[AMRadio] B&W 5100B

Tom Elmore tom at telmore.com
Mon Oct 11 14:26:25 EDT 2004

The B&W 5100 transmitter I recently acquired does not have the original
modulation transformer but a replacement made by Stancor.  When I modulate
the transmitter with a 400 Hz tone about 50 % into a dummy load I see a
significant decrease in power at least 25 watts. I measured this with both
my Bird and Coaxial Dynamics wattmeter.  I don't see a return to normal
power until I remove the modulation. I'm wondering if the primary and
secondary on the modulation transformer are possibly backward. The original
owner included some documentation for the mod transformer that shows it's
possible to use either the primary or secondary winding connected to the
plates of the modulator tubes in my case 6146's for different types of
transmitters and impedances.

----- Original Message ----- 
From: "Donald Chester" <k4kyv at hotmail.com>
To: <amradio at mailman.qth.net>
Sent: Monday, October 11, 2004 9:32 AM
Subject: Re: [AMRadio] B&W 5100B

> >It's normal for power to deflect downward on AM during modulation. You
> >to have a peak reading wattmeter to see the peaks..... Sounds like you
> >good modulation.
> >
> With an average-reading wattmeter, theoretically the power should shift
> upwards 50% with 100% modulation with a sinewave tone.  An rf ammeter in
> line should kick upwards about 22%.  A field strength meter should show no
> change, since it reads average voltage output.
> With voice modulation, the upward shift in power will be much less, since
> the duty cycle of a voice signal is much less than that of a pure sine
>   The rf ammeter will show little, if any deflection with voice
> Whistle into the mic or play some music, and the meters will show more
> positive  deflection.
> If the carrier level (field strength) meter, rf ammeter or wattmeter show
> any downward deflection during modulation (voice, tone or music),
> is wrong.  The transmitter is showing poor modulation linearity.
> There may be a slight downward shift in carrier strength due to less than
> perfect voltage regulation of the power supply, especially with plate
> modulation.  This should be no more than 5 percent.  The power and rf line
> current should still kick upwards.
> Don K4KYV
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