[HBR] Fw: Question about homebrew receiver

Tue, 19 Aug 2003 10:39:52 -0500

Here is a note I just received and I thought I'd pass it by the experts.
Please respond to Don directly since he's not on the reflector and copy
the reflector.

73 Kees K5BCQ

----- Forwarded Message -----
From: "Kelly, Don" <[email protected]>
To: "'[email protected]'" <[email protected]>
Date: Tue, 19 Aug 2003 10:14:22 -0500
Subject: Question about homebrew receiver
Message-ID: <[email protected]>

I saw your HBR receiver site. Your presentation of those rigs is great. I
could not find a better tube RX site anywhere on the web.

I have almost completed a homebrew tube receiver and I have one small
problem I would like to ask you about if you are still in that "busines".
My rig is basic 40 meter. Will use front end upper band to 40 converters
for 20, 15, 10. Same in the "twin" transmitter only backwards (will use
RX LO and mix up).
I use a 6BA6 RF amp with gang cap to tune front end. This is going to be
my question. 
My LO runs 3.0MHz to 3.35MHz with 4MHz IF using micro processor crystal
in lattice filter. 
The LO is buffered by a 6C4 cathode follower with double tuned bandpass
filter to reduce harmonics. Going to change to triple tuned as there is
still a slight birdie that I hear on quiet band.
The mixer is 6BE6 pentagrid mixer. I probably didn't need to buffer the
LO due to good isolation characteristic of 6BE6 but since I am going to
use RX LO for transmit I thought this to be wise.
There are three stages of IF all of which will be controlled by AGC (not
sure whether to use IF or audio yet)...no AGC on RF amp planned yet but I
think I will make change.
I use 12AT7 product detector. MY BFO is single 2N2222A solid state
Hartley with low pass filter in output. Birdies originally appeared at
harmonics of LO and BFO and these filters all but wiped them out.
AF pre-amp is 6CG7 with 6005 power amp. 
I used a lot different things. I used toroids in place of pi wound chokes
with great results as it made thing smaller. The RF chokde after the
product detector is an FT82-75 works somehat as a low pass audio filter. 
The LO coil is also toroid T68-6 which has performed tempature wise much
better than the literature would lead one to believe. I purchase a new
100pF cap form Cardwell Condesor supported on both ends and
mounted all the components in a PC board box. I positioned all the LO
components on standoffs as solid as I could. I temperature adjusted for
several weeks until I was satisfied. After initial warmup it is SSB
stable for hours at a time even though the chassis heats up...the
isolation in the PC bd box and because I kept heat away from this area
makes it very stable.
I hope this is not too much info but a little background. 
My only real problem is that when I tune the front end I get an
occasional "squiggling" type of noise. Usually when I tune off the
resonant freq and it is not bothering operation. I realize this is
probably some HF or VHF parasitic. I placed resistors in parallel with
the front end coils and it reduced it. I think this problem occurred in
part because I used toroids in the front end and I am a victim of Q. I
see in many of the schematics on your site the placement of a small
resistance in series with first tuned coil and grid of RF amp. Sometimes
a choke is wound on this small resistance I assume to suppress
parasitics.
I can find no info on this fix or if there is a better more current
method for reducing this problem....such as ferrite beads, best trimmer
caps to use, etc.
If you are not busy or if you are still interested in tube receivers
could you help me or refer me to someone with some info? All my dad's old
tube books have no reference and the ARRL Handbooks show the use of this
fix but give no background.
If you are busy with other things thanks anyways and I appreciate your
site for us tube builders. 
Don Kelly, KA5UOS 
Edmond, OK      73003 

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