[AMRadio] Re: DDS VFO projects


sbjohnston at aol.com sbjohnston at aol.com
Mon Apr 9 15:03:52 EDT 2007


Don wrote:

>You need to bypass the cathode of the 12BY7 to ground when the VFO is 
in use.
>The stage will be more stable and possibly deliver better output.
>Simply connect a 0.01 mfd capacitor, similar to C5 and C7, between the
>cathode (pin 1 of the 12BY7) and the unused contact on S1.

Very slick idea to use the other pole of the switch.    I did test 
bypassing the cathode as you describe when I was testing the VFO input. 
  I tried both .001 and .01 temporarily in place and it was no different 
than the existing 330 pF alone, so I didn't pursue it further.

Steve WD8DAS

sbjohnston at aol.com
http://www.wd8das.net/
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-----Original Message-----
From: k4kyv at charter.net
To: amradio at mailman.qth.net
Sent: Mon, 9 Apr 2007 1:12 PM
Subject: [AMRadio] Re: DDS VFO projects

  > From: SBJohnston at aol.com
 > Subject: [AMRadio] DDS VFO projects
 > To: amradio at mailman.qth.net
  >> In response to some inquiries, here's a webpage summarizing my DDS 
VFO
 > projects...
 >
 > http://www.wd8das.net/DDSVFO/ddsvfo.html
 >
 > Steve WD8DAS

  You need to bypass the cathode of the 12BY7 to ground when the VFO is 
in use. The stage will be more stable and possibly deliver better 
output. Simply connect a 0.01 mfd capacitor, similar to C5 and C7, 
between the cathode (pin 1 of the 12BY7) and the unused contact on S1.

  On mine, I use a low impedance VFO about 10 ft. from the transmitter. 
The VFO runs about 250 milliwatts into a 50-ohm load. I inject the rf 
directly to pin 1 through a 0.05 mfd mica coupling capacitor, and short 
pin 2 directly to ground. I disconnected the wire from the junction of 
C3 and C4 that goes to the cathode. This converts the 12BY7 to a 
grounded grid buffer stage. Your DDS VFO may not have enough output to 
do this without an amplifier between the DDS unit and the 12BY7, so 
your circuit may work better, but I would make sure the cathode is at 
rf ground. Turning S1 back to the crystal positions in yours restores 
it to normal crystal control.

  I disabled the crystal control function altogether in mine, and used 
one of the holes that originally had C1 or C2, to hold remounted C9. As 
I recall, I had to replace C9 with another capacitor with a longer 
shaft because the original was screwdriver adjustable. But now I can 
adjust the tuning of L3/C9 with a tuning knob from the front panel, 
without having to go inside the transmitter, remove the cover of the 
oscillator unit and adjust the slug in L3. I had to remove some turns 
 from L3 to get it to resonate in the 160m band. Then I adjusted the 
slug by setting C9 at mid range, and adjusting the slug in L3 for 
resonance at 1900 kHz. I used trial and error when removing the turns 
by removing just a few turns at time until I could bring it into 
adjustment with the slug, with leeway in both directions of the tuning 
range of the slug.

 Don k4kyv
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