On Jun 4, 12:38*am, K7ITM wrote:
On Jun 2, 10:01*pm, Roy Lewallen wrote:



This might shed a little light on the discussion.

Consider a diode, forward biased by a 10 mA DC current source connected
directly across it. The characteristics of this particular diode are
such that the voltage is exactly 0.7 volt.
...
The lengthy discussion about tube plate resistance has muddled the DC
operating point (equivalent to the diode DC bias) and the plate
resistance, which is the plate's AC or dynamic resistance. They're
related just the same as for the diode. And just like the diode, if you
were to send an AC signal to the plate of a biased tube through a
capacitor, you'd find an AC voltage and current which are in phase
resulting in power being delivered to the tube, and an increase in plate
dissipation in the amount of that power.

Roy Lewallen, W7EL

I'm happy to see that Roy posted this clarification about dynamic
versus static resistance. *I haven't read all the posts here, but
those others that I have seemed to completely skirt the issue of the
meaning of "plate resistance."

Further to Roy's posting, though, you must realize that a device with
more than two terminals is complicated by the fact that you need to
specify that the dynamic resistance is measured with other terminals
held constant. *So, for example, plate resistance of a tube is
normally defined as the partial derivative of plate voltage with
respect to plate current, with the grid-to-cathode voltage held
constant, and of course at some particular plate voltage (and
corresponding current). *For a tetrode, the screen-to-cathode voltage
must also be constant.

The reason for this is that you will get completely different answers
if you don't hold the voltage of the grids constant with respect to
the cathode. *For example, if you set up a circuit with a tube with
the grid grounded, and an appropriate resistor between the cathode and
ground (not bypassed), to establish a desired bias point, then a
change in plate current will be accompanied by an essentially
identical change in cathode current, and a corresponding change in
drop across the cathode resistor, which represents a change in grid-to-
cathode voltage. *Measuring dV/dI at the plate in such a circuit will
yield a much higher resistance than if the grid-to-cathode voltage is
kept constant. *It is, in fact, a good way to make a constant current
source.

There is no particular relationship that must be maintained between
the load resistance you present to an amplifier element -- a tube or a
bipolar transistor or a FET -- and the plate/collector/drain (dynamic)
resistance of the device. *For example, it's common to operate a push-
pull pair of 6146's in audio service with a load around 5000 ohms
(plate to plate) at 500V plate supply voltage, 185V screen voltage,
and perhaps 30mA plate current. *That's effectively 2500 ohms to each
plate of the pair. *But at that operating point, the plate resistance
of a 6146 is around 600V/30mA, or 20k ohms (roughly, from plate
characteristic curves in my old RCA transmitting tubes manual). *On
the other hand, a triode will have a much lower plate resistance,
likely similar to the load resistance--but again, not really related
to it.

Another rather interesting thing happens, though, when you connect an
RF load through a tank circuit such as a PI network. *It's quite
possible for a pi network to transform a 50 ohm RF load so it presents
a reasonable (say) 4k ohm RF load to the plate of a tube like a 6146.
But that same pi network will in turn transform the plate resistance--
or rather, the net impedance of the tube in its particular circuit
configuration (grounded grid or grounded cathode, and invariable some
amount of feedback whether you wanted it or not), along with the DC
feed (RF choke) in parallel, to quite possibly an impedance not very
close to 50 ohms, and likely rather reactive. *I can, without much
difficulty, design a PI network that will yield a source resistance at
the output connector that's a lot less than 50 ohms, using a tetrode
amplifier tube, while giving a very decent transformation of a 50 ohm
load to a desired load at the plate of the tube.

And as if that weren't enough, I can modify the output impedance
further by application of feedback; this is more commonly done at
audio frequencies, where amplifiers designed to drive loads like 4
ohms an 8 ohms have output impedances in the area of a small fraction
of an ohm. *But it's also done with RF amplifiers, often for reasons
unrelated to the output impedance, but also to control the output
impedance so that it IS very close to 50 ohms (for example in
instrumentation, where it may be important).

Cheers,
Tom

I'm going to break with my earlier decision to no longer respond to
this thread in order to respond to Tom, K7ITM.

Tom, if you review my earlier posts, you'll find that I agree with you
concerning plate resistance Rp as the partial derivative of the Ep/Ip
relationship when both the grid and screen voltages are held constant.
Although Rp is definitely a factor in determining the value of load
resistance RL, it is by no means the total source resistance RL, as
others have insisted.

Walt Maxwell, W2DU