|[AMRadio] VFO vs XTAL|
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
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
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
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.
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