a follow up...
-
: From: 2 <2@vc.net>
>The swamp resistance value would have to be
>changed. The indicated swamp R value appears
>to be similar to the average dynamic tube
>impedance.
-
: 50-ohms might fly.
-
Just to be sure the point is not lost... The alpha input
circuit parallels a swamp resistor network with the
tube(s) cathode(s).
-
The swamp R values are/were probably chosen to
allow convenient matching to the rf input transformer
secondary, to "trade" a higher drive requirement well
in line with many of the common transceivers and to
add a measure (no pun intended) of stability.
-
Most of the time, it is mucho betta practice to use a
proper "LC" circuit.
-
The Alpha type circuit provides low swr to the exciter,
is also relatively broadband, is low in cost and physical -
mechanical part requirements and easy to install.
-
It is by no means the best, but it works much better
than nothing. Distortion is present, but not considered
excessive by the mfgr. A "trade off is made."
-
: The sticky wicket is that the cathode impedance is far
: from constant. It varies from virtually infinity to approximately
: one-half of the mfg's spec impedance. A transformer is not
: going to work very well in impedance hell.
-
In one half cycle of operation, we have a resistor network
and a parallel cathode impedance equal to the xmfr
secondary. All seems well for this half cycle...
-
The alternate half cycle is (for the example) pretty much
just the resistor network and some C. So the toroidal rf
xmfr secondary "load" halves. Not great, but much better
than nothing. The swamp R network remains in circuit,
some of the secondary fixed loading is maintained. The
average swr back to the exciter is "reasonable." There is
no flywheel effect provided by the rf input transformer.
-
My interest in your suggestion was the simple series
circuit to provide the missing tube impedance
in the alternate half cycle. Still, no flywheel effect is
provided. It is still not the best choice, but possibly a
better option if it works. Contributions by the additional
series D&R circuit would probably have to be compared
and measured.
-
>Your suggestion taken to the next step, would
>be a series R & UF Diode paralleled with
>the current circuit. The R value chosen to be
>similar to the tubes parallel drive impedance
>value.
-
: I do not see how such a circuit would do the job.
: Perhaps a road map would help.
-
It would not provide flywheel, just "better" exciter
matching in the alternate half cycle. Impedance
hell would potentially be less hell in the alternate
half cycle.
-
>The current amplifier drive requirements well
>suit the output of many common Amateur
>Transceivers. Most Amateurs will probably
>over drive the amplifier with the swamp
>R circuit removed or revised to provide only
>half cycle loading.
>
: A fool and his money are soon parted.
-
The over drive protection might be or have been
a method of real world manufacture self preservation.
-
A major draw back of an amplifier warranty period,
is that it allows many operators to plug in and hit the
gas before they read and understand care and
feeding.
-
The bullet proof valve amplifier has yet to be
made en'masse.
-
>Some of the relative gratis stability
>provided by the lower existing swamp R circuit
>might go away. But it might not matter with this
>ground grid layout.
>-
: However, if series cathode resistors are added, things
: might be less squirrelly.
-
The Alpha circuit seems to work well for exciter
matching, trading drive power for low swr and moderate
but increased distortion. Alpha claims the distortion meets
requirements for Type Acceptance.
-
ciao for now
skipp
-
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