Tuning loading RF amplifiers tune PA tank adjustments

 

 

 

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TOF Amplifier Tuning Aid

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PA Converts DC to RF

Arcing in amplifiers

 

 

Tuning Pulser Systems

Tuning Steps

This page might be a bit long, but it explains how and why you
need to adjust the amplifier in a certain way. If you find it confusing and have
suggestions to improve it, please email me at this address:

 

Important!!!!! In grounded-grid amplifiers, grid current meter is the single most important indication of proper final loading and operation!! Grid current indicates proper tuning better than any other single parameter. Output power is a very close second after grid current.   In grid-driven tetrodes or pentodes the most important parameter is screen current, with control grid current a close second, and output power a close third.

 

Note: All amplifiers used to amplify amplitude varying signals should have a TOF system. To ensure proper tuning,  less-experienced tuners should also have a proper power output meter. True peak-reading meters are best, such as the AWM-30. The AWM-30 will accurately indicate peaks with any tuning pulser, CW or FM carrier, or voice signal. The TOF system will ensure proper grid current to prevent amplifier saturation or flat topping, and minimize splatter.

 

 

(This section is optional reading.
You can SKIPDOWN to tuning steps.)

Tuning Worries

Besides splatter, destructive things sometimes occur when amplifiers are mistuned
or improperly operated. There are two overall damage mechanisms,  excessive
temperature (heat or slow component failure) and excessive voltage (high voltage arcing
or instantaneous component failure).

Excessive temperature permanently damages components. Excessive
heat is the most common source of tube damage. Heat is always a time function of dissipated power. Heat damage
is dependent on the thermal mass
of the thing being heated, the initial temperature, how long heat is applied,
and how much heat is applied over that time. Heat damage always relates to temperature
rise, which is a function of heating power
over time. The time limit is determined by ability components or elements to absorb
heat (thermal mass)
or move heat out to other areas (dissipate heat). Heat damage always takes some finite time, even if
it is a very short time for small objects with low thermal mass. Heat
damage is sometimes cumulative, with failures eventually occurring after many
brief overloads spread over time, each overload not being enough by itself to
cause catastrophic failure.
Heat damage is generally not cumulative until a certain temperature threshold is
reached, where a materials starts to change. In almost all cases, electrical or
dissipation overloads below a certain resulting temperature will not cause life-shortening in
components.

For example, as long as the anode does not exceed a certain
temperature limit vacuum tube life is virtually the same despite nearly infinite
numbers of severe anode dissipation rating overloads. The same is true for many FET’s
or semiconductors. This is why
tubes and FET’s can be operated far beyond published dissipation limits without
noticeable life reduction.

Reducing temperature does not always extend component life. As a matter of fact, some vacuum tubes actually have a
pronounced reduction of life when operated at low temperatures! The cathode of
a 3CX1500A7/8877 and the anode of a 3-500Z are just two of many examples where low
temperature operation rapidly accelerates device failure. Carbon resistors, on the other
hand, are damaged by small thermal overloads over long periods of
time, because damage accumulates over time as the base material changes. The
slow material change is why carbon resistors placed near hot running vacuum tubes, or carbon resistors
with
high operating dissipations and temperatures, age down in resistance over time.

Excessive voltage breaks down components. Arcing can destroy things in
small fractions of a second! Arcing comes from too much voltage. Arcing causes
instantaneous failures by punching a hole through insulation, very rapid
failures by surface damage (such as carbon tracking a surface), or fairly quick
damage by concentrating heat like a welder does. Arcing can ruin insulation or damage surfaces. 

For example, FET’s, band switches, and tuning capacitors are
most frequently damaged by arcing. Switch contacts can be ruined in fractions of
a second, FET’s almost instantaneously. Tuning capacitors, because of larger
mass at arc points, are slower to damage but still might fail in a few seconds. 

