Post by coffelt2Ah, yes, "(negative modulation)"!
You seem to be "hep" on old stuff, do you remember "negative peak
clipping"?
I thought I was in Heaven in about 1958 with a single 2E26 final on 15
Meters.
In class C, with a pair of 6L6 modulators, push-pull, class AB1, and
used a VR
tube across the modulation transformer secondary clipping the negative
peaks, while
allowing the positive peaks to go "over" 100%.
Technical gurus of the day poo-poo'd the scheme. It looked a little
rough on the
scope, but unsolicited signal reports said it "packed a lot of
modulation".
I am thinking it must have been a little like more modern amateur
"speech
processing"
I also seem to remember those modified 1646's getting almost 20%
suppressor
grid modulation after tinkering around with power supply voltages.
Old Chief Lynn, W7LTQ
Hi Lynn,
A pair of 6L6's to modulate a 2E26... there was enough audio to drive a
6146 with a some to spare and enough to really overmodulate a 2E26! :-)
Negative peak clipping was based on a diode with its anode connected to
B+ and its cathode connected to the output side of the modulation
transformer. This would allow the final's voltage to drop to only a few
volts during negative modulation peaks. Later refinement was to add a
gaseous voltage regulator tube between the diode's cathode and the output
side of the transformer. In this case, the final's voltage would only
drop to a voltage determined by the regulator tube. Today a silicon
diode might be used, and zeners would replace the VR tube. The result of
this circuit was that the RF would never be completely cut off during
negative modulation peaks. Positive modulation peaks would be passed
allowing higher positive modulation. This is what gave the higher "talk
power." It still caused distortion, but much less than that if the
negative peaks were not clipped.
Everyone's voice is asymmetric. If you look at the voice waveform with
an oscilloscope, you will see that peaks on one side of zero are often
considerably higher that the opposite polarity. The average is still
zero, however. The peak is caused by harmonics in the voice being in
phase with each other. This property can be used to advantage in
amplitude modulation by having the peaks occur on the positive
modulation. Usually all that was needed to put the peak on the right
side was to reverse the connections to the modulation transformer or to
reverse the connections to the microphone.
I am sure Scott Dorsey knows more about this than I do, but CBS produced
two devices called the Audimax and Volumax that shifted the phase of the
audio as a function of frequency. Another term for these devices is
phase rotator. Kahn Communications also was in the market with its
SymmetraPeak. To the ear, the sound was unchanged, but to the
transmitter, the peaks became symmetrical. There is an excellent
discussion of these devices on James Tonne's (W4ENE) website
http://www.tonnesoftware.com/appnotes/allpass/allpass.html and on Gary
Blau's (W3AM) website http://www.w3am.com/8poleapf.html. {A biased
opinion here Jim's site contains some _excellent_ free software.}
As to being "hep" on old technology, I appreciate the compliment. As an
undergraduate, my university taught tubes and transistors. By the time I
got to graduate school, tubes were no longer taught. By the time I got
out of graduate school, integrated circuits were the "in thing" and
microprocessors had just begun. I do have a good collection of older
engineering books, however. I am constantly amazed by the technology of
the late 1920's and the 1930's. And it was all designed without the
benefit of computers! While I haven't used it in years, I still have my
K&E metal log-log-decitrig slide rule, and the bamboo rule I used while
in high school.
73, Barry WA4VZQ