Whether its a triode or a tetrode, grounded grid or grounded cathode
configuration, the effect of load VSWR will affect the output circuit,
connected to the plate, to the first order. Normally, a tube amplifier
is designed to operate with output network presenting a real impedance,
which is essentially resistive load to the tube. This ideal load can be
sketched as a straight line across the constant current characteristic
curves, with the center point being the B+ DC voltage, and the actual RF
voltage is cycling back and forth across that quiescent point. The slope
of this line is determined by the transformation of the network, as it
defines the plate load. At the low voltage end, the currents are high,
and vice versa for the high voltage end of the RF voltage cycle, all the
way to cutoff in class C, B or AB in varying portions of the sinewave.
The Pi- or Pi-L or cavity, or whatever is used is the transformer that
matches from 50 ohm load back to this ideal impedance that the tube is
designed to operate into. If the amplifier is operated off tune, without
readjusting the plate tuning and coupling/loading capacitors (or
inductors or sliding piston in a cavity), then things aren't so easily
modeled. When an amplifier is operating into a mismatch, with reactive
component such as 45-j20 ohms, it also reflects through the output
matching network to present a reactive load to the tube. It may be
capacitive or inductive, depending on the phase shift through the
network, but nevertheless it is not a pure resistive load anymore. It
can be described as an elliptical load line instead of a straight line.
Over a 360 deg cycle of RF, the plate voltage/current moves around this
ellipse The result of this, on average, is to have a net higher RF
current or higher voltage somewhere in the network and at the plate.
This reduces efficiency usually and the amplifier has to work harder,
hotter, and output is lower. Also, the network components are stressed
more as there will be higher voltages or currents appearing at spots,
compared to a purely resistive loaded circuit. Dummy loads are the best
you could ask for.
Anyway, this is how i think about these things. And, as stated below, if
you go too far off normal, the components in the matching network are no
longer able to compensate to bring the real part of the impedance to the
optimal point of the tube for a given plate voltage and power. That,
along with the reactance that gets transformed back to the tube, are all
bad things, tube or transistor. But how far is bad, requires doing the
math of calculating the circulating currents and voltages.
73
John
K5PRO
Re: [Amps] Acceptable SWR for Tube Amps
Datum: 2025-02-15T12:00:49+0100
Von: "Michael Tope" <W4EF@dellroy.com>
The difference between the case Lukasz is describing and what you are
describing, Dennis, is subtle but important. When specifying what level
of VSWR a conventionally tuned amplifier can handle, you are indicating
the range of antenna mismatch the output tank can transform to the plate
impedance the tube wants to see when tuning the amplifier into that
particular antenna mismatch. For this case, the tank circuit is in
essence also serving as a limited range antenna tuner.
For example, say you tune-up the plate and load controls on a
conventional amplifier with a 50 ohm load so that you get optimum tuning
(i.e. the tube sees an impedance that produces an optimum combination of
efficiency, output power, and linearity). Now, you switch from that
ideal 50 ohm load to a higher VSWR (e.g. 2:1) . In that case, you have
the luxury of re-adjusting the tune and load controls to attempt to get
the impedance presented to the tube back close to what it was seeing
when the amplifier was driving a perfect 50 ohm load. As the VSWR gets
higher, eventually you will get to a point where you run out of tuning
range in the tank circuit (e.g. the capacitance range of tune or load
capacitors is insufficient, the tank inductor starts to overheat, the
load capacitor starts to arc, etc).
An amplifier like Lukasz describes is a slightly different animal. Here
the tune and load controls for each band are preset to fixed values (the
ETO Alpha 78 when operated in "bandpass" mode is an example of this type
of amplifier). These presets can be optimized either for a perfect 50
ohm load or the impedance of a particular antenna at a particular
frequency.? Whichever is the case, as the load impedance departs from
that optimum preset value, you can NOT re-adjust the tune and load
controls to bring the impedance presented to the tube back to the
optimum. Here the antenna impedance range is limited by the range of
plate impedances that the tube can tolerate (in addition to whatever
limits the fixed tank components impose in terms of voltage and current
stress when they see a non-optimum antenna impedance).
The Alpha 78 manual states "A load VSWR of 2:1 or better is required for
safe manually-tuned operation of your Alpha 78. For safe and efficient
operation in the bandpass (no-tune-up) mode, a load VSWR of 1.5:1 or
better is desirable". The fixed tuned "bandpass" mode preset capacitors
in the Alpha 78 are pretty small compared to the variable tune and load
capacitors used for manually tuning mode. Also, the 8874 tubes used in
the Alpha 78 are sensitive to grid overcurrent. Both of these things may
factor in to the more limited VSWR range for "bandpass" mode. It may be
possible to accommodate a VSWR range greater than 1.5:1 in a "bandpass"
mode amplifier with more beefy tank components and more forgiving tubes.
73, Mike W4EF.............
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