[AMRadio] VFO vs XTAL


D. Chester k4kyv at charter.net
Thu Oct 18 08:35:06 EDT 2007


>If the VFO has a coupling cap, tie the VFO output directly to the grid

>of the buffer and see what happens.  If you don't have enough then, move
>back to the grid of the Osc.  Just be careful of driving the Osc too
>strongly.

>Jim/W5JO

In my Gates BC1-T I modified the 12BY7 xtal oscillator and converted it to a 
grounded-grid buffer.  Simple - bypass the grid to ground with a .005 or .01 
mfd capacitor.  The cathode already has an rf choke to ground, and the xtal 
feedback circuit connects to the cathode through a capacitor.  Disconnect 
the feedback lead, and replace it with a .01 capacitor, and use that 
capacitor to couple the rf from the outboard vfo directly to  the cathode of 
the stage.

That makes it very easy to convert back to stock if desired.  The vfo outut 
needs to be low impedance.  Mine measures about 1/4 watt into a 50-ohm 
resistor dummy load.  That drives the 12BY7 perfectly.

I use a similar scheme with the same vfo to drive the homebrew rig, to feed 
the type 59 input stage.  I have it wired to automatically convert to the 
conventional pentode xtal oscillator circuit with the vfo plug removed plus 
one jumper connection.

VFO's like the 122 and VF-1 are high-z output, directly from the plate of 
the oscillator tube.  You would need to add an additional stage to boost the 
power to 250mw at low-Z.  An untuned cathode follower stage might do the 
trick.

I use a highly modified T-368 master oscillator unit as my VFO.  The type 
6000 tube was replaced with a 6AG7 (socket rewiring required), with about 
200 volts on the plate. A coupling link was added to each of the slug tuned 
output  coils.  That required modification of the bandswitch, with the 
addition of another wafer section.  The addition of the cupling coil 
affected the tuning linearity of output stage somewhat, but it still gives 
pefectly uniform output across each ham band, which is all that counts, but 
the output drops off at the unused frequencies at extreme ends of each 
tuning range.  The stock unit delivered uniform output from 1.5 through 
about 20 mHz, directly into the grid of the 4-250.

My vfo is connected to the transmitter through a RG-59 coax line.  I have 
used as much as 15' of cable.  You can't do that with a stock 122 or VF-1. 
If the vfo cable is more than a few feet long, you lose output.  The output 
circuit has a slug-tuned coil at the plate, which feeds through a coupling 
capacitor to the coax line feeding the transmitter.  The capacitance of the 
coax line resonates with the coil to form a broadly resonant circuit to feed 
the grid of the 1st stage in the t ransmitter.  If the coax is too long, it 
won't hit resonance.  I once used a VF-1, and tried replacing the output 
coil with one that would resonate with a long run of coax, but it tuned too 
sharply, and wouldn't supply full  grid drive across the entire band.

Don k4kvy









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