Arcing is generally an instantaneous or rapid failure mode,
while heat dissipation damage requires time. Arcing is from excessive voltage,
and heating from excessive current. Because of the difference in cause, these
two very different failure modes do not normally occur under the same operating conditions.

A Word about Tuning Pulser Systems

Tuning pulsers, or if we are into locker-room innuendo
“tuning peckers”, reduce average dissipation. This means tuning
pulsers or “peckers” can reduce heat
damage in physically larger objects, such as vacuum tube anodes. Contrary to
some claims, tuning pulsers or peckers do not
lessen the chances of most arcing or voltage failures, unless the arc failure is
a product of long-term heating or excessive average dissipation. Claims a pulser prevents arcs or results in optimum tuning because it “emulates speech” are
made in good faith, but such claims are not
true. Peak voltage is not directly tied to average power, so a reduction in
average power through use of a pulser or “pecker” tuning aid will not reduce peak voltage
or prevent arc damage or failure.

By definition, optimum peak current and optimum peak voltage have to
occur at the same tuning condition, or an amplifier cannot be properly loaded! Tuning always has to be done at
full peak power, and that means we have full peak drive power available for
tuning.

What surprises me more than anything is how or why so many fell into a
trap, thinking how a pulsed single-tone audio signal (into a transmitter’s
microphone or line audio input) somehow emulates
a slowly varying multiple-tone voice, while another pulsed
single-tone applied via the CW key jack cannot emulate voice. There certainly are
cases where otherwise knowledgeable people do not understand the difference between
audio
tones converted to RF in a SSB transmitter and CW signals.

A steady single audio
tone into a SSB transmitter microphone input produces exactly the same RF output spectrum and
characteristics as a CW carrier, RTTY carrier, or unmodulated AM or FM carrier. A
pulsed audio tone produces exactly the same spectrum and loading as a pulsed CW
dit, providing the repetition rates and pulse length within response limits of
the transmitter.

Also there seems to be a lack of understanding of failures and stresses. The
only thing pulsing can do, for a given peak envelope power level, is reduce heat. Pulsing
will not
reduce arcing, although pulsing can make the arc occur in bursts, reducing arc heating
of larger metal surfaces.

Audio pulsers increase bandwidth of the tune signal, and offset the actual RF frequency
from the dial setting by the pitch of the audio injection tone.
This might
annoy operators on adjacent SSB channels, although any and all over-the-air tuning can
be annoying to others. Signal bandwidth of an audio tone pulser is determined by any audio harmonics
present in the audio signal, plus shape and time of rising-and-falling
tone envelope edges. Audio
pulse bandwidth is limited by SSB filter and audio stage bandwidth.

CW pulse
bandwidth is determined by CW keying system rise and fall
characteristics. CW bandwidth is free from audio tone harmonics and generally
much narrower than SSB. CW pulsed tuning can be just as effective as a single-tone
SSB pulse, but because of slower rise and fall and elimination of harmonic
distortion, is almost always less disturbing off-frequency.

Final tuning results are identical for CW pulses (ditter) or SSB pulses
(pecker), while both have identical potential problems. Potential problems include
meter response, making sure full peak exciter power is produced in the
tuning process, and ensuring the amplifier is not saturating or non-linear.
Neither method produces better end results, since both methods are equally
critical for pulse shape, pulse level, and pulse rate.

Pulse tuning is not very often a good final step.
No matter which pulse generation system is used, steady-carrier grid current should be checked
as a final step at maximum drive or you
should have a TOF system.

Pulsed audio tone methods and CW pulse tuning aid methods, when applied correctly, provide
exactly the same results.
Overall, except for bandwidth, the two systems are generally identical in
results. Neither method has a consistent advantage in tuning results. Depending on meter response and equipment, some systems will require
nearly 100% duty
carrier, while other systems might
get away with few percent duty cycle. Any pulse duration and injection method will produce
identical results providing transmitter response limits are not reached, if a
TOF is used, and if meters indicate
RF pulse peaks correctly.

