On Sun, 13 Jun 2010 05:17:54 -0700 (PDT), Richard Fry
wrote:

the PA tube -- whose non-linear characteristics

From Mendenhall:

"VHF amplifiers often exhibit a somewhat unusual characteristic when
tuning for maximum efficiency. ... If the amplifier is tuned exactly
to resonance, the plate load impedance will be purely resistive and
teh load line will be linear."

73's
Richard Clark, KB7QHC

On Jun 13, 12:12*pm, Richard Clark wrote:
On Sun, 13 Jun 2010 05:17:54 -0700 (PDT), Richard Fry
wrote:

the PA tube -- whose non-linear characteristics

From Mendenhall:

"VHF amplifiers often exhibit a somewhat unusual characteristic when
tuning for maximum efficiency. ... If the amplifier is tuned exactly
to resonance, the plate load impedance will be purely resistive and
teh load line will be linear."

'Unusual'? Certainly not unexpected once one thinks about it. For a
given controlled power, the minimum dissipation in the controlling
device will occur when the minimum voltage occurs at the time of
maximum current, and vice versa, i.e. the voltage and current in the
load are in phase, or equivalently, the load impedance is resistive.

I would expect this to be a fundamental characteristic, and not just
for VHF amplifiers.

The 60Hz folk are trying to achieve the same result as they strive
for a power factor of 1.

....Keith

On Sun, 13 Jun 2010 10:03:40 -0700 (PDT), Keith Dysart
wrote:

'Unusual'? Certainly not unexpected once one thinks about it.

Hi Keith,

The "unusual" was lost in the ellipsis that I will reveal:
"... The highest efficiency operating point does not exactly coincide
with the lowest plate current because the power output continues to
rise for a while on the inductive side of resonance coming out of the
dip in the plate current. ..."

Continuing, Mendenhall presents the problems of trade-offs between
what would seem to be maximum power for worsening characteristics in
performance - the goal is what he calls minimizing synchronous AM
versus Efficiency.

Synchronous AM is a problem that can be introduced by measurement
equipment, as Mendenhall relates: "The input impedance of the
envelope detector must provide a nearly perfect match ... 30dB return
loss ... to the sampling line."

This sidebar relates to what Owen characterized as "Usability" where I
have recited the objective technical specification to reduce the
subjectivity of the term.

There's more that could be said, but I am waiting to see if Richard is
willing to subscribe to his own reference's writings. If not, and
Mendenhall doesn't mince words on the topic, then as Perry Mason would
observe "The D.A. is impeaching his own witness!"

I've always loved Perry Mason.

73's
Richard Clark, KB7QHC

On Jun 13, 12:35*pm, Richard Clark wrote:

I am waiting to see if Richard is willing to subscribe to his own reference's writings

I responded to you some 20 minutes before you posted, and now await
your response.

Also please comment on whether or not a Class C amplifier operating on
a linear portion of its transfer curve will function as a linear
amplifier.

RF

On Sun, 13 Jun 2010 11:04:50 -0700 (PDT), Richard Fry
wrote:

I am waiting to see if Richard is willing to subscribe to his own reference's writings

I responded to you some 20 minutes before you posted, and now await
your response.

So your subscription to your own authority is contingent upon me?

I'm flattered.

Also please comment on whether or not a Class C amplifier operating on
a linear portion of its transfer curve will function as a linear
amplifier.

"If the amplifier is tuned exactly to resonance, the plate load
impedance will be purely resistive and the load line will be linear."

I would note that the bulk of his paper concerns frequency products -
something the bears quite intimately upon linear operation.

Your question is rather opaque in that the appearance of subtext is
undoubtedly bound up in the term "linear" which Mendenhall has already
employed within the norms of amplifier design.

How linear is linear? Give me a technical criteria (subjectivity
abounds and is not very informative except for statements of fashion).
Express the least departure from a line response in percent that you
accept as being linear. If it exceeds Mendenhall's design or
commercial product, then I am afraid you won't be satisfied with any
answer.

73's
Richard Clark, KB7QHC

On Jun 13, 1:20*pm, Richard Clark wrote:

How linear is linear? *Give me a technical criteria...

A truly linear device does not produce r-f intermodulation products
when signals of different frequencies are applied to it at the same
time.

For many practical uses a linear device does need to be perfectly
linear. Let's say that the IM products need to be only - 20 dB w.r.t
the lowest amplitude value of the various input frequencies. The
Mendenhall paper I quoted conclusively shows that even such nominally
linear performance is not an attribute of Class C vacuum tube r-f
amplifiers -- especially at, and near the center frequency of the PA
output tuning/matching network.

That Mendenhall paper also shows that such r-f amplifiers do not
present a functional, 50+j0 termination to r-f energy entering via the
tx output connector. That is the reason why such energy is present in
the PA plate circuit, giving rise to r-f intermodulation and/or
changing the power dissipation there.

RF

On Sun, 13 Jun 2010 12:16:40 -0700 (PDT), Richard Fry
wrote:

On Jun 13, 1:20*pm, Richard Clark wrote:

How linear is linear? *Give me a technical criteria...

Let's say that the IM products need to be only - 20 dB w.r.t
the lowest amplitude value of the various input frequencies.

Medenhall's paper shows "RF spectrum showing distortion products" that
are 30dB down.

His statement: "If the amplifier is tuned exactly to resonance, the
plate load impedance will be purely resistive and the load line will
be linear." is attached to that linearity which is greater than you
expect.

73's
Richard Clark, KB7QHC

On Jun 13, 11:12*am, Richard Clark wrote:

From Mendenhall:
...

Would you please respond in your own words directly to my
previous statement (repeated below)?

If you believe by your understanding of the clips you quoted from
Mendenhall that a conventional, single vacuum tube Class C r-f
amplifier provides a functional termination of 50+j0 ohms for energy
applied to the output connector of the transmitter, THEN PLEASE
EXPLAIN WHY THAT TERMINATION ALLOWS SUCH SIGNALS
TO REACH THE PLATE OF THE PA TUBE -- whose non-linear
characteristics created the _measured_ r-f intermodulation products
and other performance data given in the Mendenhall paper I quoted.

RF

On Sun, 13 Jun 2010 10:14:21 -0700 (PDT), Richard Fry
wrote:

Would you please respond in your own words

Hi Richard,

I conform to Mendenhall to the specific statement I responded to.

If you believe

What I believe has been succinctly laid out in my subscribing to
Mendenhall's explicit statement.

THEN PLEASE EXPLAIN WHY

I don't see that elaboration is going to improve what has been
presented.

Yes, it is a difficult concept that many struggle with and few have
had experience in making a sufficiently accurate determination of.
Consult Walt's 333 line posting and examine how experience comes to
bear and through my recitations reveal the dovetail fit to theory.

The Only Explanation Possible:

Here's a modest proposal, Mendenhall constructed a power amplifier
that is within the technical grasp of many here to achieve at a modest
workbench. The design is quite spartan. The design is quite
understressed (there is nothing "forcing" a conclusion). The design
conforms to all engineering standards.

Build your own. [It feels strange to have to offer that option to a
group of Hams.]

Having this amplifier before you, observe all the variables, play with
them. Measure their impact on NOhms. Account for the heat with
direct measurement and note what does not conform to convention.

When that is finished the real work begins. Calibrate your tools and
repeat this for accuracy.

73's
Richard Clark, KB7QHC