Western Electric 407A-4, Instruction Manual

Page: 70 / 136

total plate current should be b etween 4. 55 and 4. 7 amperes .
The plate current of V2D ma.y be increas ed or decreased by
changing the setting of the "Inter-Grid Coupling" c ontrol .
This requires a s light readjustment of the "Grid-Circuit
Tuning" control to maintain the 90-degree phas e relation.
If the plate current of V2D at 50 kilowatts output can­
not b e made high enough by adjustment of the "Inter-Grid
Coupling" contro l, lowering the gri d l oad resistance RlE by
short-c ircuiting s ome of the. res i stance cards will have the
same effect . This res istance should not b e re duced to lower
than 2.80 ohms .
It is als o pos s ible to adjus t the distributi on of load
between the tubes at the carrier output by varying the bias
supply voltage to one tube or the other by means of the taps
on the bias rectifier trans formers . It is usual ly found,
however that best results are obtaine d with the spe c ifie d
bias es of approximately 485 and 1200 volts .
With the required carrier output, ob serve the plate
current of the modulating amplifier . rhis should be be­
tween 0 . 5 and 0. 8 ampere total for the two tubes . I t may
be adjus ted to within these limits by changing the numb er
of turns on L30C at the high-potential end, or by changing
the pos ition of the tap which c onnects terminal 6 3 to this
coil . For each new adjustment on L30C, condens er C6C should
be adjusted to minimum plate current.
Place link Dl 7A of the oscil lator-amplifier in the 1-2
pos iti on and observe the feedback current as indicated on
the 11V9 Plate" setting of the te st meter in the os cil lator
amplifier Unit . The taps on the feedback coil LSD should be
adjusted to give a feedback current , at the full carrier
output, of 8
±
1 milliamperes .
Adjus tment of Radio-Frequency Protective C ircuit
The adjustment of this circuit cons ists in choos ing
6 3