Pulser Duty and Repetition Rate

Any pulser system has two critical adjustment parameters. The
critical characteristics are pulse
repetition rate and pulse duty cycle. Optimum duty cycle and pulse repetition rates are
defined by system response to pulses. Optimum duty cycle is NOT defined by some
imagined “voice duty cycle” emulation. A
single-frequency pulsed audio tone is no different than a pulsed dot on CW, for
the same pulse (dot) repetition rate and pulse (dot) duration.

Emulating speech requires at least three
test tones of syllabic, lower voice tones, and upper voice tones, and is really
only especially useful if we look at output on an expensive and complicated
spectrum analysis or frequency domain device. Speech emulation is a lab-type
performance proof procedure, not a tuning aid. Two-tone tests are most commonly
used, but do not load the system at a syllabic or speech-pause rate. Two-tone
tests, and even notched noise tests, fail to show many power supply and bias
regulation problems. 

An audio injected pulsed single tone, other than rise-and-fall rates which
create “click” sidebands and wide
bandwidth) and harmonic distortion, is not any different than just running a keyer on dots.
Once any pulsed system is
inside the bandwidth of the modulation system and meters, results are identical.
Staying within limits of power detector meter response and the modulation
system is critical, as is making sure the exciter reaches full peak power long
enough for accurate meter readings.

An ideal pulser would allow adjustment of pulse rise and fall times, pulse
repetition rate, and pulse duty cycle. It would always force
output level to the absolute maximum PEP power from the transmitter on CW or SSB
(whichever is highest). By allowing adjustment of pulse rise and fall, pulse duration, and
pulse rate, the pulser could be set within metering and ALC system limits.

It cannot be stressed enough, peak power from a pulser should always be
slightly greater than maximum PEP power ever expected on SSB (or CW). Any
critical meters
would need to fully respond to the peaks.

other than heat, the most reliable tuning method is a steady carrier. With a
steady carrier, meter response is no longer
an issue. Meters will always work. This is why we should always do a final
confirmation check with a short carrier at full peak power.  Remember, peak
envelope power equals average power with a carrier.
 

Basic
Operational Theory (this section is optional reading
SKIPDOWN)

The
output device in
your amplifier has a
certain optimum
available voltage
swing, and has
limited current
available. It
is important that
load
impedance presented
to the output device
matches the optimum
values of available RF voltage
and current from that device. When we
adjust the tank
circuits (or
auto-tuners) in our
power amplifiers
(PA), we are
really setting or
adjusting the load
impedance presented
to the output
device. Here’s what
happens when we
tune:

1.)
If load impedance
presented to the
output device is too
low, current is
excessive and
efficiency suffers.
This is also
called

over-coupling. This causes too much
heat. Heat is a long
term problem that
takes a finite time
to cause damage. It
is generally NOT instantaneous
damage, although
tube anodes or
transistor
junctions can be
overheated to the
point of damage in
a matter of 15-30
seconds in some
cases. This is the
case where we use
too little loading
capacitance.

One
good thing about
over-coupling is screen
or control grid
current is reduced,
and this protects
the most sensitive
and easily damaged
parts of vacuum
tubes. Another
advantage in tube
amplifiers is
linearity generally
is a bit better with
slight over-coupling.
There
is slightly less splatter or
distortion.

2.)


If load impedance
presented to the
output device is too
high,
 current
is reduced but voltage
will be
too high.
This is called

under-coupling.
This is the case
where we use too
much loading
capacitance. Efficiency is
normally very good,
heat is reduced or
remains in a
normal range for
the level of
output power
produced. Voltage
can increase well
above the supply
voltage limits, up
to several times
the dc supply
voltage in extreme
cases. This is the
worse scenario
because severe
damage can be

instantly caused by
arcing or voltage
breakdown of
components, and
damage can be
instantaneous even
with very slight
over-voltage. Worse
yet, once an arc
starts, it causes a
dielectric failure.
The dielectric
failure destroys
insulation, creates
sharp points or
surface
irregularities that
reduce voltage
breakdown, or the
arc ionizes air or
creates a plasma.
All of this works to
sustain the arc even
after voltage is
reduced to safe
levels.

