[AMRadio] coupling cap on the moduation transformer secondary

Bob Bruhns bbruhns at erols.com
Wed Aug 17 12:05:47 EDT 2005

Do whatever works for you.

You may actually get better low-end response with a
smaller cap, because the coupling capacitor and
modulation inductor make a high-pass L-C network that
can have a peak before rolloff, depending on impedance
and part values.  This peak can supplement a sagging
low end, although rolloff below the peak will be
faster.  Hopefully it will be low enough in frequency
that it does not matter.

A peaky high-pass filter will certainly mess up phase
response, which would affect low-level negative peak
clipping and such.  And a high-pass filter will also
leave the modulator unloaded at the bottom end.  This
loading issue should not matter much if the modulator
tubes are triodes.  If your modulator tubes are
tetrodes or pentodes, you could apply some negative
feedback from the plates or the transformer output, to
lower the effective plate resistance and control the
unloaded output.

Mostly, it's the sound that matters.  See what works
best and sounds best.  If it doesn't screw things up,
the right capacitor value could add a few dB of
response a little bit above low-end rolloff, and that
could work perfectly well for your voice.

More capacitance will extend the low end response, the
low-end phase response will be better.  The slight
low-end peak will be lower in frequency, and it may be
gone altogether.  Less capacitance will make low-end
response worse, and the peak would rise in frequency
too.  If you want furnace rumble, you might find a
combination of capacitor and inductor values that
produces a resonant peak for you down around 20 or 30

  Bacon, WA3WDR

----- Original Message ----- 
From: "John Lyles" <jtml at losalamos.com>
To: <amradio at mailman.qth.net>
Sent: Tuesday, August 16, 2005 3:21 PM
Subject: [AMRadio] coupling cap on the moduation
transformer secondary

> I was just about to say this, Don already responded
to it. Besides, finding a compact oil filled capacitor
of 2-3 kVDC rating is much simpler if you use a few uF.
You don't need 10 Hz low end response, thats wasted
power since most receiver/speakers won't reproduce
that. Also, the capacitor is just another source of
stored energy in your radio, to deal with from a safety
standpoint. More is not always better.
> > Message: 3
> > Date: Tue, 16 Aug 2005 03:00:55 +0000
> > From: "Donald Chester" <k4kyv at hotmail.com>
> > Subject: Re: [AMRadio] AMPX article on mod reactor
> > To: amradio at mailman.qth.net
> > Actually, broadcast transmitters use much less than
than, and still boast of
> > flat response down to 30~ or lower.  RCA
transmitters typically use 1 mfd.
> > Gates uses 2 mfd.  I use 4 mfd in my homebrew rig @
4000 ohms modulating
> > impedance.
> >
> > 73, Don k4kyv
> >
> >
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