[AMRadio] Low modulation Gates BC-1T

D. Chester k4kyv at charter.net
Mon Feb 26 18:14:08 EST 2007

Mr. T., W9LBB said:

"As a last ditch effort...   in the W9LBB Surplus Heap I have a modulation
AUTOTRANSFORMER (Wilcox was big on those) that matched a pair of 450THs to
Gawd only knows WHAT in the final! The primary CT is internally tied to the
B+ supply side of the secondary, near as I can figure. It's about 150 
pretty ratty (needs derusting of the case bottom and paint, and a general
cleanup). The good part is that the impedances on primary and secondary are
clearly marked on the rusty case. The beast is oil filled, and the PA plate
winding has at least two taps as I remember."

I have one of those.  I think mine is filled with tar, not oil.  The rig was 
a Wilcox modulator for a ground-to-air aviation base station.  It ran a pair 
of 450TL's, driven by a pair of 845's.  It should work fine in the Gates, 
since the rig uses a common power supply for modulator and rf  B+.  I would 
still use the modulation reactor with it.  Regardless of impedance, use the 
higher impedance tap.  That will give a turns ratio closer to that of the 
Gates mod transformer, which is about 1.35:1 total turns ratio.  The main 
problem is that you will not have the tertiary winding to modulate the 
driver stage along with the final, but maybe it will make do until you can 
find a stock replacement transformer.  There should be plenty of those lying 
around somewhere, since so many stations used the later BC1 series, so you 
should eventually be able to find a replacement.

I mounted my BC1-T mod xfmr on ceramic standoffs and put a spark gap across 
the primary, each side of midtap to modulator plate.  Those xfmrs look a 
little anaemic to me.

"But you CAN'T have the interstage transformer for the 450TH grids (matches
807 drivers, I assume in Class A).  ;o)   Good P-P interstage transformers,
especially ones that'll handle 25 watts of audio, and almost impossible to
find these days!"

If that was the same modulator mine came out of, it matchse a pair of 845's 
to the 450TH grids.  It should have about a 5:1 stepdown turns  ratio, since 
the 845's had about 1250 volts on the plates, while 807's would have about 
600 volts.  An 807 driver transformer would have about a 1.6:1 stepdown 

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