Under-coupling, or
having the loading
capacitor closed
too far for the load
impedance and/or drive
power, increases grid
current and
splatter. It creates
a very hard form of
non-linearity where
the device switches
into non-linearity
very quickly, and
the sharp transition
into non-linearity
or gain reduction
creates a very wide
bandwidth splatter.

If we have
a coupling error we
would like it to be
slight over-coupling
in the
PA output device. It is better
to see a little too
much device plate,
drain, or collector
current than too
much voltage at
reduced supply
current. We also do
not want excessive
grid current in
vacuum tubes.

For
this reason, almost
all “pre-tuned”
solid state
amplifiers are
over-coupled to the
load. They are
actually optimized
for a higher than
normal load impedance by
slightly
over-coupling the
output devices
to the load.

SWR or Reflected Power Myth: We often hear people claim reflected power burns up as heat in the power amplifier stage. This is not true at all. The only effect of reflected power is it changes the loadline of the output device. This can either increase PA device RF voltage swing, or it can increase PA device current. If the voltage increases heat generally is reduced, but the PA can arc. If the load mismatch is of a phase angle that increases current, PA device heating increases because conduction angle and peak current increases. In one case heat increases, in the other heat decreases. An SWR mismatch only requires the matching network be readjusted to restore the proper loadline at the output device. In an adjustable pi-network or pi-L network system the only effect of SWR is in current in the inductor(s) and voltage across the loading capacitor, so long as the network can be adjusted to proper load at the output device. in other words if you can retune the network and don’t exceed voltage breakdown of the loading capacitor, your amplifier is very likely OK for any SWR.

Improper and Proper Loading of Amplifier (read this section)

Grid current is the single most important indicator of proper tuning.

• In a grounded grid amplifier, the control grid indicates proper tuning
• In a tetrode amplifier, the screen grid current indicates proper tuning

Improper tank adjustment,
antenna system
failures, and excessive drive are equally harmful to component
life. Improper tank adjustment, antenna system failures, and excessive drive either create splatter (and in extreme cases cause keyclicks) on adjacent frequencies,
or they cause
excessive heat in
the output devices
or components in the
system. Regardless
of the cause,
amplifiers are
quickly damaged by

excessive

tank voltages


or excessive device currents. Watch the meters!

(In some amplifiers,
particularly with low power on the
lowest frequency
bands,


proper loading
cannot be achieved.)

Undercoupling

When the output capacitor (load capacitor) is meshed too far (too much
capacitance), especially at high drive power levels, the amplifier will be
under-coupled. Under-coupling is
the very worse thing
to do to any
amplifier because failures can occur in a matter of seconds! There are
several signs of
under-coupling in a
grid-driven tetrode
or grounded-grid
amplifier. Watch closely for the following:

1.) When
the drive power,
using a steady
carrier, is slowly
increased the grid
current (either
screen or control
grid) will
at some drive level suddenly rapidly
increase. The
sudden rapid grid current
increase will be
disproportionate to
the plate current
or drive power increase!

DO NOT
go past the point
where grid current
starts to rapidly
increase with
small changes in
drive power level.

2.) Too much grid current, either screen or
control grid, is a clear sign you have the loading control too far meshed or
closed.

In a
grounded-grid
amplifier or a grid
driven tetrode
amplifier, the grid
current meter
(control grid in the
triode, screen grid
in the tetrode) is
the most reliable
indicator of
improper loading
and/or tuning. Be
especially watchful
of
disproportionately
high grid currents
compared to anode
currents or drive
power, or a rapid
increase in grid
current with a
modest increase in
drive power.

