> It's just another kind of balanced bridge, so it should improve
> stability at the VHF resonant frequency too, provided that phase-
> inverting transformer is sufficiently broadband. More specifically, it
> won't make VHF stability worse than the un-neutralized condition
> unless the phase of the feedback has somehow reversed itself between
> 30MHz and 80MHz (which is easy enough to ensure).
I'd make sure I had a neutralizing system, and use a suppressor
like the Ameritron AL811H uses if I was refurbishing a Warrior.
There are two distinctly different stability problems with tubes like
811's and 572's.
One is the high feedthrough capacitance that makes such tubes
unstable at upper HF. This is what neutralizing cures. It is a
problem every designer faces, especially when multiple tubes are
parallelled. It is aggravated by anything that increases HF gain.
This is the root reason why Svetlana 572's take off in amplifiers
while the TR relays are disengaged and the amplifiers are on 40
meters and higher. The input circuit and tank are not loaded, so
gain is very very high. The Svetlana tube, unlike other tubes, does
not go into cutoff when biased with 15 or 20 volts of negative grid
voltage. Instead the tube is drawing very light quiescent current and
has maximum gain. The internal feedback capacitance makes a
TPTC oscillator, and the tube runs away on the frequency the tank
is set at.
When an oscillation is at the operating frequency of a tank without
any load, voltages in tank components can get extremely high. It
can damage the bandswitch or other components.
Yaesu's are particularly bad because they have the "bogus"
neutralizing circuit that applies feedback from the output of the tank
to the input, and they have very low cutoff bias.
As with many or most things in the world, it takes more than one
error to make a severe problem. In this example, Yaesu made a
design error with feedback...Svetlana built a tube that does not fully
cut off with the normal bias...and Yaesu chose a poor biasing
scheme.
Take any one of the three away and the problem disappears.
Svetlana's "Yaesu cure" is to bias the tube into cutoff heavier,
although I'd personally rip out the feedback and install a correct
system along with changing the bias. A three step cure is always
the best correction for three step problems.
The second and separate issue is VHF parasitics, which are
problems because the grid is long and wide, the grid leads are long
and thin, and the anode lead is also long and thin. This combines
to cause the grid to be parallel resonant at some low frequency
where the tube still has significant gain.
When the parallel resonance of the grid's internal structure is very
close to a frequency where the anode circuit present a modestly
high impedance at the anode, the tube can become unstable. This
happens at somewhere around 90 MHz in the 572/811 if the grid is
well grounds, and at a lower frequency if external grid leads from
the socket are long.
This low VHF frequency, compared to a frequency near 200 MHz
in a 3-500Z, is why the 811 and 572 are so difficult to stabilize
without over-heating the suppressor on ten meters with a simple
R/L suppressor in 811's and 572's.
By the time you have enough parallel inductance to shift VHF
current over into the resistor, so the resistor makes up a
substantial portion of the anode impedance, the inductance is so
large an appreciable amount of ten meter current flows through the
resistor.
The simple cure for that is to add a small series capacitor in series
with the resistor, so the current change with frequency is at least
twice the rate of current shift in a conventional system as
frequency is reduced. If the resistor has intentional series
inductance, the rate can be increased to much more than than four
times the rate, because the resistor path of the suppressor can be
made series resonant at the problem frequency where the grid is
parallel resonant.
With only a few extra components, all stability problems can be
cured and tank system unloaded Q at ten meters can actually be
increased to improve harmonic suppression and efficiency!
Of course a second patch-method would be just to throw
broadband loss at the system and make the loss able to dissipate
ten and fifteen meter RF without melting down. Then you don't have
to actually understand or solve the problems. You can just cover
them up with loss and pretend like you solved them, as long as
you don't mind the loss of unloaded tank Q and some power
output. Toaster wire from old toasters works well for this second
method, since it survives high temperatures.
73, Tom W8JI
W8JI@contesting.com
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