> >Or you can put a series capacitor in series with the R and cancel
> >the reactance at the frequency of instability.
> >
> An interesting idea, Tom.
> A 100-ohm, 3w Matsushita MOF resistor has c. 11nH. At 100MHz, this
> amounts to 7-ohms of reactance. At 100MHz, roughly 200pF would be needed
> to cancel 7-ohms of Xl. . However, the typical 200pF doorknob cap. has
> approx. the same amount of L. . I don't know of anyone who has tried
> this approach.
Every Ameritron amp is being converted to this approach, mainly
because the non-inductive carbons are getting almost impossible
to fine.
The AL-811H has had this as a standard suppressor for about three
years now.
A side benefit is heating of the resistors is reduced, and output
improved, on ten meters.
If you look at the current shift from resistor to inductor as frequency
is reduced, it shifts over at about twice the rate in a properly
designed system. That's because the capacitor is increasing the
impedance of the resistor path at the same time the inductor is
dropping in impedance.
> Any current transient in the anode's VHF-resonant circuit, produces
> damped-wave ringing.
Dampened wave ringing, by definition and action, can NOT have a
peak higher than the initial exciting pulse. Because of that, the
harmonic voltage does not exceed the peak voltage at the normal
operating frequency.
I have heard dozens of reports where a big-bang was
> heard when the ZSAC switched on or switched off -- with no RF being
> applied.
That doesn't surprise me, and it doesn't have to be a parasitic. As
a matter of fact, it is highly unlikely to be a parasitic.
> When a Tune-C intermittently arcs how can one determine what the
> frequency of the arc was?
Look at the dial of the transceiver driving the amplifier. In virtually
100% of the cases, the arc will be caused by excitation at that
frequency.
If you really want to learn a lesson, connect a modern HF
transceiver using high power output devices to a good peak storage
wattmeter and send a CW "dit" at 20 watts in QSK.
The 20 watt output suddenly of one fancy radio becomes 300 watts
or more, when you capture the overshoot on a good meter.
I owned a certain $3500 transceiver for exactly three days, before I
had to send it back for a refund. Everything, including PL-259's,
arced over when ever I tried to reduce drive and run my larger triode
at normal amateur power levels. That PA takes 30 watts to
produce 1500 watts, and the "new rig" would bang it with 360 watts
on some bands for a several RF cycles.
Either an old-time engineer in Japan was trying to capture the
vintage sound of dampened-wave spark transmitters or cathode
keyed DX-100's when he worked on that ALC system, or he didn't
know what he was doing.
I've found two rigs with absolutely no overshoot. The ICOM 751A,
and the FT-1000D. There are probably more, I just haven't seen
them all. BTW, my Drake T4XC overshoots to 200 watts before
settling down to 120 watts...so this isn't a new problem.
73, Tom W8JI
w8ji@contesting.com
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