[Boatanchors] Johnson Valiant II VFO

[Glen Zook]

There are several things that can also help stabilize
the Johnson VFOs.

The first is to replace the 6AU6 with a 6AH6.  The
tubes are virtually identical except that the 6AH6 has
a slightly different internal capacitance which may
require a slight "touch up" of the variable trimmers
to get the frequency calibration back on.  Heath did
this in the Apache transmitters to stabilize the VFO. 
The very early ones had a 6AU6 but that was changed
early-on to the 6AH6.  No circuit changes are needed
and this really helps (also works great in the Heath
DX-100 series and the VF-1).

The next is to change the 0A2 regulator tube to an
0B2.  This drops the screen voltage to 108 volts which
also helps stabilize the unit (again works in the
Heath VFOs).

Along with dropping the screen voltage it also helps
to use the 0A2 removed from the screen of the
oscillator to regulate the plate voltage on the tube. 
This requires an additional 7-pin tube socket but that
can be mounted on a small bracket outside of the VFO
compartment (ditto on the Heath).

The final modification is to install a small 6.3 volt
filament transformer to keep the heater ("filament")
of the oscillator "on" all the time.  This is easy to
do, just wire the primary after the fuse but before
the off/on switch (Heath again).  A number of various
"boat anchor" manufacturers did this to cut down the
time required to stabilize the VFO in both
transmitters and receivers.

Glen, K9STH

Website:  http://k9sth.com


 
____________________________________________________________________________________
Never miss an email again!
Yahoo! Toolbar alerts you the instant new Mail arrives.
http://tools.search.yahoo.com/toolbar/features/mail/