[AMRadio] Johnson Valiant Modulator Plate Lead Question


Geoff/W5OMR ars.w5omr at gmail.com
Sun Aug 12 14:03:53 EDT 2007


Grant Youngman wrote:
>> 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.
>   

Another way would be to check the 'phase' of the audio on an o'scope and 
follow it all throughout the audio chain until it gets to the grids of 
the modulators.

Most important, observe that the proper 'phase' is being applied after 
the phase-splitter process.

This discussion came up this morning, on 75m AM on 3.880.

We do tend to have some rather 'technical' discussions in South Texas, 
on 3.880.

Another place you might want to look Bill, is an article written by AM 
Reflector list participants WA5BXO, K4KYV and WA3WDR at
www.qsl.net/*wa5bxo*/asyam/aam3.html

--
73 = Best Regards,
-Geoff/W5OMR



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