[AMRadio] Class AB and B audio XFMRS


Jim Candela jcandela at prodigy.net
Mon Mar 13 10:59:15 EST 2006


Bacon,

   You stole my thunder with the Class A modulator
using a push pull modulation transformer1 I was going
to try to do exactly as you described on my Gonset
G-50 that uses two 6L6's in parallel Heising to
modulate a 6146. The hope was for clean 100%
modulation down to at least 100 hz, and no core
saturation since the modulator DC field will largely
be canceled by the 6146 final PA current. I got a nice
25 watt multi-match transformer too, where I was going
to use the mod side of the primary as an auto
transformer to get around heising modulation
limitations. It remains to be seen if the ratio of
currents, and turns ratio options work out such that I
can run normal RF output, and get full modulation with
near zero net DC flux in the core. If this works out,
then maybe I can remove (or decrease) the gap from the
core to increase the primary inductance. 

   While searching for transformer stuff, I ran into
the following link that appears to have a lot of
basics about transformers, design, and construction
along with reverse engineering. This is from a tube
guitar amp dude. That said, we AM'ers have a lot in
common with them with our push pull modulators.. in
most cases neither are Hi-Fi, and we both run into
clipping.

Jim
WD5JKO
link below:

http://www.geofex.com/Article_Folders/xformer_des/xformer.htm

--- Bob Bruhns <bbruhns at erols.com> wrote:

> Actually you would want an off-center tap to B+
> with the single-ended Class-A technique.  The
> modulator tube should be closer to the tap and the
> output should be further away.  That way you can
> get 100% or more modulation.  You would finagle
> the turns ratio and modulator bias for DC balance
> and the desired modulation.  Assuming you have
> asymmetrical audio, positive peaks would peak at
> more than 2X unmodulated quiescent, but the
> negative peaks would only go down to zero.  It's
> probably not too difficult to do.
> 
> But actually, a series modulator would be better
> way to go.  One of those QIX sorta-class-B
> thingies if you want analog modulation.  No big
> audio transformer, caps and chokes, etc, in the
> way.  Or some switched modulator if you want to go
> digital.
> 
>   Bacon, WA3WDR
> 
> 
> ----- Original Message ----- 
> From: "Jim candela" <jcandela at prodigy.net>
> To: "Discussion of AM Radio"
> <amradio at mailman.qth.net>
> Sent: Saturday, March 11, 2006 7:34 PM
> Subject: RE: [AMRadio] Class AB and B audio XFMRS
> 
> 
> >
> >
> > Geoff, W5OMR said: More iron is better.
> >
> > Reply by Jim, WD5JKO:
> >
> >   Well not exactly. We are only part done when
> we take our mod transformer
> > that has a gap in it, and pass the DC through a
> choke instead of the
> > modulation transformer secondary. To get the
> full benefit from this we need
> > to eliminate the gap. If the mod transformer had
> E & I laminations with all
> > E's together, and I's together, then if
> possible, remove these E & I's and
> > then interleave them. This will increase the
> inductance of the windings a
> > bunch, and allow the same amount of iron to work
> down to a lower frequency
> > for the same amount of power. The P-P load
> impedance at the low end will be
> > better too, so distortion will go down as a
> result. In the example given
> > with the single ended audio output xfmr, the
> primary inductance (with gapped
> > transformer) only increased about 30% by
> removing the DC current from the
> > CCS. Combine this with eliminating the gap, and
> the inductance would go up a
> > whole lot more.
> >
> >    Now instead of using a zillion Henry choke to
> feed the DC current, and
> > capacitive couple the audio as Geoff is doing,
> why not go back to that
> > gapped mod transformer, and add an additional
> winding, and use a power Fet
> > CCS (constant current source) to buck the DC
> field? If done well, maybe the
> > gap can be lessened, or eliminated? Sure the FET
> CCS would need to be nearly
> > ideal from DC to at least 5Khz, or it's mere
> presence would introduce
> > distortion.
> >
> > Jim
> > -----Original Message-----
> > From: amradio-bounces at mailman.qth.net
> > [mailto:amradio-bounces at mailman.qth.net]On
> Behalf Of W5OMR/Geoff
> > Sent: Saturday, March 11, 2006 5:28 PM
> > To: Discussion of AM Radio
> > Subject: Re: [AMRadio] Class AB and B audio
> XFMRS
> >
> >
> > Jim candela wrote:
> >
> > >
> > >John E. Coleman (ARS WA5BXO) wrote:
> > >
> > >
> > >
> > >> I have been doing some experimenting here on
> the side and have found
> > >>a interesting thing that I've seen before but
> forgotten about.  I thought
> > I
> > >>might share it here with you guys.
> > >>
> > >>
> > >
> > >
> > >
> > >Geof W5OMR Wrote:
> > >
> > >I can't believe that no one else has replied to
> this, on the reflector.
> > >
> > >From: "Patrick Turner"
> <info at turneraudio.com.au>
> > >Subject: Re: SE Output transformer design idea
> > >Date: Monday, August 25, 2003 8:06 PM
> > >
> > >
> > >Jim WD5JKO pasted in the following:
> > >
> > >This from an old thread about single ended
> audio transformers, and how some
> > >folks deal with the high DC flux from the class
> A audio output tube. In
> > many
> > >respects this is the same problem we AM'ers
> face with push pull modulators
> > >where the mod transformer has to bear the RF
> final plate current without
> > >core saturation. Pay attention to the stuff
> about using a DC constant
> > >current source into one winding to buck the
> flux from the Class A output
> > >tube plate current. It may not be practical,
> but it sure is interesting...
> > >
> >
> > What I surmise about their discussion is that
> they've basically came to
> > the same conclusion that John has...
> >
> > More iron is better.  For us, that means we need
> more iron at the Class
> > B Modulator level.  All of the Henries of
> reactance wont matter, if the
> > modulation transformer iron isn't suffiecient
> enough.  Not necessarily
> > for passing low frequencies.. rather, having
> enough iron in the
> > transformer, to not have to worry about
> cross-over modulation, ergo,
> > distortion.
> >
> > Good post, Jim!  I enjoyed it, and am looking
> for more.
> >
> > --
> > 73 = Best Regards,
> > -Geoff/W5OMR
> >
> >
> >
> >
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