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[Amps] Grid Dipping the Pi network in a new amp?

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Subject: [Amps] Grid Dipping the Pi network in a new amp?
From: "Chuck Curran" <>
Date: Sat, 14 Jul 2007 16:55:07 -0500
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Hi Guys:


I am close to completing a new amplifier, using a pair new 3-400Z's I have
carted around since 1969.  I have run into a situation today that is at this
time puzzling me, and I need advice.


This amp is set to run with 2800 VDC plate voltage, under load, and uses a
B&W 852 tank coil.  I installed vacuum variables, a 290 pf tune cap and a
1300 pf load cap.   I got to the point today where I installed the
connecting straps between the tubes, RF choke, input cap tank coil and load
cap.  I was pleased with the results and pulled out the very, very old Heath
Grid Dip meter, circa 1956.  I couldn't get a dip anywhere, with the band
switch set for 80 meters and vacuum variables at maximum capacitance.   Yes,
I then individually checked each cap and the tank coil using a small L/C
meter, with the following results:


Tune Cap 288 pf

Tank Coil 6.9 uh

Load cap  1293 pf


No shorts or opens wee detected.  I removed the connection to the tubes,
i.e. the plate blocking caps and tried again, with the same results.


I do not yet have the required padding caps installed to get the exact
capacitance, but I expected to be able to obtain a dip within .25 MHz or so.
I got no dip at all, anywhere in the 2-5 MHz range.  My calculations are as


Source Impedance:    1800 

Load Impedance:           52

C1  Tune Cap:             274 pf

Tank Coil                         7.06 uH

C2  Load Cap            1415 pf


Actually I did not calculate the above, I put the Post Versa log away and
can't find it!  I used


I checked for opens and shorts, all is OK and now I am simply puzzled.  The
amps I built 37 years ago I just plugged in and ran, but I decided to grid
dip this one, simply to make sure things were OK.  Maybe I should just smoke


I am using a tuned input from a SB-220, which I successfully dipped, so I
think that the meter itself is OK.


Should a Pi net really dip, or not?


Thanks in advance,


Chuck     WA9POU

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