
But only for air core coils. Toroids would mean back to the MFJ again,
or one of the other half dozen ways of measuring I have.
## The b+k has spring loaded 3/4" wide slots on front, so bannana
jacks/test leads don't have to be used at all. Any strap up to 3/4"
wide, can be used as a test lead. Also, things like .01 uf caps can
have their leads stuffed deep into the same spring loaded slots,
eliminating the stray L from their 20 ga long leads.
Jim VE7RF
73, Larry
Larry - W7IUV
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