Paul,
The modulation transformers in these kind of rigs are a big compromise.
They must carry the RF current of the final amp through the secondary
winding, and therefore must do so without saturation. In order to do this,
these transformers do not interleave the E & I laminations, and instead use
the E's together, and I's together like a filter choke with a fish paper gap
in between. This reduces winding inductance, and increases the flux
saturation point. As a consequence of this, low frequency response suffers
as the modulators see a decreasing P-P load impedance as the af frequency
drops below 500 hz.
So what can you do? The easiest thing is to lightly load your final, and
that alone should improve things. Your modulation percentage should go up,
as well as the frequency response will broaden (low end). You can extend the
LF response a little more with negative feedback from the modulation
transformer secondary back to a low level stage so long as there is not too
much phase shift. An alternative is to take the WA1HLR technique and
interleave the mod transformer so that there is no gap. Then add a outboard
modulation choke (> 30 Henry), and a coupling capacitor of (> 5 uf) in such
a way that the final PA plate current goes through the choke, and the
modulation transformer only has to deal with audio. This requires a big
external modification to the Valiant, so many will not like the idea.
On last parting thought is that the Valiant Hi B+ filter capacitor may be
too low in value such that as the modulation frequency drops, you
increasingly modulate the power supply while decreasingly modulate the final
RF amplifier. Think of this as a see-saw. Most ham rigs of this vintage
could use more B+ capacitance than stock especially if you extend the audio
modulator response below 100 hz.
Regards,
Jim
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] Behalf Of Paul Baldock
Sent: Sunday, August 28, 2005 10:09 AM
To: amradio@mailman.qth.net
Subject: [AMRadio] Johnson Viking Valiant Modulation Transformer
I'm renovating a Valiant. I've made the usual audio chain mods to improve
the frequency response.
However, when I monitor the modulated wave form on a scope I notice that as
you take the audio lower in frequency that the modulation gets distorted.
Its OK down to about 500Hz, but below that gets progressively worse. The
drive to the 6146 modulators is reasonably distortion free.
Is this a feature of the original modulation transformer? Is a better
transformer available?
Thanks
Paul - KW7Y
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