Never tune, peak, or dip the
amplifier at reduced drive power, and then
attempt to operate
or attempt to suddenly apply
full drive! If you
are going to make a
mistake, make the
mistake by having
the loading control
too far open or unmeshed…not
too far closed or meshed! At
least with the
loading control too
far open, you will
not cause an arc,
blow out a
bandswitch, or
damage a tube grid. You have slightly more time for mistakes and corrections
when the loading capacitor is open too far than too far closed.

Most Common Tuning Error 

Too much grid current is almost always a sign of a loading control that is
meshed or closed too far for the amount of drive power. This is hard to see on
SSB, and best to view on CW.

The most common
amplifier tuning or
loading error is adjusting
an amplifier at low
or reduced drive power as a last
amplifier tuning
step. When we load a radio or
amplifier at reduced
drive as a last
tuning step, we establish that
power level as the
absolute ceiling for
drive and output power. Final loading
at reduced drive
results in a loading control too-far meshed. This can cause



arcing, splatter,
and excessive grid
current.  

Ideally
(if possible)
we should make the
final tuning and
loading adjustments
at or near maximum
exciter drive
power. Some amplifiers drive too easy to do this, so we should always pay attention to
factory instructions and
avoid exceeding
factory amplifier tuning
current limits,
especially for
control and screen
grids. Grid current is
especially important to watch because grids
often do not have
sufficient thermal mass to
absorb large overloads
even for short time periods.
Excessive grid
current in metal
oxide cathode tubes
(ceramic tubes with
indirectly heated
filaments) like the
8877 and 3CX800A7 can damage tubes in
less than a few seconds; whereas most anodes will tolerate severe
overloads for 15 seconds and longer. It
is better to let the
large anode or plate in a tube take the brunt
of any mistuning
heat, which means with any mistake it will be better to
over-couple or have
the load control
capacitance slightly
lower than optimum.

The last few tuning
steps should
always be:

• Load the amplifier to maximum obtainable output at full exciter drive
  (without exceeding
  amplifier short
  term overload
  ratings)
• After that, advance the loading control
  very slightly beyond
  that point (towards less
  capacitance). 

ALWAYS load your amplifier for maximum obtainable power, and reduce
drive to rated,
safe, or desired
operating power levels!  This ensures minimum
voltage and current in the tank and maximum possible linearity
(best
signal quality). High grid current is a
strong indicator of excessively light
loading in grounded grid amplifiers.

Voltage Sag

Voltage sag, unless accompanied by significant conduction angle changes, does
not affect loading setting. Voltage sag will not cause mistuning.

Voltage sag does not cause mistuning because voltage and current decrease at
about the same rate. While sag does reduce power, it does not normally affect
optimum tuning position. Even drastic changes in voltage, such as going from CW
operating voltage to SSB voltage in a Heath SB220, has only a slight effect on
optimum tuning point. If properly loaded on CW at maximum available drive, the
amplifier will remain acceptably tuned at SSB voltages.   

Exciter Transients or Power Overshoot

Maximum available carrier drive might not result in sufficient drive
for tuning. This is
especially true when an exciter has transients or
power overshoot
from marginal ALC
response.

Transients or overshoot
appear on the
leading edge of the
RF envelope, on the
leading edge of
speech or CW
transmissions. This
is the time when the
transmitter is going from zero power towards full power.
Since the ALC
circuit has no
stored voltage at
this moment, the
exciter runs full
throttle for an
instant. This effect
is missed by most
power meters.

Once the ALC
comes up, the hang
time of the ALC will
hold the exciter
gain back. Transients and/or overshoot will generally disappear.

Transients and
overshoot, being of
short duration and
infrequently
occurring, make it
impossible to tune
correctly at maximum
drive. With transients or
ALC overshoot, it
is impossible to tune your amplifier properly
by simply tuning for
maximum output with a carrier,
a tuning-pulser,
a whistle, or normal speech. We cannot just tune for maximum output and expect the
amplifier to be properly loaded when the exciter has
leading edge ALC transients!

