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Re: [Amps] HV Switch Mode Power Supplies

Subject: Re: [Amps] HV Switch Mode Power Supplies
From: "Will Matney" <>
Date: Mon, 27 Jun 2005 01:48:46 -0500
List-post: <>

Yes I understand that point too but here's what I'm trying to think about. In 
SSB where if there's no modulation, there's hardly any power being consumed so 
the plate voltage would raise a good bit. Then when you modulate the amp, it 
would swing very wide I would think. What would this do to the plate impedance, 
or how hard would that make one to tune easily? Generally the Pi network, or 
especially a Pi-L, is tunable over a certain impedance range. Running on AM/CW, 
a carrier would be there to drag the voltage down somewhat, but then after 
modulation, say at 4X the carrier, it would swing a good bit too but not as bad 
as on SSB I would think. In other words, I don't know how one would deal with 
that amount of plate impedance swing? You know you wouldn't want the tubes to 
be drawing 400 mA at idle just to keep the voltage down to 13% regulation, so 
on SSB or AM what would one do?



----- Original Message -----
From: "Ian White GM3SEK" <>
Subject: Re: [Amps] HV Switch Mode Power Supplies
Date: Sun, 26 Jun 2005 23:35:46 +0100

> Will Matney wrote:
> >
> > I would think it would be easy as some of the large commercial 
> > switch mode supplies work with feedback regulation. It would be 
> > just a form of a voltage regulation circuit like used on 
> > everything else I would think. Adding a bleeder resistor to drag 
> > it down would be prett wasteful after I looked into it. A 4000 
> > ohm 250 watt resistor would do it to get 250 mA.. Only thing is 
> > this wastes power just to get it down some. Here the problem is 
> > that by viewing the curve, at 250 mA, there's a 500 Vdc swing. 
> > For say a 300 volt swing equivelant to about 13% transformer 
> > regulation, there would need to be a 400 mA draw from a bleeder. 
> > That's a bunch! I'm not sure how accurate this curve they have 
> > published is though.
> >
> The point K5AND was making is that you don't needed a bleeder, so no
> power is wasted.
> When the amp is on (TX), the standing current in the PA tube brings the
> voltage down to the level where the regulation is quite good. When the
> amp is off (RX), the HV supply is switched off completely. On
> switch-over, the supply comes up in milliseconds so it's part of the
> normal RX-TX sequencing.
> --
> 73 from Ian GM3SEK         'In Practice' columnist for RadCom (RSGB)
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