Back in July there was a lengthy discussion here about
an amp's input circuit. Now I have been finding that
I have no way to really know the amp drive power from
my exciter rig. I thought I did by having a VFD peak reading
watt meter to set the drive, then switch that into the amp.
However, W1HIS, Chuck (who has the same sort of Henry
amp that I have) and I have been doing a lot of comparative
tests on our amps. He has pointed out issues which
impact this; I believe he is correct! His points:
"1. Unless the load VSWR is exactly 1.0, the forward power
indicated by any directional-coupler-wattmeter exceeds the
net power delivered to the load.
2. Because the Henry amp's input is a _nonlinear_ load, for
which voltage is not proportional to current!, all (simple) bets
are off!! The load is nonlinear because the grid part of the
cathode/grid circuit rectifies.
3. How a nonlinear load appears to a wattmeter depends strongly
on the voltage and current waveforms, which are nonsinusoidal
and which change depending on the length of the transmission
line and any filters, tuned circuits, impedance-matching, etc., etc.,
between the exciter and the load (the Henry's cath/grid circuit). These
nonsinusoidal waveforms have harmonic content, and the harmonic components
typically are reflected from both ends of the line and
form standing waves. Thus, waveshapes are different at different
points on the line. Garden-variety wattmeters do not sample the
entire waveforms and do not compute power, really; they use
diode rectifiers, which tend to respond to peak rather than rms
voltage, and so on; and they infer approximately what the
power _would_ be, were only the waveforms sinusoidal."
Next, Chuck ran an experiment with his Icom driven amp:
"Transceiver: Icom IC-775DSP, which is all solid-state. Adjusted
its output so that the VFD indicated 99 W when the Henry was
bypassed. The Icom's internal antenna tuner was switched OUT.
[Below I repeat the experiment with the Icom's internal
antenna tuner switched IN. Whether the Icom's internal antenna
tuner is switched in or out turns out to make a big difference,
although it "shouldn't", because in each case the Icom was
adjusted to deliver 99 W to the dummy load as indicated by the
VFD.]
1.6 Indicated PA voltage....4380 or so as read on
amp meter (voltage as selected using the "CW" bias position)
1.7 VFD Pout reading into Bird dummy load....1639 watts
But with the Icom's internal antenna tuner switched IN and the
Icom's power readjusted for 99 W to the dummy load as
indicated by the VFD, then VFD Pout reading into my Bird dummy
load is NOW 1896 watts!
Obviously (IMO) a nonsinusoidal reflected wave, very rich in
harmonics, returns from the Henry's cathode (input) circuit; and
these harmonics are absorbed or reflected by the Icom
completely differently depending on whether the Icom's
antenna tuner is switched in. So the voltage and current
waveforms delivered to the Henry are very different.
1.8 VFD indicated vswr....1.20 here, regardless of Icom
tuner in/out.
1.9 Bird meter on the amp Controller Unit.......2000 watts
when my VFD indicates 1896 W.
1.10 Now, lower input drive power to bring indicated output
power down to 1500 watts steady CW key down....VFD reads
either 1486 or 1529, evidently allowed values
for the digital readout in this area.
"Drive" power, VFD indicated with amp by-passed....88 Wtts."
Chuck then summarizes his and my comparative measurments:
"1. With the amp biased for CW, I get less or more power than
you do, with the same 99 W of VFD-indicated drive power,
depending on whether the antenna tuner _inside_ my exciter is
switched out or in, respectively. IMO this proves that the Henry's
grid circuit generates and sends a nonsinusoidal reflection back
to the exciter.
2. Neither your nor my VFD indicates VSWR = 1.0 (as it should)
when looking into a 50-ohm Bird termination.
3. The wattmeter on your amp controller reads higher than your
VFD; my controller panel meter is also optimistic, but not by
so much.
4. I always see less grid current and more plate current than you,
for roughly the same RF power output. Probably nothing to worry
about. Our tubes may be biased differently.
5. My filament voltage(7.4) is lower, perhaps 'cuz I changed the fil.
transformer primary tap (to the top) to lower the filament voltage.
Per manual, filament voltage is supposed to be 7.50 plus/minus
5%, in other words 7.125 to 7.875 ? [KH7M filament at 7.7 or so
per meter on amp controller unit].
6. My TUNE dial readings are the same as yours, but my LOAD
dial readings are much higher than yours; indicates higher load
C, lighter loading.
7. Both of us seem to be getting reasonable amounts of output
power to the Bird loads (1600 to 1900 watts with "CW" bias and
2400-2500 W with SSB bias) with about 100 watts in. The key
word being "about".
8. You might be able to get a valid measurement of the drive
power required by the Henry, by using an "antenna tuner,"
ahead of the linear amp and inserting a drive-power meter
between the exciter and said tuner. "
Is there any real benefit to bringing the line between the
transceiver and the amp input "flat" using either a tuner,
or perhaps putting in a 3dB or so pad?? Suppose there is
no real particular benefit to know the actual drive power
in any case! Not sure where to obtain such a pad to
handle drive powers up to 150 watts or so anyway such
that half of that power is dissipated and about 70 or so
"real" drive power is used. Would the output signal be
cleaner with such an input tuner, or the use of a pad
to bring the line flat, free of the non-linearities?
Complete detailed measurement data on each of our
3CX3000A7 amps can be supplied if you might be
interested.
73, Jim KH7M
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|