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 -----
From: TexasRF@aol.com
To: kk7uv@bresnan.net ; amps@contesting.com
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).
73,
Gerald K5GW
In a message dated 3/22/2008 6:26:27 A.M. Central Daylight Time,
kk7uv@bresnan.net 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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