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

[STEVEN & NANCY FRAASCH]

Chuck:

I don't know where you are coupling the grid dip to the network (I assume to the tank inductor), but you need terminations.  Try adding a 50 ohm load at the output, and place a 1800 ohm resistor from the plate to ground, leave the tube in place so that you have the tube parasitics in place.  Obviously you are doing this with the thing unplugged.

I do this all of the time, but I use an Autek bridge.  I connect to the amp output and connect a termination emulating the source impedance from the plate to ground and it works fine.

73,

Steve, K0SF

----- Original Message -----
From: "Chuck Curran" <ccurran at wi.rr.com>
To: amps at contesting.com
Sent: Saturday, July 14, 2007 4:55:07 PM (GMT-0600) America/Chicago
Subject: [Amps] Grid Dipping the Pi network in a new amp?

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
follows:

 

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
http://my.athenet.net/~multiplx/cgi-bin/pinet.main.cgi

 

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
test.

 

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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