[Amps] IM distortion and such

[Peter Chadwick]

Tom said:
>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 
products).<

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?
73
Peter G3RZP