[Amps] Measuring tank coil inductance

Jim Thomson jim.thom at telus.net
Thu Apr 25 23:04:11 EDT 2013

Date: Thu, 25 Apr 2013 18:39:22 -0700
From: Bill Turner <dezrat1242 at yahoo.com>
To: Amps <amps at contesting.com>
Subject: Re: [Amps] Measuring tank coil inductance

ORIGINAL MESSAGE (may be snipped): 
On Thu, 25 Apr 2013 17:29:02 -0700, you wrote:

>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:

If you're going to use the AADE unit to set the capacitance in-circuit, be
sure to disconnect the plate choke. 

Bill, W6WRT

##  whoa.  A typ 90 uh plate choke on a  80-10m amp will  eat 10 pf  from the C1 tune cap on 80m..
A typ 180 uh plate choke will eat 40 pf  from the C1 tune cap on 160m band. 

##  You can see that effect asap  when you wire in the load resistor  between anode and chassis...
with mfj  on the output.   Tweak tune and loads caps  for  1:1 swr on mfj.   Then disconnect hot end
of each cap.... and measure how many pf each cap is.  This is all with the plate choke in the circuit, and
bypass cap from base of plate choke to chassis. 

##  OK, then remove the plate choke, or disconnect  top end of plate choke.  You will see asap that the
swr on the MFJ has jumped to 2:1   Retune the C1  tune cap to bring swr back down to 1:1 

##  Disconnect hot end of C1 +C2 tune cap..and measure how many pf it is.   You will see that the C1  tune
cap has INCREASED in value.  On 80m,  with a 90uh plate choke used, it will increase by 10 pf.  On
160m, with a 180uh plate choke used, it will increase by  40 pf.   On 160m, with a 90 uh plate choke used.
it will increase by a whopping 80 pf. 

##  The tune cap has to be increased a bit to cancel out the XL  of the plate choke on the lowest band used.
It will be less as you move higher in freq. 

##    Hot ends of BOTH  tune and load caps  have to be disconnected,  to measure the C of each cap,
or else the two caps will be in parallel, and any lcr meter will  simply read the sum of the 2 x caps. 

later......... Jim   VE7RF
via the tank coil between em.      

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