Richard Fry wrote:
"If the source resistance of a tuned r-f PA stage was truly
non-dissipative, and the tx simply supplied less power into poor
matches, how would that explain the catastropic failures to the output
circuit components often seen when high power transmitters operate
without suitable SWR protection into highly mismatched loads?"

The PA is a switch. Almost no voltage across it when it is closed and no
current through it when it is open. Some of its impedance is dissipative
and some is non-dissipative.

A conjugate match to its total impedance is the way to deliver maximum
power from the transmitter to its load. Alexander H. Wing wrote on page
43 of "Thansmission Lines, Antennas, and Wave Guides":
"If a dissipationless network is insrted between a constant voltage
generator of internal impedance Zg1 and a load ZR such that maximum
power is delivered to the load, at every pair of terminals the
impedances looking in opposite directions are conjugates of each other."

An operating transmitter is normally adjusted for conjugate match with
its load.

Normal plate dissipation occurs when electrons strike the anode and
there is little damage to the tube when the current and cooling are
within limits.

Let an arc strike across the transmission line and it may effectively
become a short circuit which may impose an enormous mismatch in an
instant to the transmitter. That`s why a d-c supply is often connected
in series with a relay coil across the transmission line. The arc
completes the d-c circuit energizing the relay which breaks the
interlock circuit. The transmitter instantly is shut down until it is
manually restarted.

Tubes are often destroyed by internal arcs if overloads don`t act in
time.

Best regards, Richard Harrison, KB5WZI

(Richard Harrison) wrote in news:10691-484F0717-
:

....
The PA is a switch. Almost no voltage across it when it is closed and no
current through it when it is open. ...

The OP's question related to Class B and Class C.

Have you hopped on another tram?

Your statement is definitely not true for Class B RF PAs. In fact it is
difficult achieving theoretical Class B efficiency with tetrodes running on
relatively low HT (eg 750V as in many transceivers).

If what you state was true, you would expect conversion efficiencies in
excess of 95%... and that just doesn't happen with practical electron
devices.

Practical Class C electron devices cannot achieve those efficiencies at
maximum continuous output power. The anode voltage waveform in practical
continuous Class C amplifiers with resonant loads is hardly the rectangular
pulse waveform that you suggest. In fact, if you observed it with a CRO you
would be hard put to argue that it wasn't an almost pure sine wave.

Owen

Owen Duffy wrote:
"Have you hopped on another tram?"

Maybe. I agree. Radio power amplifiers have clean sinusoidal outputs,
otherwise they would generate unacceptable harmonics. All except Class A
amplifiers which have continuous plate current flows and a maximum
efficiency of 50%, have plate currents which flow in pulses.

In Class B, the plate current in individual tubes flows in pulses of
approximately a half cycle. Actual efficiency is about 60%.

In Class C, the plate current pulses last less than a half cycle.
Practical efficiencies are in the range of 60 to 80 per cent.

Best regards, Richard Harrison, KB5WZI

(Richard Harrison) wrote in
:

Owen Duffy wrote:
"Have you hopped on another tram?"

Maybe. I agree. Radio power amplifiers have clean sinusoidal outputs,
otherwise they would generate unacceptable harmonics. All except Class
A amplifiers which have continuous plate current flows and a maximum
efficiency of 50%, have plate currents which flow in pulses.

In Class B, the plate current in individual tubes flows in pulses of
approximately a half cycle. Actual efficiency is about 60%.

Pulses is not very descriptive, it is definititely not rectangular
pulses as implied by your earlier posting.

For a single ended RF common cathode valve PA with resonant load, an
active device that is perfectly linear and zero saturation voltage,
theoretical efficiency is 79% in Class B.

If saturation is only 20% of the supply voltage, that figure drops to
63%.

For HV tubes, saturation of about 10% of the supply voltage is
reasonable, and efficiency is that case is 71%.

Then the output circuit loss need to be factored in, and 90% is not an
unreasonable output circuit efficiency.

The comination of 10% saturation and practical output circuit arrives at
64% efficiency overall (ie plate and output circuit) ... but tetrodes on
low voltage can't usually deliver low saturation voltage and won't reach
that figure.


In Class C, the plate current pulses last less than a half cycle.
Practical efficiencies are in the range of 60 to 80 per cent.

Again, not rectangular pulses, not nearly.

For most practical valves in continous mode, they cannot develop their
rated maximum power at very small conduction angles without exceeding
rated cathode current, so there is often little benefit in operating at
conduction angle much below 120 degrees.


Owen

I wrote:
"In Class C, the plate current pulses last less than a half cycle.
Practical efficiencies are in the range of 60 to 80 per cent."

Owen Duffy wrote:
"Again, not rectangular pulses, not nearly."

A pulse does not need to be rectangular. According to my electronics
dictionary:
"Pulse - 1. The variation of a quantity having a normally constant
value. This variation is characterized by a rise and decay of finite
duration. 2. An abrupt change in voltage, either positive or negative,
which conveys information to a circuit. (See also Impulse.)"

A rectified sine wave could properly be called a string of pulses being
constantly off between pulses then rising and falling between pulses for
a short finite duration.

Best regards, Richard Harrison, KB5WZI

(Richard Harrison) wrote in news:20731-4850859D-
:

I wrote:
"In Class C, the plate current pulses last less than a half cycle.
Practical efficiencies are in the range of 60 to 80 per cent."

Owen Duffy wrote:
"Again, not rectangular pulses, not nearly."

A pulse does not need to be rectangular. According to my electronics
dictionary: ...

Yes Richard, pulse can mean all things to all people... but you did say
"The PA is a switch. Almost no voltage across it when it is closed and no
current through it when it is open." That implies a rectangular current
pulse... and I think we are now agree that is not the case for Class B or
Class C (which were the stated scope of the OP's question).



Owen

This illustrates, I hope, only one, small point:

Owen says:

For most practical valves in continous mode, they cannot develop their
rated maximum power at very small conduction angles without exceeding
rated cathode current, so there is often little benefit in operating at
conduction angle much below 120 degrees.


Recall that valve/tube diodes in rectifier service were not used with
capacitive filters because the rated peak to average current ratio was
something like 1.5:1. Modern, solid-state diodes can have ratios of 40:1
and can be happy with capacitive filters in rectifier service.

Peak current limits of high-voltage, high-power tubes is a real physical
limit. The cathode can only emit so much.

73, Mac N8TT
--
J. McLaughlin; Michigan, USA
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