[AMRadio] Re: AMRadio Digest, Vol 46, Issue 23

Geoff/W5OMR ars.w5omr at gmail.com
Tue Nov 13 15:01:06 EST 2007

D. Chester wrote:
> With a normal mod xfmr turns ratio of about 1.5 to 1, I would say run 
> the modulator plate voltage around 25-50% above the final amp plate 
> voltage, if that fits within the ratings of the tubes.  There is 
> nothing magical about the p-p load impedance or the plate voltage or 
> plate current of a tube, as long as you stay within the manufacturer's 
> recommended maximum values of current, voltage and plate dissipation.
> With a  lower turns ratio, say 1.2:1 or 1:1 (the famous and abundant 
> RCA surplus mod xfmr), there would be little need to run the 
> modulators above PA plate voltage.  But you might want to add a couple 
> of extra tubes and  run the modulators in push-pull parallel, since a 
> single pair might not work very well at that  low a p-p load impedance.

That would explain why, when I had an 810 modulator, that I had to run 
(4) in push-pull parallel, to get the same amount of audio as I was 
getting out of a pair of 811's.
That, and I'm not entirely sure those 810's were as good as they could 
have been.  But, I was using the ol' Buzzard 5,500 ohm 1:1 RCA 
Transformer, with the tag that reads "Secondary: 0.198mA Max current".  
Meaning, you could only run 200mA of current through the secondary, 
before the core was saturated.

Driving your AM Rig without a scope, 
is like driving your car at night, without headlights. (K4KYV)

73 = Best Regards,

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