Exactly! I might mention that if you plan to operate SSB, and have grid
current, there will be a voltage drop across the 250 ohm rheostat that adds to
the bias supply voltage. While not a really big deal, it will have an effect on
linearity. The effect follows Ohm's law so you can calculate for yourself if
it is worth consideration.
If you connect a high voltage transistor as an emitter follower between the
bias supply and grid circuit, the bias voltage shift at the grid will be
greatly reduced. Back in the days of the original circuit you are working
with,
there were no high voltage transistors and this was not an option. In this
circuit, the bias supply provides operating bias voltage and the grid current
is
shunted to ground through the transistor. Naturally the transistor has to be
rated greater than 100v and be able to handle worst case grid current.
I hope this is not more than you wanted to know!
73,K5GW
In a message dated 3/22/2008 9:34:50 A.M. Central Daylight Time,
kk7uv@bresnan.net writes:
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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