[AMRadio] Johnson Valiant Modulator Plate Lead Question


Grant Youngman nq5t at tx.rr.com
Sun Aug 12 13:44:58 EDT 2007


> Is there any truth (or significant value in performance) to 
> this concept that it does matter which plate lead connects to 
> each modulator tube?  Appreciate any advice the list 
> membership might have. Bill, w0ng.

Yes.

The male voice waveform tends to be very asymmetrical.  If the phase is
incorrect the radio will likely hit 100% negative modulation before getting
to 100% positive.

There is more than one way to skin the cat.  You can (1) just reverse the
mic leads -- thus reversing the phase.  Or you can (2) swap the phase by
changing plate leads on the modulator (or by swapping the leads on the
driver transformer, etc.).  Or you can use an all pass filter in the audio
chain which will remove the asymmetry altogether.

In any case, the objective (and you should have a scope to check which is
which) is to get highest possible positive peaks before you hit 100% on the
negative peaks.

Grant/NQ5T



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