> ...snip.....
> (Mr. Rauch:)
> >Someone asked about why tetrodes aren't used. Rich gave a cost
> >example. Let me give a technical reason.
>
> The cost example was PENTODES, Mr. Rauch.
My mistake, I should have said pentodes.
> >.......That's
> >why a high MU tube is more stable in a regulator, ...
Higher Mu tubes generally have more stable Rp values. Rp in this
case is the slope of the anode to cathode resistance as voltage
drop across the tube is varied. The idea regulator tube would have no
slope, or a Rp of zero ohms.
> Does this make sense?
> 'High Mu' (Mu = 100 to 200) triodes need much grid current--and much
> drive power--in order to draw their maximum rated anode current.
That makes sense, but the application of that to a regulator is
nonsense. The important parameter in a regulator is Rp, not grid
current. Tetrode's and pentodes have lower Rp than triodes, as a
general rule.
> >........... It is also more difficult to
> >obtain high efficiency with a pentode .....
>
> Outside the Rauchian World, it is universally acknowledged that pentodes
> are more efficient than tetrodes.
Another personal remark.
The highest efficiency amplifiers are generally low mu grid driven
triodes (for example, RCA made a low mu triode AM BCB 5 kW
transmitter that produced over 90 percent anode efficiency. The
selection of a low mu-triode was no accident).
Pentodes are generally selected for low IMD. Rp determines the PA's
sensitivity to tank adjustment, tubes with low values of Rp require
more careful matching of the tank to the operating dynamic resistance
of the tube in order to produce the highest efficiency, and more
careful waveshape matching of the time varying anode resistance to
the tank voltage swing.
A high Rp tube offers the advantage of less critical anode impedance
matching, if the dynamic resistance is driven low enough and sharply
changes. A sharp dynamic change in plate to cathode resistance (by
driving the grid with a steeply sloped waveform) in a pentode or
tetrode results in wasted efficiency when driving a resonant tank,
because the dynamic resistance is too rigid (due to low Rp) and the
tank's Q prevents it from changing voltage fast enough.
There's no secret to this. Anyone using a tetrode or pentode PA knows
the tuning is more critical than a low Mu triode. That's why low mu
triodes are used in industrial PA's where the operator has no idea
how to tube the PA, or when the load varies (such as in HF ovens,
that are used in food processing).
The irony of this all is the IMD improvement in SSB operation can
be very small from a low Rp fussy to tune pentode down to a low mu
easy to tune triode. This is especially true in the real world, where
the operator may adjust the PA less than perfectly.
73, Tom W8JI
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