Hi Tom,
> The dc plate voltage is the peak RF voltage in a very hard
switched PA,
If we want to be precise, we would have to see in which class the
amplifier operates. Hard switched in class C or class E? While the
lowest instant voltage depends on the tube saturation, the highest in
some cases and situations can be quite a bit more than twice the DC
voltage. In linear service, the p-p voltage is always less than twice
the DC voltage, but in nonlinear, high efficiency service I would not be
suprised to see a p-p voltage of even more than three times the DC voltage.
> so RMS plate voltage would be about .71 times anode voltage
That figure would be correct for an amplifier operated in fully linear
mode, and it assumes the tube will saturate at zero volt, which in
practice doesn't happen, so in real life the RMS voltage is even a bit
less than that. But that's only in linear service. In various non-linear
classes the RMS voltage can be higher, because the voltage into a
resistive load at the plate would approach a square wave and thus has
the RMS level much closer to the peak level. Into a tank circuit, it's
close to a sine wave, but with an amplitude such that the average
matches the average of the square wave. This can actually drive the
plate negative! So, a lot depends on how the amplifier will be operated.
> but this ignores harmonics.
Yes, but I think it's pretty safe to ignore them for the purpose on
hand. Firstly, most of the current in the caps is tank current, which
has only a very small harmonic contents. The high harmonic contents of
the plate current is not very important because the plate current is
such a small part of the total tuning cap current. And secondly, the
current handling ability of capacitors increases at higher frequencies,
which further reduces any current handling problem coming from harmonics.
> So it seems the DC rail would probably be a very
conservative estimate when the PA is properly loaded.
Yes. Taking the DC voltage as equivalent to RMS is actually a worst case
assumption, valid in some high efficiency classes of operation. If the
caps will survive this level of RMS voltage, they are safe. In class B
the RMS will be significantly lower.
> When the PA is not properly loaded the circulating currents
can be higher, and the peak voltage can be higher than the
anode voltage!
Yes, but I think it is not necessary to design an amp with tuning caps
that tolerate THAT level of current. For the simple reason that such a
condition can last only a very short time, or the tube will blow up. And
the caps are rated for AVERAGE RF current, and can safely handle a brief
overload in this regard!
By the way, we can get even more peace of mind if we consider that an
amplifier in ham use will run in ICAS, and typically in SSB, with an
average much below the peak. So it turns out that those 100pF caps in
practice will just be loafing along!
In short, taking the DC plate voltage as equivalent to RMS applied to
the tuning caps is technically imprecise, but in practice provides a
simple and useful reference to do a quick check of whether the caps will
survive. The method has a built-in safety factor for class B amplifiers.
It is my opinion, and of course I accept differing ones, that it would
be overkill to calculate the tuning cap current to a high degree of
accuracy, when the only purpose is to find out whether the current is
within the capability of the caps, or not. If the simple method based on
DC voltage says that we are using only half the rating of the caps or
so, it's simply a loss of time and effort to do any additional math! I
would rather put the amp on the air and run a nice comfortable ragchew!
Manfred.
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http://ludens.cl
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