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,
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