[Amps] What are the drawbacks of using a tube running, relatively low voltage and high current?

[Manfred Mornhinweg]

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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