On Jun 7, 12:19*pm, K7ITM wrote:
On Jun 6, 7:29*pm, walt wrote:
...
And Tom, why would the source be reactive when the pi-network is tuned
to resonance? And because the source resistance of the (tube) power
amp is non-dissipative, its reflection coefficient is 1.0 by
definition, and so it cannot absorb any reflected energy, and
therefore re-reflects it.
Walt, W2DU
If you allow that the thing driving the pi network has an effective
source impedance different from the load that the pi network presents
to it, then clearly the output impedance seen at the other end (the
"50 ohm" end) of the pi network won't be 50 ohms. *Try it with some
numbers; for example, assume a pi network that transforms your 50 ohm
load to a 4k ohm load to the amplifier output, and assume an amplifier
output stage that looks like a 20k ohm source. *Design the pi network
for a loaded Q of 10. *I believe you'll find that the source impedance
seen by the 50 ohm load is about 11+j18 ohms.
As Wim has pointed out, requiring an amplifier to be loaded and driven
in a very particular way unnecessarily dismisses some very important
classes of amplifier. *What do you do, for example, with a linear
amplifier? *What do you do with an amplifier that drives a voltage
very hard (and for which a simple pi network is inappropriate for
matching to a load)? *Perhaps an even more basic question is: *why
exactly do we tune a pi network to present a particular load to an RF
amplifier stage? *Why should we operate a 6146 with, say, a 3000 ohm
plate load? *Why not 1000, or 6000?
And what if I set up a tube and pi network for operation such that the
apparent output source impedance is 50 ohms (while driving a 50 ohm
load), and then I add feedback to the amplifier in such a way that the
operating conditions are not changed, but the impedance looking back
into the plate is changed?
How did we get to the source resistance of "the (tube) power amp"
being non-dissipative? *I know there are some of us who don't buy into
that...
Cheers,
Tom
Hello Tom,
Well, the reason I chose to discuss only the tube amp with a pi-
network filter and impedance transformer is that that arrangement is
the only one I've measured, and that I'm not sufficiently acquainted
with other arrangements to discuss them.
As to your question, why not use plate loads of 3000, 6000 or 1000?
Because the transceivers I measured using two 6146s in parallel are
Kenwood TS-830S and Heathkit HW-100. I don't remember the exact plate
load with the HW-100, close to 1400 ohms, while the TS-830S was 1400
ohms. I had no control over those plate loads because the plate and
grid voltages were preset, and 1400 ohms is the RL I measured at the
input of the pi-network when the grid drive and the network were
adjusted to deliver precisely 100w to the load 50-ohm load.
Now concerning the non-dissipative source resistance of the tube-type
power amp. There are two separate resistances in the amp, the cathode-
to-plate resistance, Rpd, that accounts for all the dissipation in the
tube; and the output-source resistance that is non-dissipative. It is
a common myth that an RF power amp cannot have an efficiency greater
than 50% when conjugate matched to the load, because half the RF power
is dissipated in the source resistance. This is not true, because when
the amp is operating properly, resistance Rpd is less than the output
source resistance, thus allowing more power delivered to the load than
that dissipated in the plate-to-cathode resistance. The output source
resistance is derived from the voltage-current ratio E/I that appears
at the output terminals of the pi-network. A ratio, as such, cannot
dissipate energy, but the load it feeds does.
Using an example from Terman's Radio Engineer's Handbook I explain
this phenomenon in great detail in Chapter 19 in Reflections, and
further in Chapter 19A, an addition to Chapter 19. Both are available
on my web page at
www.w2du.com. Chapter 19 appears in 'Read Chapters
from Reflections 2', and 19A appears in 'Preview Chapter from
Reflections 3'. The entire Chapter 19 appears in Reflections 3, which
is now available from CQ. I invite you to review these Chapters.
Walt, W2DU