Hello to All:
Here is the problem I am dealing with. I built a home brew amp using a pair
of Eimac 3-400Z's, which I obtained brand new in 1968. I hauled them around
for years, became active again in Amateur Radio and built this amp.
I used available information in 2008 to calculate the values needed for the
PI-NET output circuit, and then found a NOS B&W 852 tank coil at a Ham Fest
and bought it. I found that this tank coil had proper inductance, relative
to my needs, on all bands except 80 meters. I modified this tank coil to
have exactly what I believed I needed. If you look at a 2007 ARRL Handbook
in the PI-NET area, page 18.7, you can see my values are very close to what
is listed for a plate load impedance of 2100 ohms, which was about what I
expected during construction.
Upon completing the amp, I fired it up on 75 meter SSB and it ran
beautifully. I was getting about 62% plate efficiency, so I thought all was
well. I tried it on 20 meters, into a dummy load and as I recall, it ran
fine. I do not use it too often, mostly running just my exciter.
Recently I tried to load it up on 40 meters, but I immediately discovered a
horrible efficiency, something in the 25% range. Nice cherry red glow to
the plates! So that is the issue and reason for this posting, I am trying
to understand why my 40 meter efficiency is so bad. (80 meter was still
great at this time.)
Now for a bit of past history. I used the formulas for PI-NET component
calculations available to me at the time. They were based on the updated
formulas offered by Elmer Wingfield, so I felt I was in a safe zone. I also
have a General Radio 1656 Impedance bridge, which allowed me to measure the
tank coil values on each band, to what I perceived to be a very accurate
level. As a point of information, I obtained an original B&W sales brochure
and it gave the inductance values of their 852 coil on each band. My
measured values were very close "except" on 40 meters, so I convinced myself
that my impedance bridge was running OK. The PI-NET uses a Comet 5-500 pf
tuning cap and a Russian 15-1200 pf load cap.
For my calculations, I used an on-line calculator and obtained the following
for a Q=12, plate load of 2100+ohms:
Band
C1, in pf
L, in uh
C2, in pf
As Advertised B&W 852
MY Modified B&W 852
80 Meters
246
9.44
1352
7
9.3
40 Meters
123
4.72
676
3.72
4.5
20 Meters
61
2.36
338
2.34
2.34
15 Meters
41
1.57
225
1.34
1.34
10 Meters
31
1.18
169
0.95
1
The first columns above were the targeted area, the far right column is the
measured inductance as installed. The amp is actually running a bit above
2800 VDC with a plate current of 650 ma during tune up. I can vary the
plate voltage up to 3300 VDC, just as a FYI. I originally did the
calculations assuming a slightly lower plate voltage, but the 2800 VDC is
what I got in the end.
OH, another interesting point. When loaded on 80, I am actually using a
measure 280 pf on tune cap and 1630 pf on load, not real close to expected
values----
Since trying the amp and finding the issue on 40 meters, I tried using the
currently available on-line calculators, and my numbers are not matching as
closely as I would have expected. However, whether it ends up with a Q of
10 or 12, I would not expect that to result in an efficiency of 25% on 40?
I have checked the components using both digital and the trusty General
Radio 1656 Impedance bridge, where possible. The caps are all measuring in
a consistent manner between all three meters, I can not check the inductance
on multiple meters, the digital types do not have the required capabilities
in uh. They can not go that low, at least the ones I own.
The input circuit uses an LDG AT-100AMP auto-tuner, which does a real nice
job, my SWR is always down around 1.3-1.4 or better. I can drive these
tubes to my targeted value of 250 ma on grid current, on both 80 and 40, and
beyond - no shortage of drive is present. I have typically set up the
linear on 80 meters to 1200 watts output, the tubes have no issue on getting
there.
I have a significant number of pictures, and feel at this point I know the
circuit quite well. If anyone might have any comments or questions to aid
me, it would be appreciated. I have pulled the tank coil and again measured
the inductance with it sitting on the bench. Unfortunately, all seems well.
No issue was detected. I am thinking of putting it back in and hanging a
load resistor of 2100 ohms from plate to ground and sticking my MFJ-259B
meter on the output, and seeing if I can resonate on 7.175 MHz - that is my
next idea, anything you might have as a better approach?
Oh I do find a peak in output while tuning on 40 meters, just can't think of
anything else to try so that is why I through that out---
Thanks,
Chuck W9POU
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