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Re: [Amps] Grid Bias Supply followup

To: <>
Subject: Re: [Amps] Grid Bias Supply followup
From: "Steve Flood" <>
Reply-to: Steve Flood <>
Date: Sat, 22 Mar 2008 08:34:11 -0600
List-post: <">>
I think I see now - I had to redraw it to understand.  During receive when the 
T/R switch/relay is open, the -100vdc appears on the grids in reference to the 
filament(cathode) which is at DC ground via the filament transformer 
center-tap.  Is that correct?   And during transmit, the T/R switch/relay 
closes and the rheostat is a divider that provides the operating bias voltage 
to the grids.

  ----- Original Message ----- 
  To: ; 
  Sent: Saturday, March 22, 2008 7:53 AM
  Subject: Re: [Amps] Grid Bias Supply followup

  Perhaps the -100vdc is used for cutoff bias in receive?

  A typical t/r circuit might use a set of relay contacts to lift the + end of 
the rheostat from ground on receive thus allowing the full -100vdc to appear on 
the center of the rheostat and tube grid(s).

  Gerald K5GW

  In a message dated 3/22/2008 6:26:27 A.M. Central Daylight Time, writes:
    As a followup to my question about the grid bias supply on the G.E. Ham 
News 813 amp (and the B&W LPA-1 is nearly the same design):  

    The bias supplies for these amps use a 120:120 transformer, half-wave diode 
rectifier, C/RC filter (40uF/500 ohms/40uF) and a 500-ohm, 25-w rheostat load 
for the adjustable bias.  These supplies are said to provide zereo to -100v 
bias.  Yet, the settings for CW and SSB only use -19 volts and -5 volts.  Why 
then, is it necessary to have -100 volts available?  Seems like a much smaller 
supply would suffice.

    Thanks, Steve KK7UV
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