>Even a curved line does not necessarily produce
IM3 or IM5. (harmful IM3 is 2 * F1 - F2, or 2*F2-F1, IM5 is
3*F1-2*F2, 3*F2-2*F1 and so on through all odd-order
If it's a square law curve, for example, there are NO third order products (at
least from that stage). Then again, some curvature can lead to cancellation and
an improvement in IMD at some power level.
The data sheet absolute maxima are the ones that really count - exceed those,
and you can have problems getting warranties honoured. (Cynicism suggests
getting warranties honoured can be difficult whatever you do...)
Note also that in a lot of older tube data sheets ' Class B operation' was
applied to AM linear service, so a zero signal anode current of zero didn't
matter - especially as they were usually talking of a modulation index of 80%
at the most. Fortunately, this somewhat misleading approach tended to die out
in the 1960's.
Slightly off topic, I never understood where the 572B came from for linear
service. It seemed to appear in the 1950's: I sort of assume as a bigger
modulator tube, being a pair of 811A, but that seems a bit late in the day for
a purely audio tube. If designed as an RF tube, it surely would have been given
a better structure?
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