Let’s assume the exciter is rated to deliver 100 watts, but has momentary
peaks or transients of 160 watts while the ALC or power control loop “takes
hold”. Power surges of 160 watts, too short to register on normal power
meters, occur at the start of every transmission.  Of course, if
we don’t
run the exciter wide open and reduce power to 50 watts
the problem actually
gets worse!
In this example the
transient peak would
still reach nearly
to the same 160
watts, but the
amplifier would be
tuned for 50 watts
drive! This is bad
news for splatter
and for components
in the amplifier.   

This is why the maximum power setting of the exciter should
generally be used while
tuning. If the
exciter has far too
much drive for the
amplifier, we need
an attenuator or an
amplifier better
matched to the
exciter.

The loading
control should
always be advanced a reasonable amount beyond (further open) the
actual maximum
output power setting. This will allow the

amplifier tank system to handle
transients without

arcing or component failure.

Easy-to-Drive Linear Amplifiers

Some hobbyists and manufacturers tout
“very low drive” as an advantage, claiming it
offers “cleaner signals”. Nothing is further from the truth. 

Exciters almost always provide the best IM performance when operated at a
time-averaged peak power a reasonable amount below full output, rather than very low
levels. At low power levels, exciter performance is dominated by cross-over distortion.
This is where bias
non-linearity or
device input
threshold induces
distortion. The ALC
system also adds
cutoff bias to early stages.
This bias increases distortion in
ALC controlled
stages. At
very high levels, gain compression or
negative bias shift becomes an issue. Exciters
typically do best when operated in the area of 60-80% of rated power.

Worse yet, low drive amplifiers are especially susceptible to damage from
exciter overshoot or transient problems. Transients and overshoot
peak power remains almost
the same level regardless of exciter power control settings. As exciter
operating power levels are reduced, the percent of power overshoot becomes worse.      

The most undesirable situations
are those where exciter power greatly exceeds
(by more than twice) an amplifier’s normal drive power
limit. Not only does this reduce system IM performance,
amplifier drive transients are aggravated. Amplifiers should be designed or
selected to match the exciter’s maximum power output, or an external attenuator
used to bring the amplifier’s drive requirement up to the exciter’s full power
level. Low drive
amplifiers are, as a
general rule, bad
news.

Amplifiers
Without Enough
Loading Capacitance

Some amplifiers
do not have enough
loading capacitance.
The loading or
antenna coupling
control is all the
way at maximum
(capacitor fully
meshed) for maximum
output power, making
it impossible to
“peak” the output.
Opening the loading
capacitor up more
just reduces the
output power, no
matter what the
drive level. This is
over-coupling that
cannot be corrected.
It can be caused
by several things:

1.) The loading
capacitance is
inadequate through
bad or improper
design. This
is common on the
lower end of the
lower bands in
some amplifiers.
For example the
Kenwood TL-922
(which works
better in the old
Japanese segment
of 160 meters,
above 1900 kHz).
Another amplifier
that had poor
tuning range on
160 and 80 meters
was the original
AL80, the Dentron Clipperton, and
several Amp Supply
amplifiers.

2.) The output
power level you
are tuning at is
lower than the
design target. As
power is decreased,
the
maximum-power-output loading
capacitance
setting always
increases. In
other words as
drive is reduced
and we re-tune,
the output power
“peaks” with more
and more loading
capacitance.

3.) A padding
capacitor has
opened up.

4.) A tank
inductor has
shorted between
turns or does not
have enough turns.
(Common in Dentron
amplifiers on
lower bands, where
loaded Q is often
20 or more.)

5.) Antenna system
impedance at the
amplifier is too
low, or is
slightly inductive
rather than being
resistive or
capacitive.

 

 

 

since July 2004