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
[EMAIL PROTECTED]
http://www.wd8das.net/
------------------------------------------------
Radio is your best entertainment value.
------------------------------------------------
-----Original Message-----
From: [EMAIL PROTECTED]
To: [email protected]
Sent: Mon, 9 Apr 2007 1:12 PM
Subject: [AMRadio] Re: DDS VFO projects
> From: [EMAIL PROTECTED]
> Subject: [AMRadio] DDS VFO projects
> To: [email protected]
>> 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
______________________________________________________________
AMRadio mailing list
List Rules (must read!): http://w5ami.net/amradiofaq.html
List Home: http://mailman.qth.net/mailman/listinfo/amradio
Partner Website: http://www.amfone.net
Help: http://mailman.qth.net/mmfaq.html
Post: mailto:[email protected]
To unsubscribe, send an email to [EMAIL PROTECTED] with
the word unsubscribe in the message body.
________________________________________________________________________
AOL now offers free email to everyone. Find out more about what's free
from AOL at AOL.com.
______________________________________________________________
AMRadio mailing list
List Rules (must read!): http://w5ami.net/amradiofaq.html
List Home: http://mailman.qth.net/mailman/listinfo/amradio
Partner Website: http://www.amfone.net
Help: http://mailman.qth.net/mmfaq.html
Post: mailto:[email protected]
To unsubscribe, send an email to [EMAIL PROTECTED] with
the word unsubscribe in the message body.
