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Re: [Amps] Grid Bias Supply followup

Subject: Re: [Amps] Grid Bias Supply followup
Date: Sat, 22 Mar 2008 11:00:55 EDT
List-post: <">>
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 
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 
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!
In a message dated 3/22/2008 9:34:50 A.M. Central Daylight Time, 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 
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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