Roger,
> How does this negatively affect the components in a Pi-L network for say
> 160 through 10, 6 meters, or say a strip line on two?
I would say that using a lower plate voltage affects these components
POSITIVELY! Since the tube's capacitance stays the same, the lower load
impedance resulting from the lower voltage and higher current makes the
tube's capacitance less important, comparatively to the plate load
impedance. This allows you to use a lower Q for the tank circuit, which
is often advantageous for cutting down losses or reducing size. Note
that a lower voltage does not FORCE you to use a lower Q! It only gives
you more range of freedom to choose the Q that's optimal for your
intentions!
> I'd think the same parameters would be present in running a pair of
> tubes in parallel.
You can see it that way.
> How about input capacitance? Given a tube with say over 100 pf input Vs
> one with 20 pf. Then there is the out put and transfer capacitance.
The capacitances stay the same, but both the output and transfer
capacitances have less effect when using a lower voltage. If the tube is
a triode, you will need higher grid drive, and that will increase the
effect of input capacitance. But with triodes anyway being usually
driven via the cathode, which is at low impedance, this is not a problem
usually. If instead the tube is a tetrode, you can keep the gain up by
increasing the screen voltage, but there are limits to this, because at
some point the screen will exceed its safe rating.
Now comes the other side of the coin, the drawbacks you asked for: You
cannot lower the plate voltage at will! Very soon you will reach the
point where the tube can no longer cleanly produce the required output
power. This is because the cathode cannot emit unlimited amounts of
current. You will need to see how much current the cathode of your
chosen tube can emit, and select the minimum acceptable plate voltage
according to this. Also take into account that as the tube ages, the
cathode emission gets lower! So, a given tube used at higher plate
voltage will normally remain usable a longer time than a tube used at
the bare minimum plate voltage, for a given power level.
It's all a compromise, and very dependent on frequency. If you are using
a low capacitance tube at low frequencies, there is really no problem in
using relatively high plate voltages, to get long life, good gain, and
best efficiency. If instead you are hard pressed by the tube's
capacitances forcing you to use an unreasonably high Q, lowering the
plate voltage will ease the situation, at the cost of reduced tube life,
and possibly forcing you to use the tube at a lower power level than the
one it would be capable of.
If you prefer using really low voltages, let those old fashioned tubes
alone, and use FETs! With high power RF FETs, you have a choice of
operating voltages in the range of about 40 to 350 volt.
Manfred.
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