[Milsurplus] BC-230 TX: 6AQ5 Subs- An Update

[David Stinson]

OK, got a chance to look at the 6AQ5 substitutions 
in the BC-230 again.
While using the rig on CW is pretty settled
(I worked 15 states on 40 with it during the 
Novice Rig Roundup), there was some confusion 
on getting it to work properly on phone.  
First I changed the bias to the modultors and that got 
the modulation working, then next time it wouldn't etc.

The issue was the proper connection of the 6AQ5 
cathode.  With the VT-52s, bias was developed for 
them from current through the PA grid resistor and is
piped-over via resistor #100, a 100K.
The 6AQ5s need -12 on the grids when triode-connected
(6AQ5 is actually designed to be triode-connected as a
television vertical stage).
With the 6AQ5 cathodes connected to Pin 1 of the VT-52 
socket, they are at A+ of (surprise) +12 volts.
So I disconnected the end of resistor 100 that went
to the PA grid circuit and grounded that end (just
a matter of lifting one end and moving it to the next
connection over).  This puts the proper -12 VDC 
potential on the 6AQ5 grids to ground and the 
rig modulates well.  

So on all four 6AQ5s, be sure to connect the cathode
 to VT52 socket Pin 1.  I also put ceramic monolithic
.1 @ 50V cap across the 6AQ5 filament to keep both 
ends at the same RF potential.  Don't know if that's
really needed but it seems like a good idea.
Haven't investigated the 5763s but probably 
will go similarly.
Need to look at how to do this with the 24Volt 
transmitter, too.
73 DE Dave AB5S