[AMRadio] Johnson Valiant Modulator Plate Lead Question


w0ng at comcast.net w0ng at comcast.net
Sun Aug 12 14:13:19 EDT 2007


Geoff,

Thanks for the advice. It's appreciated.

73
Bill
w0ng

-------------- Original message -------------- 
From: Geoff/W5OMR <ars.w5omr at gmail.com> 

> 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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