Globe Scout Page
Bias and Sequenced Grid Block CW Keying (soon)
Bias and Grid Block Keying
The power transformer in the Globe Scout is a major limitation. The
transformer only has one high voltage winding. This means all lower voltages,
such as supply voltages for low level speech and RF stages, are
derived from the 500 volt high voltage source with dropping resistors. This is a
good strong transformer, it just lacks the HV winding taps to be an ideal
transformer for all the required voltages.
The power transformer voltage is 600 Vac. Normally a choke input supply gives
about .9 times RMS voltage minus rectifier drop, or about 500 volts.
Bleeder resistance is too high to hold the filter output voltage at normal
voltage of 500V, so unloaded voltage soars a bit higher.
Worse yet, all stages run from dropping resistors, so all the low voltage
tubes and screen grid of the 6146 go to 500 volts or more when the key is up!
I found a novel way to correct this while supplying negative voltage for grid
bias. I moved filter choke CH-1 into the center tap of the power transformer on
the ground side of SW-1, and
rectified the AC appearing across the filter choke to provide a negative DC
I moved the 6146 screen feed from the junction of CH-1 and CH-2 to the bottom of the plate choke
RFC-3, on the lead common
to the 6L6 plate.
The 6AG7/6F6/6V6 oscillator and audio stages operate from a much stiffer
voltage divider and the center of the two electrolytics, which now tie into the
center tap of the bleeder/voltage divider. This eliminates low voltage soaring
while the key is up.
Even if you do not change to grid block keying, adding this circuit and
biasing the 6146 grid negative will improve output power and reduce open key
voltage on the CW jack. This modification will allow moving the 450-ohm bias resistor from the
6146 cathode into the modulator plate circuit.
The tank circuit
in the Globe Scout
series does not work
very well on lower
bands as built. The
365 pF loading
capacitor is much
too small for lower
bands, but it can
easily be fixed
without tearing up
the rig. An easy patch
without destroying the rig is to solder a
160pF or 220pF 1000V mica capacitor across the
loading capacitor. Make up a
680 pF 1000V
capacitor in a PL-259 connector also.
working 40 and higher, use the standard RF output port.
When working 80 or 75, use the
When you work 160 meters, use the doublet port with the 680pF screwed into
the empty regular RF output port.
Correction For Oscillator Pulling
Remove the ground
from the driver
from the crystal
socket. The original ground point puts buffer RF plate current from the “OSC
TUNE” capacitor back into the crystal.
Route the tuning capacitor ground
ground wire (bare buss wire at
ground lug of osc capacitor) to the
ground where the
grounded. This is
only a partial cure,
but it is much
better than doing
Meter shunt is
R9, 0.4 ohms