Jim,
### has anybody actually used a tuned circuit in any
of these proposed configurations ??
I have often tuned up solid state amps. While I have never so far seen
one that has tune and load controls on the front panel, or anything like
that, many solid state amps are tuned designs. That includes almost all
VHF and higher amps, and most HF amps designed for single-band use, such
as CB radios, single-band ham transceivers, and commercial/industrial
things like RF welders.
> I just cant fathom
manually tuning up a SS amp..... unless you were
really careful, and pulse tuned it with a low 3-10 % duty
cycle, and also starting with low drive power.
It's not as touchy as you imagine. Most typically it's a matter of
taking a screwdriver and tuning two trimmer caps for maximum output.
It's good to watch the current at the same time, because there is often
a range over which the power output almost doesn't vary, but the current
does. In those cases you tune for the lowest current on the max power
plateau, or in some cases such as solar-powered VHF repeaters you tune
for best efficiency.
And if a trimmer runs out of range, you simple bend the turns of the
coil a bit together or apart, to change its inductance, and then
readjust the trimmers.
I have done it this way with dozens of ham repeaters from brands like
VHF Engineering, RPT Electronics, Hamtronics, lots of VHF radios by
Motorola, General Electric, and the various Japanese brands, countless
CB radios, and of course with my homebuilt HF and VHF radios and
amplifiers, such as the little 5-watt 40m QRP transceiver, or the 80W FM
stereo broadcast transmitter, both published on my web site.
It's no rocket science.
With tube amps and their high Q networks, starting at full power and
with the network seriously out of tune can melt the tube plate, or the
envelope. I have seen both. In transistor amps instead, with their much
lower Q networks, they are never totally out of tune. It's really only a
touching-up what one has to do! So it's much harder to dramatically
exceed the normal dissipation level.
> This
would be one application, for a 6:1 or 10: 1
vernier drive, with well calibrated large diameter skirts.
Absolutely not! The loaded Q in these networks is far lower than what's
used in tube amps, and for that reason the tuning is very broad and
quite uncritical.
And will
it handle higher swrs ? Or do we have to go through the
rigmarole of a manual or auto tuner ?
All such tuning networks do handle somewhat elevated SWR. How much,
depends mostly on their Q. Since the minimum required Q is typically
much lower than in tube circuits, most designers would indeed use a low
Q to reduce size, cost, touchiness, increase bandwidth, etc, and this
will allow only a limited range of load mismatch to be tuned out. But
there is absolutely nothing preventing the designer from deliberately
using a higher Q in the matching network, achieving a behavior closer to
that of a tube amp: Ability to match crummier loads, along with more
sensitive tuning (here your vernier might come in!), and more loss in
the tank, requiring a big coil to keep loss at bay.
But then the typical autotuner anyway allows only a 3:1 SWR range or so.
That level of SWR doesn't require a very large Q in the matching network.
Think of it like this: Instead of having low voltage transistors
followed by a broadband transformer, a set of low pass filters, and then
an autotuner, you would have high voltage transistors and then just
the autotuner, doing away with all the other stuff in between! But then
you need an autotuner having good enough harmonic suppression. The most
commonly used T or L networks don't, but a slightly more complex tuning
network does. This slightly more complex autotuner (or manual tuner)
goes directly between the high voltage transistors and the antenna,
allowing to work into an SWR of 3:1 or so.
Tuning can be done at reduced supply voltage and limited supply current,
so that the transistors cannot be thermally overstressed. Once tuning is
complete, the full supply voltage is applied. With an EER system, this
is trivially simple to do, since the variable power supply is already
present. Instead a conventional class AB linear system would need the
supply voltage switching to be added, but that's quite simple too.
When the transistors are selected according to conventional wisdom, that
is, that their dissipation rating should be well over twice the maximum
output power, I would even feel comfortable running such an amp without
further protections (but paying attention while operating). Instead in a
high efficiency amp that has transistors rated for a dissipation roughly
similar to, or even way lower than the output, additional protection
circuitry would be required to keep the transistors alive in case of
mistuning, a bird sitting on the dipole tip, a coax connector coming
loose, or whatever.
Manfred
========================
Visit my hobby homepage!
http://ludens.cl
========================
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|