Date: Sat, 10 Jun 2017 05:51:41 -0400
From: "Roger (K8RI)" <k8ri@rogerhalstead.com>
To: amps@contesting.com
Subject: Re: [Amps] LDMOS availability
<Like Bill and Jim(s) I disagree. You really aren't running class C all
<the time. You are varying the bias from class to change from class C to
AB 1 or 2 either continuously or in steps. I question the effect this
would have on the actual linearity. With the bias / class changes, the
drive required changes and the gain changes.
Even with normal amps, is the output a faithful reproduction of the
input? With the current state of the art for ham gear, I doubt it even
in class AB1. Many of the signals I hear on the bands went through some
stage that wasn't linear. Dynamic predistortion we have finally
surpassed the signal quality of the old tube rigs. HOWEVER, as I've
asked before, how much deviation can we accept in the output from being
a faithful reproduction of the input before we no longer see it as being
linear.
73, Roger (K8RI)
### The concept of sliding bias has been around in the audio world for quite
some time. Krell uses it to slide their audio amps from Class A....down to
Class AB.
That way, their Class A amps are not running like blast furnaces, with low
drive levels,or
during pauses etc. Audio amps are not like RF amps, where on ham RF amps,
the idle is cut off
on RX. Audio amps are running Class A all the time, as soon as you turn it
on.
## My Denon stereo amp uses sliding bias, but in the opposite fashion. With
real low signals, it
runs Class A.... up to aprx 5-7 watts. Any more, and it slides into Class
AB, where its more eff, and
less heat... like from 8-120 watts.
## Yaesu shoulda done that on their Class A xcvrs, when in Class A mode.
IE: Slide it into class AB,
between words, pauses etc, so its not sitting there...sucking 10A @ 30 vdc =
300 watts. Key the ptt,
say nothing, and you end up with 300 watts being dissipated. Talk..and its
now 75 watts out..and 225 watts diss.
They coulda dumped a lot of heat using a simple sliding bias scheme. On the
yaesu 5000 + 9000, that front panel
adjustable bias control only works while in Class A mode. Crank it on way
and its pure class A. Crank it the other
way, and its adjustable from Class A....down to class AB. And either way,
its still only 75 watts out. You have to switch
to Class AB, to get the full 200 watts out. That was yaesus method to reduce
heat while in Class A mode. Which
sorta defeats the Class A concept.
## I have not done this ..yet, but I will try either reducing the idle
current, when in Class A mode.... or increasing the idle when in Class AB
mode. Im certain that a compromise can be made. Class A is over
kill...while AB is less than optimum. I have 8 x bias adjustments
in total on my yaesu MK-V. 4 for the driver, and 4 for the final. 2 are
for driver, 2 for final..for Class A, then repeat for AB.
## right now its sucks 2A in AB...and 10A in A. I will experiment with
say 3-5 A in class AB. The idea being a compromise between
oem A + AB. The mating Driver bias will be tweaked in the same ratios.
## when listening on a 2nd MK-V...3 ft away....and off freq, the difference in
splatter between A + AB is astounding...and thats running 75 w pep
out in either mode. The MK-V in AB, when run at 75 w , is a lot cleaner vs
200w in AB.
## Y/ I /K needs to introduce xcvrs with the pre-distortion built
in...without the 20 msec latency. That 20 msec latency on the ANAN 8000 would
drive me nuts.... no way to monitor yourself when using headphones.... which
kills it for me right then and there.
later... Jim VE7RF
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