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Re: [Amps] GPT-750 power supply

To: Steve Flood <>,
Subject: Re: [Amps] GPT-750 power supply
From: Radio WC6W <>
Date: Thu, 9 Feb 2012 08:47:24 -0800 (PST)
List-post: <">>
Hi Steve,
  If you don't mind heating up the room ...

  You could use what I call a "smart bleeder".

  Take a big power FET (IXYS IXTX5N250 or similar), suitably heatsinked, and 
connect it in series with a "small" (10K / 1KW?) bleeder configured such that a 
minumum of 300 mA, or so, is drawn from the supply in standby.  As the load 
current increases the FET circuit & bleeder draws less current leaving more 
available to the amplifier than a conventional configuration.

  As the suggested FET is only rated for 2500V either something like a 50K 
resistor would need to be connected across it, or a string of zeners connected 
to the FET gate, to stay within rated drain voltage.

  I have implemented this on smaller supplies and can send you a schematic 
direct if you desire.

73 & Good morning,
  Marv WC6W


--- On Thu, 2/9/12, Steve Flood <> wrote:

> From: Steve Flood <>
> Subject: [Amps] GPT-750 power supply
> To:
> Date: Thursday, February 9, 2012, 5:03 AM
> I've been attempting to design a
> power supply using the big (100 lb) TF-193
> transformer from a GPT-750 transmitter, the rating of which
> is 2800vdc @
> 750mA CCS, which is about what I'd like to get out of the
> supply for my
> intended ICAS use with CW and SSB only.  (No AM or
> The transformer was designed for a choke-input and it has
> been a big problem
> locating a suitable choke, particularly the actual 750mA
> 8-25H swinging
> choke used in the GPT-750.
> I have a few big chokes but none rated for 750mA.  The
> closest is a 12H
> smoothing choke rated 600mA CCS which would probably be fine
> for current,
> but the 12H is not enough inductance as it would only
> provide around 23%
> regulation (in combo with a 12uF cap and 30k bleeder) as I
> calculated using
> (Duncan PS design software).  It seems I need something
> more like 25 or 30H
> to get down below 10% regulation.
> So then, am I overlooking something in  this design,
> Or should I just consider selling/trading this transformer
> for something
> more modern for a capacitor-input supply?
> Thanks,
> Steve KK7UV
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