[AMRadio] Re: B&W 5100B Audio Mods


Bob Bruhns bbruhns at erols.com
Fri Oct 22 01:05:48 EDT 2004


Tom, a 6146 needs its screen modulated along with its plate, or it won't modulate well at all.  If your transmitter provides screen voltage to the 6146 output stage from another source, it needs some kind of modulation.  Somewhere around 60% mod on the screen is enough.

Sometimes this is done with a separate winding on the mod transformer (which you don't have with the A-3894).   If screen voltage comes from a lower voltage supply, you could get self-modulation using about 150V through a choke of about 50 henries, to give around 20K impedance at the low frequency end.  The screen draws less current at higher plate voltages and more current at low plate voltages, so an impedance in series with the screen causes self modulation.  Surprisingly, the screen tends to modulate itself by about the needed amount despite the varying impedance of the choke over frequency.

In my Viking II, I used a 20K resistor from unmodulated B+ (600V), and the screen self-modulated pretty well.  See "The Self-Modulated Screen" in AM Press/Exchange issue 98 ( http://www.amfone.net/AMPX/98.htm ).  Also check "Class C Optimization for Ultra Low Distortion" by WA1KNX, in AM Press/Exchange issue 71 ( http://www.amfone.net/AMPX/71.htm ). Maybe you could put the screen dropping resistor to a tap on the mod transformer secondary. But, if the 5100 uses a low voltage screen supply and a series choke, you would need to add a clamp tube if you switched to the dropping-resistor design.

Maybe you could whip up a screen co-modulator that would put out 150VDC with an adjustable amount of modulation, that tracked the modulation in the plate voltage.  Then you could experiment with different amounts of screen modulation.

  Bacon, WA3WDR


----- Original Message ----- 
From: "Tom Elmore" <tom at telmore.com>
To: "Discussion of AM Radio" <amradio at mailman.qth.net>
Sent: Thursday, October 21, 2004 8:27 PM
Subject: Re: [AMRadio] Re: B&W 5100B Audio Mods


> The unit I have has a separate B+ supply for the screens. So would this be
> self-modulating then?
> 
> -Tom
> ----- Original Message ----- 
> From: "ne1s" <ne1s at neandertech.com>
> To: "Discussion of AM Radio" <amradio at mailman.qth.net>
> Sent: Wednesday, October 20, 2004 8:56 AM
> Subject: [AMRadio] Re: B&W 5100B Audio Mods
> 
> 
> > i am not familiar with the 5100B, but I would be VERY surprised if the
> > screens weren't modulated as well as the plates - it is absolutely
> necessary
> > to achieve 100% modulation of tetrode, pentode, or beam-power calss C RF
> > stages. This is usually done with this class of transmitters (~100W
> > commercially-manufactured ham AM rigs)with a screen droping resistor from
> > the modulated plate supply. It can also be done with a dropping resistor
> > from the unmodulated B+, or an audio choke from a voltage source equal to
> > the desired screen voltage; in these cases, the screens self-modulate.
> >
> > GL,
> >  -Larry/NE1S
> >
> > Tom Elmore writes:
> >
> > >     Does, anyone know of a source for audio mods for the B&W 5100B
> > > transmitter. I wonder if anyone has tried to modulate the screens in
> > > addition to the plate of the finals. It appears the screens use a
> different
> > > power supply than the plates of the finals.
> > >
> > > Thank You
> > > Tom Elmore KA1NVZ
> > > Anchorage  Alaska
> > >
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