[Amps] Fwd: L-PI amp network

Mon Dec 19 11:06:42 EST 2022

---------- Forwarded message ---------
From: jim.thom jim.thom at telus.net <jim.thom at telus.net>
Date: Mon, Dec 19, 2022 at 8:04 AM
Subject: Re: [Amps] L-PI amp network
To: Jim Kinney <jimkinney at bellsouth.net>

I use a B+K  875B   It measures down to just .1 pf    and also down to just
.1uh.  (mouser)   Comes with a pair of 3" test leads.

I have also seen some cheap offshore lcr meters, that read down to .01pf
and also .01uh.

In any case, the stray pf and uh in the test leads has to be cancelled out.

I also have the older  B+K  875.    Can't zero out the uh on the pair of 6"
test leads.  Shorted they are .66 uh.     So had to subtract .66 uh
from all readings.

Sometimes I use the 3" leads..and sometimes a single 3"..and also a single
6"...and sometimes a pair of 6" leads.

The 875/875B both have spring loaded copper plates, so if testing stuff
like say .01uf caps,  test leads are unplugged, not used.   The .01uf cap's
wire leads are stuffed directly into the  spring loaded pair of copper
plates on each side.  Then it's real fast to test a bunch of em.

The newer  B+K 875B measures AC resistance, NOT dc resistance.  It has a 2
ohm scale.... and measures down to .001 ohm.  It does all ac
resistance measurements at 1 khz, using a square wave.  It's useless for
measuring say the pri resistance of a dahl plate xfmr.   The 1 khz sq wave
won't make it through to the secondary.

BUT it can be used to measure the ESR of electrolytics...at 1 khz.   Typ
the ESR of lytics are spec'd / stated at 120 hz..and also 10 khz.  Readings
done on the 875B at 1 khz will read lower vs 120 hz.  I have to take the
readings,,,and divide by 3.1  to get the actual ESR at 120 hz.  No big
deal.  Both versions of the 875 will read D factor on caps.  This comes in
handy when measuring a caseload of identical lytics.   You can spot a bad
one asap.

On both versions, it will read down to .1 pf  if on the 200 pf lowest
scale.  On the next scale up ( 2000 pf), it only reads in 1 pf increments.
same deal with uh. Below 200 uh, reads in .1 uh increments.

The 875 will read up to 2000 uf.   The 875B will read up to 20,000uf.   I
have and use lytics ( 450 vdc types), that are 10,000 uf.  But can't
measure thr 43,000 uf @ 50 vdc lytics.

You can get better lcr meters that will read ESR at several different
freqs, like 100/120 hz, and also 300/360 hz ( used for 3 phase)  and also
higher like 10-50 khz.... for switching supplies etc.

For my purposes, at the time the 875 and later on 875B was ample.  But a
lot of cheaper lcr meters now on the market, that work superb.   Others may
well chime in.

Before I got it, I was bumbling about with everything.  Being able to map
out a vac cap from A-Z, in just 3.6 deg increments is a treat.  I still use
the old groth counters that divide each turn into 100 increments  in
addition to the 00-99 turns.   Ditto with mapping out an air variable with
it's 6:1 vernier.  ( jackson bro's skirts are marked 0-100).   Ditto with
any roller coil too. mapped out in fractional turns.   Only then do I know
exactly what values I'm using.

Beware, once the coil is in between the tune and load cap, u are now
reading both caps.,  The coil between em acts like a jumper wire.   So all
these readings are done with nothing initially connected to em.  It's a
tedious process, but well worth it.

Jim   VE7RF.

On Mon, Dec 19, 2022 at 6:50 AM Jim Kinney <jimkinney at bellsouth.net> wrote:

> Great info Jim, I've been confused on the spreadsheet myself trying to
> convert an SB-220 to 6m.
>
> What LCR meter do you recommend?  I don't have a good one.
>
> Thanks & 73,
> Jim WE4S
>
> Get BlueMail for Android <https://bluemail.me>
> On Dec 19, 2022, at 8:16 AM, "jim.thom jim.thom at telus.net" <
> jim.thom at telus.net> wrote:
>>
>>
>>
>> Without a digital LCR meter, you are dead in the water. It's ur number one
>> tool when building amplifiers, or anything else.  Then you can map out both
>> the tune and load caps in fine increments and make up a cheat sheet.
>>  Ditto with measuring coil values.    handling all the current.
>>
>> Jim  VE7RF
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