[AMRadio] B&W 5100B Modulation Update

Bob Bruhns bbruhns at erols.com
Wed Oct 13 15:39:52 EDT 2004

Hi Tom,

The A-3894 rating was 120W, the A-3893 rating was 60W.   I am not
sure what the power rating of the 3891 is.  I'll assume it is good
for 60W or more.

What kind of current is the modulator drawing?  A mod transformer
with some shorted turns could cause the modulator current to be very
high for a moderate percentage of modulation.  The 600 volt B+ line
could be pulling down because of this.

It's quite common for an old mod transformer to be shorted and draw
way too much current, and not be able to fully modulate the
transmitter anyway.  It could also be mismatched in such a way as to
show a very low impedance to the modulator tubes, resulting in high
mod current.

If you disconnect the transformer and put 6.3VAC from plate to
plate, you should see about 3 to 6VAC on the output side.   If you
are in doubt as to the tap configuration, measure the voltages and
write it all down.  Look for the highest voltages...  connect the
full windings, and try again, etc.

If a mod transformer has split windings, make sure you hook them up
in the proper polarity.  Measure secondary voltage with 6.3VAC
across the whole primary, and then measure secondary voltage with
the 6.3VAC from CT to either end.  You should get twice as much
secondary output voltage with the 6.3V from CT to one end.  If the
secondary winding is split, make sure you see two times as much
voltage from end-to-end of the secondary as you see from the CT to
either end.

Ultimately, you want a full-primary to secondary voltage ratio
between 2:1 and 1:1.  One winding will usually have more turns than
the other.  Generally you want the largest winding on the primary
side, and the smaller winding on the secondary side (although this
is not true for ultra-modulation).  I have done best to use the full
primary, and experiment with taps on the secondary.

2:1 voltage is 4:1 impedance.  1:1 voltage is 1:1 impedance.  As KYV
Don pointed out, the lower this ratio, the more modulation you will
get, but the lower your modulator load impedance will be, and
therefore the higher the modulator current will be for a given
output power.  4:1 impedance won't give you 100% mod, while 1:1
could give you about 150% if the tubes are up to it.

Typical commercial amateur transmitters used about a 3:1 or 4:1
impedance ratio so the modulation would voltage-limit at about 100%.
That would be a voltage ratio of about 1.7:1 to 2:1.  So if you put
6.3VAC on the full primary, you should get  about 3.5V on the
secondary.  But that's just a starting point - when you get it all
together, you probably want more like 6V on the secondary in this
test, which ought to give you about 150% modulation capability.  If
you know the transformer is OK, you can put 120V AC on it and
measure more easily, but 120V is dangerous if you may be connected
to the wrong taps, etc.

If your taps seem ok, but mod percentage is low, try 120VAC through
a 25 watt light bulb to the full primary, with no load on the
secondary.  If the bulb lights up, you have a shorted transformer.

  Bacon, WA3WDR

----- Original Message ----- 
From: "Tom Elmore" <tom at telmore.com>
To: <amradio at mailman.qth.net>
Sent: Wednesday, October 13, 2004 12:34 PM
Subject: [AMRadio] B&W 5100B Modulation Update

> Now that I have had some time to troubleshoot the B&W 5100B
modulation here
> is what I found. The B+ is around 600 volts with no modulation and
I as
> start to modulate and watch the scope when it reaches about 50%
> the B+ drops from 600 to around 400 volts.  I changed the final
> modulator 6146 tubes and see the same results. The original
> transformer has been replaced with a Chicago Standard Transformer
> Corporation # A-3891.   When I remove B+ to the final's and
terminate the
> modulation secondary into a resistive load of about 6k ohms I
still see a
> significant drop in  B+ as I start to bring modulation up. I don't
have the
> original mod transformer and the manual I have doesn't say what
> impedance should be.  I have tried various other tap settings on
> transformer secondary and don't see much of an improvement in
reducing the
> amount of voltage drop. I wonder if it is possible that this mod
> just will not work with this unit. Anyone have experience with mod
> transformers using 6146's as finals and modulators?
> Thank You
> Tom Elmore KA1NVZ
> Anchorage  Alaska
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