Western Electric 407A-4, Instruction Manual

Page: 66 / 136

quency and give a high degree of suppress ion at this har­
monic .
L2E is adjusted by measuring the input impedance of the
harmonic fi lter at the carrier frequency when the fi lter is
terminated in a non-inductive resis tance of 6 2 . 5 ohms . In
ma.king this measurement the filter is disconnected from the
transmis s ion lines at one end and from LlD at the other . The
measurement may be made with a radi o-frequency bri dge ar­
ranged for measuring either s eries or parallel components of
grounded impedances . L2E should be adjusted to give a series
or parallel input resistance to the fi lter of 75 to 80 ohms .
The reactive component should be found to be not more than
1 0 per cent.
When this adjustment has been completed, the filter
should be reco::m.ected to the transmiss ion l ines and to LlD .
Adjustment of Load Impedances
This c onsists in finding the proper tap on coil LlD to
give the desired load impe dance, approximately 600 ohms , for
the combined output of both tub es , and then the proper tap
on c oil L2D to give the des ired load impe dance, approximately
2 800 ohms , for tube VlE when this tub e is supplying the
smal ler instantaneous outputs without as s istance from tub e
V2D.
The adjustment of load impedances should be carried, out
with the water system fille d so that the res istances of the
water columns will be included in the measurement s . Choos e
a pr el iminary s etting for co ils L2D and Ll E t o give an esti­
mated reactance of 1200 ohms for each at the c arrier fre­
quency, and a preliminary setting for coil LlD to give an
estimated reactance of 200 ohms at the carrier frequency.
In choos ing the numb er of turns a simple inductance formula
may be us ed, such as that given under "Modulat ing Amplifier -
Output Circuit Tuning. 11
59