> > be a significant high RF voltage across the coil. It's peak-to-peak
> value is
> > typically in the range of VCC. For a 4KVDC amp this results in a peak
> of 2KV
> > and and RMS of about 1400V.
>
> Say WHAT!!?
>
> A typical tube amplifier with a 4KVDC plate supply should exhibit
> something in the vicinity of 3.5 KV peak RF voltage with an RMS figure of
> about 2.5KV.
Not quite. The normal properly-tuned peak voltage of a near-class B
amplifier (class AB approaching B) is about 80-90% of anode voltage in each
direction, which would translate to 3kV or more across the tank inductance.
The exact value depends on tank loaded Q, which sets phase shift across the
inductance. Q's of 10-12 result in about 130-degree shift, so the full p-p
voltage from tuning cap to ground and loading cap to ground does NOT appear
across the inductor.
But is is more than 2kV!!
Worse yet, when the PA is mistuned the peak voltage can be almost anything.
See the practical demonstration at:
http://www.w8ji.com/demonstation.htm
You'd better insulate appropriately for worse-case conditions, which is the
maximum breakdown voltage of the tank tuning capacitor times at least two or
three or whatever other component limits the peak voltage under a fault
condiution.
> Therefore, the lowest frequency of operation is the limiting condition
> and requires the most core which is why I used 1.8 Mhz for the true
> calculations.
I agree with WC6W on that!
> > Here is another dirty,little secret. This RF saturation effect is
> really
> > another form of distortion, when viewed as a circuit element. I
> predict,
> > without yet having made the calculations (which are possible but quite
> > involved) that this could also be a significant factor in rejecting
> this
> > core.
Saturation is like the "p-word", everyone uses it as a buzz-word because
they don't understand what really is going on. It makes us look
knowledgeable when we have an answer, even when the answer is way off base
from reality. A core heats, and it is saturation. An amplifier arcs, and it
is a parasitic. Easy answers, but unfortunately almost always wrong.
Saturation is virtually never an issue, unless you are running high-power
low-duty pulse systems. In normal applications you'd melt the inductor down
long before saturation and resulting distortion would be an issue.
Worry about the core heating to the point where it melts, and saturation as
a general rule will take care of itself.
The only time I've ever run into saturation issues in PA's was in fractional
percent duty-cycle pulse amplifiers, or IMD critical applications like
multi-transmitter couplers that had to maintain IM -100dBc. In a typical PA,
those problems would be meaningless.
73 Tom
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